University of Tennessee, Knoxville
Trace: Tennessee Research and Creative
Exchange
Doctoral Dissertations
Graduate School
6-1981
Electrochemical and Spectroscopic Studies of
Some Less Stable Oxidation States of Selected
Lanthanide and Actinide Elements
David Edward Hobart
University of Tennessee - Knoxville
Recommended Citation
Hobart, David Edward, "Electrochemical and Spectroscopic Studies of Some Less Stable Oxidation States of Selected Lanthanide and
Actinide Elements. " PhD diss., University of Tennessee, 1981.
http://trace.tennessee.edu/utk_graddiss/1600
This Dissertation is brought to you for free and open access by the Graduate School at Trace: Tennessee Research and Creative Exchange. It has been
accepted for inclusion in Doctoral Dissertations by an authorized administrator of Trace: Tennessee Research and Creative Exchange. For more
information, please contact [email protected].
To the Graduate Council:
I am submitting herewith a dissertation written by David Edward Hobart entitled "Electrochemical and
Spectroscopic Studies of Some Less Stable Oxidation States of Selected Lanthanide and Actinide
Elements." I have examined the final electronic copy of this dissertation for form and content and
recommend that it be accepted in partial fulfillment of the requirements for the degree of Doctor of
Philosophy, with a major in Chemistry.
Joseph R. Peterson, Major Professor
We have read this dissertation and recommend its acceptance:
Gleb Mamantov, M. H. Lietzke, Paul G. Huray
Accepted for the Council:
Dixie L. Thompson
Vice Provost and Dean of the Graduate School
(Original signatures are on file with official student records.)
To the Graduate Counc i l :
I am submitt ing herewith a dis sertat ion writ ten by David Edward
Hobart enti tled "El ec trochemical and Spec troscopic S tud ies o f Some
Les s S table Oxidat ion States of Se lec ted Lanthanide and Actinide
E lements . " I recommend tha t it be accepted in part ia l ful fi l lment of
the requirements for the degree o f Doc to o f Ph ilosophy , wi th a maj or in
Ch emis try .
J
We have read th is d i s sertat ion
and recommend it s acceptance :
Ac cepted for the Coun c i l :
Vice Chance l lor
Graduate S tud ie s and Research
ELECTROCHEMICAL AND SPECTROSCOPIC STUDIES OF SOME LESS STABLE
OXIDATION STATES OF SELECTED LANTHANIDE AND
ACTINIDE ELEMENTS
A Disserta t ion
Presented for the
Doc tor of Ph il osophy
Degree
The Univers i ty of Tennes s ee, Knoxv i l l e
David Edward Hobart
June 1981
3051973
DEDICATION
The author wi shes to dedicate this work to hi s loving wi fe ,
Barbara , and lovely daugh ter , Mi che l le, for the ir patience and under­
standing dur ing th e course of thi s endeavor .
ii
ACKNOWLEDGMENTS
The author wishe s to express his grati tude and apprec iat ion to
Dr .
Jo seph R. Pe ter son , teacher , col league , and fr iend , for he lpful
advice and support throughout the course of th is research .
The au thor
is indeb ted to Dr . Kamal S amhoun for valuab le co l l aboration and
friendsh ip.
His knowl edge and technical as s i s tance are greatly
appre c ia ted .
Sincere appreciat ion is extended to Dr . Gleb Mamantov fo r he lpful
gu idance , sugge s t ions , and enc ouragement .
Dr . Mamantov and Drs . M. H.
Lie tzke , P . G. Huray , and J. P. Young are acknowledged for he lpful
d iscuss ions and for reading and cr it iqu ing thi s manusc ript .
Spe c i a l
thanks to Mis s Caro l Proaps for typing th is disser ta t ion .
The author is gra te ful to the Department o f Chemis try , Univer s i ty
o f Tennes s ee (Knoxvi l le ) for financ ial suppor t fir s t in the fo rm of a
teach ing as s i s t an t sh ip and then as a research as s is tantsh i p , sponsored
by the U . S . De par tment of Energy under contrac t DE-AS05 -7 6ER04447 .
Apprec ia t ion is also extended to the Oak Ridge Nat ional Laboratory
( ORNL) , operated for the U . S . Department of Energy under contrac t
W-7405-eng- 2 6 with the Un ion Carb ide Corporat ion , and to the
Tr ansuranium Research Laborat ory (TRL) for the us e of their fac i l i t y.
The suppor t and cooperat ion of the ORNL Chemis try Divis ion , under
the di rec tion of Dr . 0. L. Ke ll er , Jr . , is recognized and
acknowledged .
The author is indebted to the ent ir e s t a f f of the TRL
for the ir pa t ience and te chnical support .
iii
In part icul ar , the au thor
iv
i s gra teful to Dr s . R. L. Hahn , R. G. Haire , G. M. Begun , D. D.
Ensor , S . E. Nave , and G . D. O ' Ke l ley for hel p fu l as s is tance and
d iscuss ions ; Mr . E. L. Ear ley , Captain T. C. Minton , Mr . K. Sowder ,
Mr . A. Mas s ey , Mr . J . H. Ol iver , Mr . J . Burkhalter , and Mr . J . R.
Tarrant for techn ical as s i s tance ; Ms . J. Hamby and Mrs . B .
Mercer fo r
though t fulness and pa t ience ; and to Mr . C. E . Haynes , Mr . W. D.
Carden , Mr . T. Col l ins , Mr . J . Smi th , and Mr . B . A. Power s for he alth
phy s i c s suppor t .
Apprec iat ion i s extended to the author ' s col league s , coworkers ,
and fr iends , Dr . Lieutenant D . M . Hembree , Mr . M. K. Pa s torc ich ,
Mr . V . E. Norve l l , Dr s . F . David , B. Gui l l aume , J . Bourge s , G. Bea l l ,
B . Al lard , M . Hara , Ms . I . Schauer , Ms . K. Ke l l y , Mr . J . Mo t tern , Mrs .
S . A. Morr i s , and many others for as s is tanc e and suppor t .
Spec ial ment ion is made of persons the author ha s admired and
emul ated and Who have provided leadersh ip and encour agement ,
Mr . D . B . Hobar t , Mr . A. E . Dempsey , Dr. S . Frome , Dr . G. T. Cochra n ,
and espec ia l l y , Dr. H. E . He l lwe ge .
ABSTRACT
The technique of simul taneous ob servation of elec trochemical and
spec tros copic prope r t ies ( s pec troe lectrochemis try) at optic a l ly
transparent elec trodes (OTE 's ) has been app lied to the generat ion and
charac terizat ion of some le ss s tab le ox idat ion states of selected
l anth anide and ac t inide el ements ( Ce , Pr , Sm , Eu, Tb, Yb , U, Np, Am,
and Cm) in complexing and noncomplexing aqueous so lutions .
Opt ic a l l y
transparent elec trodes us ed in th is stud y inc luded re ticu lated
vitreous carbon ( RVC ) and metal screen OTE ' s and a new OTE made from
porous me tal foam (PMF) .
Cyc lic vol tamme try at mic roe lectrode s was us ed in conj unc t ion
with spec troe lec trochemis try for the stud y of ox idat ion-reduc t ion
( redox) coupl es .
In some cases add it ional ana lyt i c a l te chniques were
app l ied for iden t i f ic a t ion of elec trochemica l ly gene rated oxidat ion
s ta te spec ie s ; these inc luded solut ion ab sorption, so l id-s tate
reflec tance,
and laser Raman spec troscopie s , X-ray powder di ffract ion
and thermogravimet ric-mas s spectral analyse s , radiochemical
measurements, and spec trophotome t ric and potent iome tric redox
titrime try.
The forma l reduc tion po tential (E 0 ' ) va lue s of the M( I II ) /M( I I )
redox coup les in 1 M KCl at pH 6 were found by vo l tammetry to be -0.34
� 0.0 1 V for Eu, -1. 18
+
0.01 V for Yb , and - 1.50 + 0.0 1 V for Sm.
Spec tropo tentios t a t ic determina tion of E0 ' for the Eu ( I I I ) /Eu ( II )
redox couple yie lded a va lue o f - 0.39 1 � 0.005 V .
Spec tropo tent ios t a t ic measurement o f the Ce ( IV ) / Ce ( I I I ) redox couple
in concentrated carbonate solution gave E 0 1 equa l to 0.05 1 � 0.005 V ,
v
vi
whi ch is about 1 . 7 V les s pos itive than the E01 value in nonc ompl exing
solution .
Th is same di fference in po tential wa s ob served for the E 01
values of the Pr ( IV) /Pr ( I I I ) and Tb ( IV) /Tb ( I I I ) redox coupl e s in
carbonate sol u t ion , and thus Pr( IV) and Tb ( IV) were stabi lized in th is
med ium.
The so lut ion absorpt ion spec tra and redox prope r t ies of these
aqueous spec ie s are repor ted .
A so l id Tb ( IV)-containing compound was
also prepared at an RVC elec trode in carbonate so lution.
The U (VI ) /U (V) /U( IV) and U ( IV) /U ( I II ) redox couples were s tud ied
in 1 M KCl at OTE ' s .
23 7Np , 243Am , and 2 48cm were stud ied in
containment gloved box fac i l ities .
Spec tropotentios t a t ic measurement
of the Np(VI ) /Np( V ) redox couple in 1 M HC l 04 gave an E0 ' va lue of
1 . 140
+
0 . 005 V.
Np( VI I ) wa s generated by elec tro lysis o f Np (VI) in
2 M Na 2 C03 at pH 1 3 , and the solution ab sorption and laser Raman spec tra
were recorded , and an E 0 1 value of 0 . 46 + 0 . 0 1 V for the Np( VI I ) /Np(VI)
coupl e was found by vo l ta mme try .
Ox idat ion o f Am( I II ) was stud ied in concentrated carbonate
solut ion , and a rever s ib l e cyc l ic vol tammogram fo r the Am( IV) /Am( I I I )
coupl e yie lded E 0 1
=
0 . 9 2 � 0 . 0 1 V i n th is med ium ; this val ue was us ed
to es t imate the standard reduc tion potent ia l (E 0 ) of the coup le as
2 . 62
+
0.01 V.
The so lut ion ab sorpt ion spec trum and redox behavior of
Am( IV) we re compared to those of Am(V) and Am( VI) in th is same medium.
At tempt s to oxidize Cm( I I I ) in concentrated carbonate so lution
were not success ful wh ich suggests that the pred ic ted E 0 value fo r the
Cm( IV) /Cm( I I I ) redox coup l e may be in error .
TABLE OF CONTENTS
CHAPTER
I.
PAGE
INTRODUCTION
A.
Background .
1
B.
Propo sed Research
3
c.
Spec troelec trochemi s t ry .
1.
2.
3.
D.
•
De scr i p t ion
Theory .
Li terature Review
E l ec tronic S truc ture . .
Oxidat ion States .
Oxidation-Reduc t ion Po tent ials
Solut ion Absorpt ion Spec tra .
EXPERIMENTAL DETAILS
A.
In troduc t ion
1.
2.
3.
4.
B.
1.
2.
3.
4.
s.
11
12
23
27
•
•
•
29
•
29
29
29
31
•
•
•
.
•
29
•
•
Spec trophotometers •
Vo ltammeter
Gl oved Boxes
•
Accessory Equipment
•
33
•
Convent iona l Working E l ec trodes
Opt ic a l ly Trans paren t Elec trodes .
Referenc e E lec trode s •
Counter E l ec trodes .
Te flon Ce l l Ho lder •
•
•
•
•
.
c.
Reagents
D.
Procedures
1.
2.
3.
4.
11
•
•
E l ec trodes and Ce l l s
4
4
6
9
.
The Lanthanide and Ac t inide Element s .
1.
2.
3.
4.
II.
1
•
•
•
General Procedures
Lanthanid e E l ement s
Ac t inide E lements
Da ta Treatment .
•
•
.
•
•
•
•
•
vii
37
•
•
•
33
34
36
36
36
40
40
41
44
48
viii
CHAPTER
I II .
PAGE
RESULTS AND DISCUSSION
. . . . . . . .
•
•
•
•
50
•
A.
Lanthanide Ele ments . . . . . . . . . . . . . . .
1 . M( I I I ) /M( I I ) Redox Coup les • • • • • •
2 . M( IV) /M( I I I ) Redox Coup les • . . . . . . .
50
50
67
B.
Ac tinide E l e ments
.
84
. . . .
. . . . .
.
•
•
•
.
84
93
107
12 5
. . . . . . . . . . .
1 33
1.
2.
3.
4.
IV.
•
Ur an iu m .
Ne ptuniu m
Ame riciu m
Cur ium •
•
. . . .
.
. . .
. . . . . .
.
. . . . . .
. . . . . . .
. . .
SUMMARY AND CONCLUS IONS
.
. .
.
.
.
.
Opt ic a l ly Tr ans parent E lec trodes
B.
Lanthanide E l e ments
c.
D.
. .
•
•
•
•
•
.
.
•
•
•
. .
.
.
•
•
•
133
135
138
•
•
. . . .
.
•
•
•
•
•
•
. . . . .
.
.
.
.
•
•
•
•
.
•
•
•
•
•
•
•
•
•
•
. .
.
•
•
. . .
.
. . . . . .
.
•
•
.
•
. . . . .
•
. . . . . . . . . . . .
.
•
•
135
135
Sugges ted Fu ture Research .
LIST OF REFERENCES
VITA . .
.
•
.
.
.
.
•
M( I I I ) /M( I I ) Redox Couples
M( IV) /M( I I I) Redox Couples
Ur aniu m .
Ne ptunium.
Amer iciu m
Cur ium • .
•
. . . . . .
Ac tinide E l e ment s . .
1.
2.
3.
4.
. . . . . . .
. . . .
.
.
.
. . . .
. . . .
A.
1.
2.
.
•
•
•
•
143
14 5
. . . . .
•
138
139
140
14 1
•
•
153
LIST OF TABLES
PAGE
TABLE
I.
II.
III.
IV.
V.
.
E l ec tronic Configurat ions o f Ac t inide Atoms and Ions
Oxidat ion States of the Lanthanide Elemen t s
•
•
•
•
•
Oxidat ion States o f the Ac tinide E l ement s
•
•
•
•
.
•
.
.
.
14
. .
16
.
.
.
17
Reduc t ion Po tentials of Higher Oxida t ion State Couples
o f Some Ac tinide E lements
VI.
13
El ec tronic Conf igur a t ions of Lanthanide Atoms and Ions .
•
•
•
•
•
•
•
•
•
•
•
•
•
28
•
Da ta for Spec tropo tentios tatic De terminat ion of
Th ermodynamic Parame ters for the Eu ( I II ) /Eu( I I )
Redox Coup l e .
VII .
.
.
.
. .
.
. . . .
.
.
.
.
. .
.
.
.
. .
.
63
Da ta for Spec tropo ten t ios tatic De terminat ion o f
Th ermodynamic Parame ters for the Ce ( IV ) /Ce ( I I I ) Redox
Cou pl e .
VII I.
•
•
•
. . .
.
.
. . .
.
.
.
. .
.
.
.
.
.
.
74
Da ta for Spec tropotentiostatic Determinat ion o f
Thermodynamic Parameters for the Np ( VI ) /Np ( V ) Redox
Coupl e . . . .
IX .
.
.
.
.
.
. . . . .
.
. . .
•
•
•
•
.
.
•
.
100
•
1 23
. .
12 6
.
Da ta from the Spec trophotometric Titrat ion of Am( IV)
w ith Ferrocyan ide Solution.
X.
.
•
•
•
•
•
•
.
.
. .
.
•
Da ta from the Po tent iometric Titrat ion o f Am( IV) with
Ferrocyanide Solu tion
•
•
•
ix
•
•
•
•
•
•
.
. .
.
.
.
.
LIST OF FIGURES
FIGURE
1.
PAGE
Standard reduc tion po tentials for M( I I I ) /M( I I ) redox
coupl es of some lanthanide and ac tinide elemen t s ,
l anthanum , and ac ti nium
2.
•
•
.
.
.
.
. . . . . . . . . . .
Standard reduc tion po tentials for M( IV) /M( I I I ) redox
coupl e s of the lanthan ide and ac tinide elements
3.
•
•
. . .
•
•
•
•
•
•
•
.
.
. . . . . .
4.
Spec trophotome ter/mod i fied gloved box as semb l y
5.
A re t icul ated vi treous carbon opt ically transparent
e lec trode (RVC-OTE )
•
•
•
•
•
.
32
. . .
35
•
•
38
•
•
39
•
•
52
•
•
•
•
•
56
57
. .
.
•
•
6.
Te f l on ce l l ho lder wi th lid in "up" pos i t ion .
•
•
•
7.
Te fl on ce l l ho lder wi th l id in "down" pos i t ion •
•
•
8.
Cyc l ic vo l tammograms o f M( I I I ) species of Eu ( A ) , Yb (B) ,
•
•
•
•
•
•
•
•
•
•
•
30
•
and Sm ( C ) in 1 M KC l
•
•
. .
.
•
•
•
•
•
•
.
•
•
•
•
9.
Cyc l i c vo l t ammogram of Eu ( I I I ) in 1 M KC 1 in an RVC-OTE
10 .
So lut ion absorpt ion spectra of Eu( I I I ) and Eu ( I I )
in 1 M HC l04 .
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
53
So lut ion absorpt ion spec tra o f Yb ( I I I ) and and Yb ( I I )
i n 1 M HC 104
12 .
25
Equipment for spec troe lec trochemic a l stud ies o f rad ioac t ive
a c t inide e l ements
11.
24
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
So lut ion absorp t ion spec tra of Sm( I I I ) and Sm( I I )
i n 1 !, HC l04 .
•
•
•
•
•
•
•
•
•
•
X
•
•
•
•
•
•
•
58
xi
FIGURE
13.
PAGE
Absorbance vs time curve for the potential-s tep elec tro l ys is
o f Eu( I I I ) i n an RVC-OTE . . .
14 .
.
•
.
.
•
. . .
.
. .
•
•
.
•
•
.
.
.
. . .
•
•
. .
60
•
.
•
61
. . . .
64
. . . . .
•
•
•
•
.
Ab sorb ance vs time curve for the poten t ial-step elec trolys is
of Yb ( I I I ) in a s i lver ama lgam sc reen OTE
17 .
•
Nerns tian plot for the Eu( I I I ) /Eu ( Il) redox coup le in
1 M KC l .
16 .
•
So lution absorpt ion spec tra of Eu ( II I ) and Eu ( I I ) a t var ious
values of app l ied po tent ial in an RVC-OTE
15.
.
. . . .
.
. . . .
66
So lut ion absorpt ion spec tra o f Yb ( I I I ) and Yb ( I I ) as a
func t ion o f time dur ing the po tent ial- s tep e lectro lys is
o f Yb (II I ) in 1 M KC 1 •
18.
•
•
. . . . . . .
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
.
•
•
•
. . . . .
.
68
.
•
•
•
•
•
•
•
•
•
70
•
•
•
•
•
73
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
76
.
.
•
.
•
•
•
•
•
77
. . . .
•
So lut ion absorpt ion spec tra of Tb ( I II ) and Tb ( IV ) i n 5 . 5 M
K2 C0 3 . . . . . . . . .
23 .
. .
So lut ion abs orpt ion spec tra o f Pr ( I II) and Pr ( IV ) in 5 . 5 M
K 2 C03 .
22 .
.
Nerns t ian plot for the Ce ( IV ) /Ce ( I II ) redox couple in
5 • 5 M K2 C 03 •
21.
.
So lut ion absorp t ion spec tra of Ce ( IV) and Ce ( I I I ) at var ious
values of appl ied potential in 5 . 5 M K2 C03
2 0.
.
So lut ion abs orpt ion spec tra of Ce ( I II) in 1 M HC l04 and Ce ( IV )
i n 1 M H2 S04
19 .
•
.
.
.
.
•
.
.
. .
•
•
•
•
•
•
.
79
•
.
•
•
81
Solut ions of Pr ( I I I ) ( A ) , Pr ( IV) ( B) , Tb ( I II ) ( C ) and
Tb ( IV) ( D ) i n 5 . 5 M K2 C03 . . .
.
. . . . .
•
•
•
•
xi i
FIGURE
24 .
PAGE
So l id- s tate re flec tance spec tra o f Tb 2 C C03 ) 3 ( A) , unknown
sol id ( B ) , and Tb 7 0 1 2 ( C)
•
•
•
•
•
•
•
•
•
•
•
•
83
•
25 .
Cyc l ic vo l tammograms of U(VI) in 1 M KC l
86
26 .
So lut ion abs orpt ion spec tra of U ( VI , V, IV , and I II ) .
87
27 .
So lut ion absorpt ion spectra o f U ( VI ) and U ( IV) as a
func t ion o f time during potent ial- s tep elec tro lys is of
U (VI )
28 .
•
•
•
.
. . .
.
.
.
.
. . . .
.
. . . . .
.
.
89
•
91
So lut ion ab sorp t i on spec tra o f U ( VI ) and U ( V ) in an
RVC-OTE
•
•
•
•
•
. . . . . .
.
.
.
.
.
.
. . . .
29.
Cyc lic vo l tammograms of U ( IV) in 1 M KC l at an HMDE .
30 .
So lut ion absorpt ion spec tra o f U( IV) and U ( I II ) a s a
•
•
•
92
funct ion o f time during the poten t ia l-step elec trolys is
o f U ( IV)
.
.
.
.
.
.
.
.
.
.
.
.
.
. . .
. . .
31.
Cyc lic vo l tammogram o f Np ( V ) in 1 M HC l04
32.
So lu t ion absorp t i on spec tra of Np ( V ) and N p ( V I ) in 1 M
HCl04 and N p ( VI I ) in 1 M LiOH
33 .
•
•
•
•
•
•
•
. .
94
.
96
.
97
•
So lut ion abs orption spectra of Np (VI ) and Np ( V ) at various
values of appl ied po ten tial in 1 M HC l04 .
34 .
.
•
99
•
Ne rns t ian plot for the Np( VI ) /Np( V ) redox couple in 1 M
HCl 04
•
•
•
•
•
10 1
35 .
So lut ion absorption spec t rum of Np ( V ) in 2 M Na 2 C03
•
•
•
•
10 2
36 .
Cyc lic vo l tammogram of Np (VI ) in 2 M Na 2 C03 at pH 13
•
•
•
104
37.
So lution ab sorpt ion spec tra o f Np (VI ) (A) and Np ( VI I ) ( B )
•
•
•
•
•
•
•
•
i n 2 M Na 2 C03 a t pH 1 3
38.
•
•
•
•
•
. . . .
•
.
•
.
•
•
•
•
•
•
. .
.
. .
.
.
Raman spec tra of Np ( VI ) and Np (VII ) in 2 M Na 2 C0 3 a t pH 13 .
105
106
xi ii
FIGURE
39 .
PAGE
Cyc l ic vo ltammograms of Am( I I I ) in 2 M Na 2 C03 a t var ious
p H va lues
•
•
•
•
. . . . . . . .
.
. . . .
•
108
•
40.
So lu tion absorpt ion spectra of Am( III , IV , V , and V I )
41.
So lut ion abs orpt ion spec tra o f Am ( I I I ) and Am( I I I ) p lus
Am( IV) in 5 . 5 M K2 COJ
42 .
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
111
•
•
1 12
•
•
•
1 14
•
1 15
•
•
1 18
•
•
12 2
•
•
•
•
Solution absorpt ion spec tra o f Am ( V ) in 2 M Na 2 COJ-KOH (A)
and in 1 M HN03 ( B)
45 .
•
•
So lut ion absorp t ion spec tra o f Am( IV) (A) and Am( IV) plus
Am( I I I) ( B ) in conc entrated HF sa turated wi th Cs F
44 .
•
•
So lut ion abs orption spec tra of Am( III ) (A) and Am( IV) ( B )
i n 2 M Na 2 C03
43 .
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
So lut ion absorpt ion spec tra o f Am( IV) and Am( I I I ) a s a
func t ion of ti trant added during a spectrophotomet ric
t itrat ion o f Am( IV) with ferrocyanide so lut ion •
46.
•
•
•
Ab s orbance vs volume of titrant added in the spec tropho tome tric ti trat ion of Am ( IV) with fe rrocyanide so lut ion .
47 .
1 24
Po tent ial� vo lume of titrant added for the potent iame tric ti trat ion o f Am( IV) with ferrocyanide solut ion
48.
.
.
.
127
•
129
.
130
•
142
So lut ion abs orpt ion spec tra of Cm( III) in 1 M HCl 04 and
Cm( IV) in 15 M Cs F at 10 . 5°C •
•
•
•
•
•
•
•
•
•
•
•
49 .
So lut ion abs orpt ion spec trum of Cm ( I I I ) in 5 . 5 M K2 C0 3
50 .
Po tential � pH d iagram for the predic ted oxidat ion
s tate spec ies of Am in aqueous solut ion . . . . .
•
•
•
.
•
•
CHAPTER I
INTRODUCTION
A.
Background
The £- trans i t ion ser ies elements , the lanthan ide s and
act inides , cons t itute nearly 2 8% of the known chemic a l elements .
Although some of these e l e ments have been known fo r more than two
hundred ye ars , l it has on ly been recen t l y , s inc e the discove ry of
nuc lear fi s s ion , that the man-made transuranium elemen ts have been
syn thesize d .
The deve lopment o f nuc lear technol ogy h a s st imul a ted cons iderab le
interes t in the s tudy of the lanthanide and ac t inide elements .
The
l anthanides are the maj or fi s s ion produc t s produced in a nuc lear
reac tor and , as ide from the ir important indus trial appl i cat ions , the
lanthanide elements exhibit the only truly ana logous chemica l behavior
to that of the transuranium elements .
A thorough under s tanding of
lanthanide chemis t ry can provide valuab le in format ion concerning the
chemical behavior o f the heavie s t ac t inides for wh ich bulk-phase
s tud ies are no t po s s ible .
occurr ing elements :
The ac tinides inc lude three natural ly
thorium , protac tinium , and uranium .
The
remaining member s of the serie s , the transuranium elements , are
synthe sized as a re sul t of neutron-c apture processes in a nuc lear
reac tor or as a resul t of high-energy par tic le bombardment in an
acc e l erator .
A thorough knowledge of the basic physicochemic a l
1
2
properties of the transuranium elements is es sent ial in view o f the
ever- increas ing dependence on nuc lear power for energy need s .
Addit iona l l y , the lanthanides and ac t inides are un ique in the periodic
c la s s i f icat ion of the elements since they are the on ly elements wh ich
exhibit £-e lectron chemical behav ior .
The chemical behavior of any element depends , to a great extent ,
upon the oxidat ion states exhibi ted by that element and upon their
relative st abi l i ties .
The electrochemic a l reduc t ion potent ial ,
usua l l y expre s sed in vo lts , is a thermodynamic measurement of the
rel a t ive s tabi l i t ies of oxidat ion state spec ies .
The reduc t ion
potential is direc t ly measurab le in many cases by us ing convent iona l
e lectrochemical analysis technique s such as potentiomet ry or
voltamme try .
Furthermore , wi th vo ltammetric techniques , it is
pos s ib le to oxid ize and/or reduce spec ies to generate new ( di fferent )
oxidat ion states in a convenient , reproduc ib le manner simpl y by
app l ying the proper elec trochemica l potential to a sys tem .
The
control o f an oxida t ion-reduc tion ( redox) process by elec trochemical
means can be much more prec ise than is pos s ib le by us ing chemic a l
reagent s . 2
With convent ional electrochemic a l techniques , the id ent ity of the
" produc t" oxid at ion s t a te is determined by the known va lue o f the
reduc tion po tential of the redox couple , or by de termining the
e lec tron s t oichiometry .
Both methods are adequate for sys tems
exhibiting elec t rochemica l reversib i l i ty under the cond i t ions o f the
experiment .
O ften t imes the sys tems are no t rever s ib l e , or the
reduc t ion po tential is not known for a part icul ar redox couple under
3
certain condi t ions ; add i t ional charac ter izat ion techniques are
requ ired in these cases .
Abs orpt ion spectrosc opy has often been us ed in conj unc t ion wi th
elec trochemic a l me thods for unequivoc a l iden t i fication o f elec tro­
generated produc ts .
However , the simul taneous appl i c a t ion of
s pec troscopic and elec trochemical technique s , cal led spect roe lec tro­
chemis try , 3
provides a much more powerful and vers a t i l e ana lyt ical
too l .
B.
Proposed Re search
A forma l reduc t ion poten t ial can be al tered by the degree o f
c ompl exat ion o f the oxid ized and/or reduced spec ies o f a redox coupl e .
A compl exing ligand wh ich pre ferentially compl exes one o f the
oxidat ion s t ates over the other can sub s tan tially change the va lue o f
the reduc t ion po ten t ial .
An oxidat ion st ate wh ich is uns t ab l e i n a
non- compl exing aqueous solution can be stabi l ized by complexa t ion .
The ob j e c t ive o f th is re search was to de termine the access ib i l i ty of
some unusual or unknown oxidat ion st ates of the transur anium elements
by spec troe lec trochemical inve s t igat ion of compl exing aqueous
s olut ions as a means to genera te and charac terize some le s s s tab le
oxidat ion s tates of selected lanthanide and ac t inide elemen ts .
Because o f the inheren t s impl icity and sens i t iv i ty of the
techn ique and the advantage of us ing sma l l sampl e vo lumes (usua l ly
less than one mL ) , spec troe lect rochemis try is we l l su i ted to the
e le c trochemical generat ion and spec troscopic charac terizat ion o f
less s t ab le oxidat ion s t ates o f the lanthanide and ac t in ide elements .
4
The technique is par ticul arly we l l su ited to the s tudy of the
transplutonium e lements since many of these are ava i l ab le onl y in
limi ted quant i t ie s .
Due to the high l y radioac t ive nature and l imited ava i l ab i l i ty of
the heavier ac tinides , exper iments were per formed us ing the
nonradioac tive l an thanides and less rad ioac t ive l igh ter ac t inides as
sub s t itutes for the transplu tonium element s .
The purpose o f these
exper�ents wa s to ver i fy the proper operat ion o f the ins t rumenta tion
and to determine the correc t exper�ental approach wi thou t
j eopardizing the fate of the l imited amoun ts o f trans plutonium
materials .
Once these experiments were compl eted , the experiments
using the transplutonium e lements were per formed in a gloved box
fac i li ty .
c.
Spec troe lec trochemis try
Here the reader is provided with a brie f description and review of
the technique s of spec troelectrochemis t ry wi th emphas i s p laced on the
app l icat ions pe rt inent to th is work .
For more detai led and
comprehens ive treatment s , the interes ted reader is directed to the
many exc e l l en t j ournal ar ticles and monographs on the
sub j ec t . 4 , 5 , 6 , 7 , 8
1.
Descrip t ion
The un ique combinat ion of spec troscopic and electrochemical
techniques has proven to be an e f fec tive approach to the stud y of
oxidat ion-reduc t ion reac t ions .
Oxidat ion states are changed
5
e lec trochemi c a l l y by the addit ion or remova l of e lectrons at an
e lec trode surface .
Spec troscopic measurements of the solution near
the electrode sur face are made simul taneou s l y wi th the electrochemical
g enerat ion process .
Spec troelec trochemic a l technique s provide a
convenient me thod for ob taining spec tra and redox po ten t ia l s and for
inve s t igat ing sub sequent chemic a l reac t ions of e lectrogenerated
produc t s . 6
Many d i f ferent opt i c a l methods have been coupl ed wi th
e lec trochemis try but the mos t frequently used technique is that o f
absorp t ion spec troscopy i n the ult ravio l e t-vis ib le-near in frared
s pectral region .
Of the three methods of absorption spec troscopy
c oupl ed wi th e l ec trochemi s t ry , transmiss ion spectroscopy , specular
reflec tanc e spec t roscopy , 9 and interna l re flec tanc e spec t roscopy , l O , l l
the method o f transmiss ion spec troscopy i s the mos t s imple and
convenient method .
Th is method involve s the pas s age of an optical beam
d irec t l y through an op t ic a l ly transparent elec trode (OTE ) and the
adj acent solution . 3 , 4 , 5 , 12
Two dist inc t l y different configura tion s of
opt ical l y transparent elec trochemica l ce l l s are commonly us ed in
conj unc t ion wi th transmis s ion spec troscopy .
The fir s t configurat ion is
s im i l ar to a convent iona l electrochemic a l ce l l in that the electrode is
in contac t wi th a solution tha t is much th icker than the d i ffus ion laye r
at the electrode surface .
In contras t , the second type of c e l l
configurat ion , the op tic a l ly trans parent th in layer electrode (OTTLE)
ce l l confines a th in layer ( <0 . 2 mm) immed iate l y ad j acent to the
e lectrode surfac e . 4 , 6
The advantage of thi s type o f ce l l arrangement
i s that al l the e l ectroac tive species wi thin the th in layer can be
6
rapid ly elec t ro lyzed and thus equ i l ibrium fo r the redox reac tion can
be qu ickly ach ieved ( typ ical ly 2 0 to 1 2 0 s ec ) . 6
2.
Theory
An electrochemic a l react ion can be represented by the fo l l owing
ne- ::t Red ,
Ox +
( 1)
where an oxidized spec ie s , Ox, comb ines with n number o f e lectrons , e- ,
to form a reduc ed spec ies , Red .
The elec trochemic a l poten t i a l produced
by the reac t ion is re lated to the ra tio of the ac tivities of the
oxid ized and reduc ed spec ies in the Ne rns t equa tion
E ; E o + RT l n
'iiY
aox
(2 )
8red
where E is the appl ied potentia l ; E0 i s the standard poten tial for the
spec i fic redox reac tion ; R, the gas constan t ; T , the ab solute
temperature ; n , the number of e lec trons exchanged ; F , the Faraday ; and
a x and ared , the ac tiv i t ies of the oxid ized and reduc ed spec ie s ,
0
respective ly.
Under mos t experimenta l cond i t ions , the temperature is
room temperature ( 25 °C) and the ac tiv ity coef fic ients of the oxid ized
and reduc ed spec ies are assumed to be unity.
Thus a simpl i fied form
o f the Ne rns t equat ion can be wri t ten
E = Eo
'
+
0 . 059
n
log [ Ox ]
[ Red)
where [ Ox) and [ Red) indic a te the mo lar concentrat ions of Ox and Red ,
'
respec tive ly and E 0 is the forma l reduc tion poten t ia l .
(3 )
7
With the use of an OTE the concentrat ions o f Ox and Red can be
determined spec trophotome t r ically by Beer ' s law
A
=
(4 )
EbC ,
where A is the absorbance of Ox or Red ; E, the mo lar absorp t ivity o f Ox
or Red ; b , the path length of the OTE c e l l ; and C , the mo lar
concentrat ion o f Ox or Red .
Combinat ion o f the Ne rns t equat ion ( 3 ) and
Beer ' s law ( 4 ) re sul t s in an expres s ion wh ich re lates the appl ied
potential to the ratio of concentrat ions of Ox and Red expressed in
terms of spec tra l parame ters
E
=
E
o'
+
0.059 l
n
og (
A
OX
/E
OX
) '
/E
A
red red
(5)
where Eox and Ered are the mo lar absorptivitie s of the oxidized and
reduc ed spec ies , re spec t ive l y .
Equat ion 5 can be used as shown to
determine the E0 and n-va lues of redox couples .
DeAnge l i s and Heineman 13 presented a more convenient form of
equat ion 5 .
These au thors se lected a wavelength wh ere onl y the oxidized
spec ies , for exampl e , absorbed .
The absorbance at this wave length was
recorded at po tent ials corres ponding to ful l y reduc ed cond i t ions (At ) ,
par t ia l ly oxidize d condit ions (A2 ) , and ful l y oxid ized cond i t ions (A3 ) •
Under ful ly oxidized condi t ions , where the elec troac t ive spec ies is
a l l in its oxid ized fo rm , the fo l lowing ho lds
(6)
8
where Ct i s the total concentrat ion of e lec troac t ive spec ies .
The
c oncentrat ion of the oxid ized spec ies at a po tential corre spond ing to
partia l ly oxid ized condit ions is
[ Ox] =
(7)
There fore , the concentrat ion of the reduced spec ies at a po tential
corres ponding to part i a l l y oxid ized cond it ions is the d i fference
between the total concentrat ion of e lectroac t ive species and the
concent rat ion of the oxid ized spec ies , i . e . ,
[ Red]
(8)
The ratio of concentrat ions of the oxid ized spec ies and the reduc ed
s pec ies is expressed as
E
[ Ox] = (A2 - A l ) / ( oxb)
-[ Red]
( A 3 - Az ) / ( Eoxb)
(9)
Sub s t i tut ion o f Equation 9 into Equat ion 3 resul t s in
(A2 - A l )
( 10 )
0.059
log
n
(A 3 - A 2 )
Experimenta l ly , the absorpt ion of the oxid ized spec ies is measured at
E =E o
'
+
•
the se lected wave length at equi l ibrium at var ious appl ied potent ial
values .
A plot of the appl ied potential , E, versus log (A2 -A l ) / (A3 -A2 )
9
for a reversible react ion yie lds a st raight line wi th s l ope equal to
0 . 05 9/n and intercept equa l to the formal reduc t ion po tent i a l , E0 1 •
This approach e l iminates the need to know the mo lar absorptivi ty of
the abs orbing spec ies , the path length of the ce l l or the to tal
concentra t ion of the e lec troac tive spec ie s .
The above de scription out l ines the theory of the spec tro­
potentios t a t ic technique for the de terminat ion of spec tra , reduc tion
potentia l s , and n-va lues for redox couples .
De sc r iptions of other
related spec troel ec trochemical technique s can be found
e lsewh ere . 4 , 1 2 , 14 , 15 , 16
3.
Literature Review
a.
Opt ica l ly transparent e lectrodes .
In 1964 the us e o f an OTE
for in s i tu spec tral ob servat ion of an electrogenera ted produc t was
firs t demons trated by Kuwana � �. 3 .
The OTE wa s a glass sur face
whi ch had been coa ted wi th a thin transparent fi lm of antimony-doped
tin oxide (Nesa g l as s ) which wa s used for bo th transmiss ion and
internal re flec tanc e spec tros copie s .
A var ie ty of th in conduc ting fi lms
depo s i ted on transparent sub s t rates for us e as OTE ' s ha s been repor ted
s ince then , inc lud ing meta l fi lms such as pl at inum l7 , 18 and
gol d , l8 , 19 and semi-c onduc tors such as tin oxide , l l , l9 ind ium
oxide , 4 carbon film2 0 ( on quartz or germanium sub s trates for infrared
s tudies ) , 2 1 and pyro lytic carbon . 22
These th in fi lm OTE ' s are
transparent due to the th inness of the meta l , met a l oxide , or
semi- conduc tor fi lm , but they do exhibit spec tral charac teris tics o f
the fi lm ma teria l .
In 19 67 Murray � .!!_ . 23 introduced the go ld "minigr id" opt ical l y
t rans parent e lec trode in a th in layer con figurat ion (OTTLE ) .
minigrid cons i s t s o f a metal (gold , 23 nicke l , 6 s i lver , 6
The
10
p lat inum, 24 e tc.) gr id with from 100 t o 2 000 wires per inch.
The
trans parency of the minigrid elec trode ( 20-80% ) is ac counted for by the
physical ho les in the minigrid st ruc ture , and there fore the spec tral
window of the se e lec trodes it no t l imited by the spec tral proper t ies of
the cons truc t ion mater ia l.
Mer cury transparent e lectrodes have been prepared by elec tro­
depo s i t ion of th in Hg fi lms on a pl at inum- f i lm OTE 25 and carbon- f i lm
OTE , 20 a s we l l as on si lver 2 6 and nicke l 2 7 minigrid s .
The mercury
imparts some of its excel lent negative po tential range charac ter i s t ics
to the sub s trate OTE wi thou t ser ious l y affec t ing the trans parency of
the electrode.
In 19 7 7 Norve l l and Mamantov 2 8 repor ted the deve lopment o f an OTE
made from a nove l elec trode ma terial , ret icul ated vitreous carbon ( RVC ) .
Th e RVC-OTE wa s qu ite different from any previous spec troe lec t rochemic a l
cel l in tha t the ce l l con figurat ion more nearly re semb led the th ick
(0.8-1.2 mm) convent iona l OTE , but it exh ib ited the spec ial
charac ter i s t ics of an OTTLE. Th i s was achieved by the unique struc ture
o f the RVC electrode ma terial , composed of glassy carbon struts in a
three- dUnens iona l network.
The porous nature and large surface- to-volume
rat io of the e lec trode al lows the RVC-OTE to behave like a th in layer
c e l l in tha t e lec trochemica l equ i l ibrium is rapid ly ach ieved , because
each electroac t ive spec ies has a re lative l y short d i f fus ion pa th to the
e lectrode sur face.
In add ition , the electric a l conduc tivity and
transparency ( 1 3 -45 % ) of the RVC-OTE are qu ite reasonab le.
b.
Appl icat ions .
Spec troe lec trochemical technique s have been
appl ied to the spec tral identi fication of electrogenerated produc t s such
as Mu-pyr az inedecaamined iruthenium ( I I , III) compl exe s , 29 n inhydrin , 3 0
11
l ith ium compl exes of 1-hyd roxy-9 , 10-anthraquinone (HOAQ ) , 3 1 v i t amin
B- l z , 24
1 , 8-d ihydroxyan thraquinone and anthraquinone anion rad icals , 32
iron ( I I , I I I ) ac ety lacetonate, 33 his-hydroxy me thyl ferrocene , l9
o- tol idine , l 3 , 2 8
permanganate ion 28 and green plant photosys tem r . 34
Th e spec tropo tent iostatic technique for the measurement of spec tra , redox
potent ials , and n-va lues has been appl ied to the study of various
metal compl exes and biological components . 1 3 , 24, 32 , 3 5, 3 6, 3 7 Add i t ional
references can be found in a recent review by He ineman and
Kiss inge r . 3 8
c.
Re l ated technique s .
Wh i le many workers have monitored elec tro­
chemical generat ion via spectrophotometry , 39 , 40 , 4 1 , 42 onl y one report
o f simul t aneous spec troe lec trochemis try of ac tinide mater ials had
appeared in the l i terature prior to the work of th is author .
Maman tov, and Whi t ing43
Young ,
reported the elec trochemic a l generat ion o f
uranium( I I I ) at a microelec trode in mol ten Li F-Be F2 -ZrF4 l ocated in
the opt ic a l path of a spec trophotometer .
D.
1.
The Lanthanide and Ac t inide E lemen ts
E lec tronic S truc ture
The lanthan ides are the four teen elements fo llowing lanthanum in
the Pe riodic Tab le in wh ich the fourteen 4 f elec trons are
consecutive l y added to the lanthanum con figurat ion .
These elec trons
are added to elec tron orb itals wh ich are lower in energy than the
valence orb i t a l s of lanthanum.
Thus the 4 f electrons occupy inner
shielded orbitals wh ich makes them, for the mos t par t , unavai lab l e for
chemic al interac tions .
This accoun ts for the remarkable sim ilarity in
the chemi cal behav ior among the individ ua l members of the lan thanide
12
ser ie s .
The e lec tron ic configurat ions o f lan thanide atoms and ions
are lis ted in Tab le I .
In the act inid e se r ies the four teen S f electrons are consecutive ly
added to the ac t inium configurat ion .
The elec tronic con figurat ions o f
act inide atoms and ions are summar ized in Tab le I I .
Comparison of
these conf igurat ions with those o f the lanthanides shows an overa l l
s im i l ar ity wi th the except ion that the lighter ac tinides ( thor ium to
americ ium) show an enhanced tendency to re tain d elec trons .
The
heavier ac tinides ( cur ium to lawrenc ium) show a tendency to re tain f
e lec trons rather than d electrons , more para l le l ing the behavior of
the maj ority of the lanthanides .
The re sul t is a greater tendency of
the l igh ter ac tinides to supply bonding elec trons , and thus a greater
tendency to exh ibit higher oxidat ion states . 44
The heav ier ac t inides
resemb le the l anthanides in the ir oxidation state behav ior . 4 5
2.
Oxidat ion S t a te s
A direc t corre lat ion be tween oxidat ion state and elec t ronic
configurat ion is the except ion rather than the rul e in the lanthanide
ser ie s . 44
The e lec tron configuration of 4fn6 s 2 i s the charac teris tic
one for mo s t of the l anthanide atom ground states ( s ee Tab le I ) .
There fore , the divalent state should be acc e s s ib le for al l of the
lan th anides wi th the loss of the two 6s elec trons .
Th is is no t th e
s ituat ion , however , s ince the tr ivalent state is the mos t common
oxidat ion s tate fo r these elements .
The loss of the two 6s electrons
is usua l ly accompanied by one 4 f electron , if a 5d electron is no t
ava i lable .
13
TABLE I
ELECTRONIC CONFIGURATIONS OF LANTHANIDE ATOMS AND IONS 1 , 2
Atomic
number
Name
Symbo l
E lectronic Configurat ion
M( III)
M(II)
Atom
M( IV)
58
Cer ium
Ce
4f l sd l 6 s 2
4f 2
4f l
4f0
59
Praseodymium
Pr
4f 3
6s 2
4f 3
4f 2
4f l
60
Ne odymium
Nd
4 £4
6s 2
4£4
4f 3
4f 2
61
Prome th ium
Pm
4f 5
6s 2
62
Samar ium
Sm
4 £6
6s 2
4f 6
4f 5
63
Europ ium
Eu
4 £7
6s 2
4f 7
4f 6
64
Gadolinium
Gd
4f 7 sd l 6s 2
4f 7 5 d l
4f 7
65
Te rbium
Tb
4 £9
6s 2
4f 9
4 £8
4f 7
66
Dysprosium
Dy
4 £10
6s 2
4 £ 10
4 £9
4f8
67
Ho lmium
Ho
4f l l
6s 2
4f l l
4 £ 10
68
Erbium
Er
4f l2
6s 2
4f 12
4f l l
69
Thulium
Tm
4f l3
6s 2
4f l 3
4f l 2
70
yt terbium
Yb
4 £14
6s 2
4£ 14
4 £1 3
71
Lutet ium
Lu
4f l4sd l 6 s 2
4f4
4 £ 14
l an ly the va lence- she l l elec trons , that is , those out s ide the [ Xe] core ,
are given .
2A dash indicates th at th is ox idat ion state is no t known .
14
TABLE II
ELECTRONIC CONFIGURATIONS OF ACTINIDE ATOMS AND ION s l , 2
Atomic
number
Name
Symbol
E le c t ronic Confisurat ion
M ( I I ) M ( I I I ) M ( IV ) M(V )
Atom
M(VI)
90
Thor ium
Th
5 f0 6d 2 7 s 2
5 f0
91
Protact inium
Pa
S f 2 6d l 7 s 2
sf l
5 f0
92
Ur anium
u
S f 3 6d l 7 s 2
5f3
Sf 2
sf l
5 f0
93
Ne ptunium
Np
5f 4 6d l 7 s 2
5 f4
5f 3
5 f2
Sfl
94
Plutonium
Pu
5f 6
7s 2
5f5
5f 4
5 f3
5f2
95
Americ ium
Am
Sf7
7s 2
5 f6
5f5
5 f4
5f 3
96
Cur ium
Cm
S f 7 6d l 7 s 2
S f7
5f6
97
Berke l ium
Bk
Sf 9
7s 2
5 f8
Sf 7
98
Ca li fornium
Cf
Sf lO
7s 2
S f lO
5f 9
5 f8
99
Einste inium
Es
Sf l l
7s 2
Sfl l
S f lO
100
Fermium
Fm
S f l2
7s 2
5 f l2
Sfl l
10 1
Mende levium
Md
5 f l3
7s 2
5 f l3
5fl2
102
Nobel ium
No
S f l4
7s 2
S f l4
5 fl3
10 3
Lawrenc ium
Lr
S f l4 6d l 7 s 2
5f7
5 f l4
l 0n 1y the va lence- she l l electrons , tha t is , those out s id e the [ Rn]
core , are give n .
2 A b lank space indicates that th is oxidat ion state is no t known or the
conf igurat ion for th is oxidat ion state is no t known .
15
The M( I I I ) s tate is the mos t stab l e one for the lanthanides in
aqueou s solution due to a fortuitous comb ina t ion o f ioniza t ion and
hydrat ion energies ra ther than a particular elec tronic configurat ion.44
In the so l id state the ionizat ion energy is coupled to the latt ice or
c rys t a l energy , and again the tr iva lent state is general ly the mos t
s table.
To a certain extent , the occurrence of oxid at ion states other
than M( I I I ) for the lanthan ides can be corre lated with their
e lec tronic struc tures and ioniza t ion potent ia ls .
As is the case wi th
the d-e lect ron trans i t ion me ta ls , there is a certain stab i l i ty
as sociated wi th an empty , half - f i l led , or fi l led she l l .
The
s igni ficance o f th is s tabi l i ty is evidenced by the exis tence o f M( I I )
and M( IV) oxida t ion s t ates o f some lanthan ides.
The oxidat ion states
of the l an thanide e lements are summar ized in Tab le I II.
The states
indicated in th is tab le are known to occur in the bulk phase , and
a lthough add i t iona l oxidat ion s ta tes have been reported , such as
inc lus ions of diva lent lanthanides in hos t latt ices , they are not
cons idered in th is l i s t ing. l ,44 , 4 5
The ac tinide elements exhibit a greater var ie ty o f oxidat ion states
than do the l anthan ides.
The present ly known oxidat ion s t a tes o f the
act inide elements are pre sented in Tab le IV.
which are known in bulk phases are shown.
On l y oxidat ion states
In general , the l igh ter
act inides (Th-Am) show an inc reased stab i l i ty for higher oxida tion
s tates wi th increas ing atomic number , whereas there is a decrease
in the s t ab i l i ty of higher oxidat ion states and an inc reas e in the
s tab i l ity of lowe r ones in the second ha l f of the series.
As is the
TABLE II I
OXIDATION STATES OF THE LANTHANIDE ELEMENT s l , 2
Ce
Pr
Nd
Pm
(2)
3
3
3
4
(4 )
(4)
3
-
Sm
Eu
2
2
3
3
Gd
Tb
nx
Ho
Er
(2)
3
-
3
3
(4 )
(4)
3
3
Tm
Yb
(2)
2
3
3
Lu
3
-
!Parentheses indic ate so l id st ate on l y or no t we l l charac ter ized .
2underl ined numbers indicate mos t stab le ox ida t ion state .
.....
Q\
TABLE IV
OXIDATION STATES OF THE ACTINIDE ELEMENTs l , 2
Th
Pa
u
Np
Pu
Am
Cm
Bk
(2)
Cf
Es
Fm
Md
No
2
2
2
2
2
3
3
3
3
(3)
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
5
5
5
5
5
6
6
6
6
7
7
(7)
Lr
3
!Parentheses indicate so l id state onl y or not wel l chara cter ized .
2underl ined numbers ind icate mos t stab l e oxidat ion state .
1-'
.......
18
case fo r the lan thanides , the triva lent state is general ly th e mos t
s tab le one for the transplutonium elemen t s .
Here aga in , there i s a pronounced pre ference in the known ac t inide
oxida t ion s t a te s for the fO , f 7 , and f l4 e l ec t ronic configurat ions .
However , s t abi l ity o f a given oxidat ion state is less c l os e l y re l ated
to elec tronic configurat ion th an to ioniza t ion energy , hyd rat ion
energy , and latt ice energy . 44
The more extensive spa tial projec t ion of the 5 f orbi ta ls , over
that of the 4 f orb i ta ls , into the ou ter or va l ence regions o f the
act inide atoms somewhat reduc es the sh ield ing of these electrons , so
the ir binding energy is le s s , and thus , in genera l , the ac t inides
exh ibit a grea ter variety of observed oxidation states than do the
l anthanide s . 44
a.
The Monova lent S tate .
In 19 73 , Mikheev � !1· 4 6 repor ted the
exis tence of a s t ab le monova l ent , ces ium- l ike ion of mende levium based
on the resul t s of co-crys tal l izat ion experiment s .
Th is was the firs t
repo rt of any monova lent ac tinide or lanthanide ion in th e bul k ph ase .
The exi s tence of Md ( I ) wa s attributed to the stab i l iz a t ion prov ided by
i ts expected 5 f l4 e lec tronic con figurat ion .
Hul e t et a l . 47
Later expe riments by
to reproduc e the co-crys tal l ization re sul t s were
unsuccess ful , and these authors conc lud ed tha t the or iginal c l a im of
the exis tence of monoval ent mende l evium wa s unsub s t anti ated .
This
conc lus ion wa s a lso suppor ted by the re sul t s of ad ditional experiments
by Samhoun � �. 48 a t Oak Ridge , Tenne s see ( in wh ich th is au thor
19
partic ipated ) and by Dav id ��. 49 at Berke ley , Ca l i fornia , us ing the
techniques of rad iopolarography50
b.
The D iva lent S tate .
and radiocoulometry . 5 1
The M( I I ) s ta te is we l l-e s t ab l i shed for
the lanthanides Sm, Eu , and Yb in both solutions and in the so l id s tate .
Tm( I I ) and Nd ( I I ) are not we l l inve s t igated .
Eu( I I ) exhib i t s a
4 f 7 (ha l f- f i l led she l l ) configurat ion and Yb ( I I ) , the 4 f l4 ( fi l led
she l l ) configurat ion .
However , the s tabi l ity o f Sm( I I ) , wi th a
4 f 6 configuration , cannot be explained by elec tronic configurat ion
a lone . 5 2
The ions of divalent Sm and Yb are uns table in water and
a re oxid i zed by air , but Eu ( II ) is qu ite s table in aqueous solut ion .
The ac t inide e lement s c a l i fornium to nobel ium are known to exi s t in
the dival ent state in aqueous solution .
Al though the tr ivalent state is
gener a l l y the mos t s table for the heavier ac tinides in solut ion , the
mos t s table state of nobel ium (homol og o f Yb ) is No ( I I ) . 53
Compounds
o f the heav ier hal t des o f Am( I I ) have been repor ted . 54 , 5 5
Am( I I ) and
Th( I I ) may a l s o exi s t in fused chloride me l ts . 56
The ins tab i l ity o f
Am( I I ) i n aqueous solution is surpr i s ing , since it i s the homolog o f
E u and exhib i t s the S f 7 configurat ion ; however , stab i l ity o f an
oxidation state is not based on elec tronic configura t ion alone .
The
behav io r of the divalent lanthanides and ac tinides re semb l e s that of
the alka l ine ear th elements .
c.
The Triva lent S tate .
The M( I I I ) s ta te is the characteris t ic
oxidation s t a te for al l of the lanthanides and is the mos t stab l e in
aqueous so lut ion .
Gd and Lu form the M( I I I ) ions because of the
a ttainment of the s table 4 f 7 and 4 fl4 configurations , res pec t ive l y .
The trivalent s t ate i s we l l known for al l o f the ac t inides wi th
20
the excep tion of pro tac t inium.
On ly Pal3 is known , wh ich is a
b lack-co lored compound , exh ibi ting th e Cel3 s truc ture , that might be
more proper l y de scribed as Pa ( IV) (I - ) 3 { e- ) . 5 7
The M( I I I ) s tate is
genera l l y the mos t stab le one for the ac tinides wi th the exception of
No .
O f the tr iva lent ac tinides , U( I I I ) is the mos t read i l y oxid ized
by air or wa ter . 4 5
d.
The Te trava lent S tate .
Cer ium( IV) is the on ly te travalent
l an thanide spec ies su f f ic iently stable to exi s t in aqueous solut ion as
we l l as in the so l id state . 5 2
In aqueous so lut ion Ce ( IV) i s a powerful
oxidizing agen t , and it s chemis try resemb les tha t o f Zr , Hf , and the
tetrava lent ac tinides .
The stab i l ity of Ce ( IV) can be at tributed
to the att ainment of the 4 f0 ( nob le gas ) conf igurat ion .
Pr ( IV) is known to exi s t in only a few so l id compounds inc l ud ing
the ha lides and the bl ack nons to ich iomet ric oxides .
The oxide sys tem
i s complex with stab l e phases ( c ontaining mixed-valence Pr ) with
formulas between Pr 2 03 and the true dioxide .
Tb ( IV) resemb les Pr ( IV)
in that nons toichiometric compounds are known such as Tb4 0 7 and
Tb 7 0 1 2 •
Both Pr ( IV) and Tb ( IV) ox idize wa ter , so no stab le aqueous
s olut ions of the se ions are known . 5 2
There is some ev idence for Nd ( IV) and Dy( IV) in the fluorination
p roduc t s of lan thanide/ alka l i me ta l mixed sal t s . Such compound s as
Cs3NdF 7 and Cs3 DyF 7 can be formed to a limited exten t . 5 8 , 5 9
Nd ( IV)
has been reported as be ing stable when incorporated in to bar ium
oxide-based hos t l a t t ice s . 60
The ac tinides thorium to ca l i fornium are known in the tetrava lent
s tate .
Am( IV) is uns tab le in mos t mineral ac id solutions .
However ,
21
Am( IV) can b e stab i l i zed in strong ly complexing aqueous
solutions . 61 , 62 , 63
Cm( IV ) has not been prepared by oxidat ion of any
s ol ut ion of Cm ( I I I ) , but rather by dis solut ion of CmF4 in concentrated
aqueous Cs F solut ion . 64
Tetravalent plutonium , l ike amer icium , tends
to disproportionate rapidly in aqueous solut ion in the ab sence o f
c ompl exing agen t s .
Bk( IV) resemb les Ce ( IV ) i n its chemical behavior
and stab i l ity in aqueous solut ions .
The te travalent ac t inides form
the stronge s t compl exes and chelates in the lan thanide-ac t inide group
o f elemen ts .
Numerous compounds of tetravalent ac t inides are known ,
the mos t common being the dioxides and te traha l ide s . 5 3
e.
The Pentava lent S tate .
Th is oxidation s t a te is known for the
ac t inides from protac t inium to americ ium .
Pen tavalent uranium ,
neptunium , plutonium , and americ ium are known to exi s t as the
d ioxo-cations , M02 + , wi th two short coval ent met a l to oxygen bonds .
Pa (V ) , on the other hand , does not form such a complex cat ion and
exi s t s in so lution as the s impl e hyd rated spec ies .
However , at Pa( V )
concentrations greater than lo -6 M , pro tac t inium forms dimers and
polymers wh ich necessari l y compl ic ate any experimental work wi th th i s
spec ie s . 6 5
Pa ( V ) i s the mos t s table form o f protac t inium.
The chemistry of the dioxo-c at ions , M02 + (M
resemb les tha t o f divalent Cd , Ca , or Zn .
=
U, Np , Pu , and Am) ,
Uranium( V ) i s uns tab le in
aqueous solut ion and d i s proport ionate& rapid ly unl ess spec i fic
cond i t ions are maintained , inc lud ing a pH of be tween 2 . 0 and 4 . 0 .
At tempts to reduce uranium( VI) t o uranium( V ) usua l l y resul t in the
immediate format ion o f U ( IV) and U ( VI ) , un less adj us tmen t s are made to
s low the di s propor t ionat ion rate . 66
Pentaval ent uranium is s table in
22
hydrofluo r ic ac id solut ions a s the UF 6 - ion .
Pu02 + and , t o a les ser
extent , Amo 2 + are also uns tab le to disproport ionat ion .
Many so l id
compound s o f these pentavalent e lemen ts are known wh ich do no t contain
the dioxo-cat ion struc ture , inc lud ing UF 5 , NpOF3 , and some tri-oxy
compounds like Liuo 3 . 5 3
f.
The Hexavalent S tate .
The elemen ts uranium to amer ic ium have
been shown to exi s t in thi s oxidat ion s ta te .
In aqueous so lut ion and
in mos t compounds , the dioxo-c at ions are present as M02 2 + .
the mos t stable form of uranium in aqueous solut ion .
uo 2 2 + i s
The stab i l i ty of
the hexavalent ac t inides decreases from uo 2 2 + , wi th the 5 f0
c onf igurat ion , to Amo2 2 + , wi th the 5 f 3 configurat ion .
Many s o l id
compounds are known where the dioxo-c at ion s truc ture is ab sent , such
as Li 6 Amo6 , Pb 3 uo 6 , uo 3 , UF 6 , and PuF6 . S 3
g.
The Heptava lent S ta te .
Rus s ian chemis ts , in 1967 /68 , reported
the exi s tence of heptava lent nep tunium and plutonium .
Kro t and Ge lman 67
repor ted that ozone oxidation of a Npo 2 2 + sol id sus pens ion in alka l i
solution re sul ted in the gradual disso lut ion o f the so l id and the
format ion o f a dark green solut ion o f Np ( VI I ) .
prepared in a s im i l ar manner .
Pu ( VI I ) can be
In alka l i solution Np ( VI I ) seems to be
reasonably s t ab l e and corre sponds to the S fO configura t ion , whereas
Pu(VII ) is uns tab le even in alka l i and is reduced by water .
These
heptavalent spec ies , of the form Np05 3 - and Puos3 - , can form sol id
compounds from so lution . 5 3
23
Am(VII ) is prepared by ozone oxidation o f Amo 2 2 + in alka l i at 0 to
7°C.
Evidence was pre sented by the Russ ian sc ientists tha t Am( VI I ) was
indeed the species formed and that it was qu ite uns tab l e . 68
3.
Oxidat ion-Reduc t ion Potent ia l s
The re lative stab il it ies of the lan thanide and ac t inide ox idat ion
s tate spec ies are reflected in the values of the reduc t ion potentials
for their redox coupl e s .
Us eful correl a t ions of s tandard reduc tion
potent ials for the lanthanide and act inide M ( I I I ) /M ( II ) and
M ( IV) /M( I I I ) couples were pub l i shed by Nugent � �. 69
These reduc t ion
potentials were derived from ( a ) direc t electrode po tential
measurements reported in the literature , ( b ) linear plots of
e lectron- trans fer ab sorpt ion bands and f-d absorpt ion bands versus
potential for the various redox couples ; and ( c ) from theoret ical
c alculat ions based on J�rgensen•s 70 re fined-elec tron- s pin- pairing­
energy theory.
and 2 .
These values are displ ayed graphica l l y in Figure s 1
The mos t striking feature of the se figures is the
d iscontinu ity Which occurs in the standard reduc tion potential as the
hal f- fi l led f- she l l is att ained for the configurat ion of the reduced
s pec ies [ Eu( I I ) and Am( I I ) in Figure 1 and Gd ( I I I ) and Cm( III) in Fig .
2] .
Thi s shows ev idence of the spec ial stab i l ity provided by the
f 7 elec tronic configurat ion .
These diagrams are par ticularly use ful for correlat ing the
s tabil itie s of spec ies in aqueous solut ion .
For example , Yb ( II ) is not
s tab le in aqueous solution ; it is rap id ly oxid i zed by water .
There fore
the po int in Figure 1 tha t corresponds to the Yb ( I I I ) /Yb ( II ) couple is a
24
ORNL-OWG 79-40327
ACTINIDE ELEMENTS
-
w
:r
z
C/)
>
4
-3
� -2
0
>
......
......
I
�
0
w
-1
0
2 ���--����-��La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb
LANTHANIDE ELEMENTS
F igure 1 . S tandard reduc t ion poten t ia l s for M( II I ) /M ( I I ) redox
c oupl es of some lanthanide and ac tinide elemen ts , lanthanum , and ac t inium .
25
ORNL-DWG 79- �0328
ACTINIDE ELEMENTS
-4
-2
�0
z
en
>
2 2
0
>
-
74
>
......
�
0
w
6
8
�0
Th Po
U Np Pu Am Cm Bk Cf Es Fm Md No Lr
A
\
\
�ACTINIDES
'
'
'
•
....
.......
'
'.t.
',
\A
'
A
I \
I
\
y
'...
'
'
'A ..
......
... ...
...
'
..__.---1.
'.t.
\
\
�
I
Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb
Lu
LANTHANIDE ELEMENTS
Figure 2 . S t andard reduc tion po tentials for M( IV) /M( I I I ) redox
c oupl e s of the lanthanide and ac tinide elements .
26
potent i a l at which oxidation by wa ter occurs , as is the case for any
point at a more negat ive po tent ial .
The va lue for the Eu ( I I I ) /Eu( II )
coup l e is less negative than that for rapid ox idat ion by wa ter and so ,
Eu( I I ) i s quite s table in water .
A potential at which water wil l reduc e
the oxid ized form of the redox couple is the point correspond ing to the
No ( III ) /No ( I I ) couple , s ince No( II ) is stab le in water and No( I I I ) is
not .
Comparison of the chemic a l properties o f the lanthanides and
act inides can be made from the corre lat ions in Figures 1 and 2 .
The points corre sponding to the Ce ( IV) /Ce ( I I I ) and Bk ( IV ) / Bk ( I I I )
coupl es are at nearly the same potential va lue , so the chemical
behavior of these two redox coupl e s is qui te similar .
Bk( IV) are s trongl y affec ted by complexat ion .
Both Ce ( IV ) and
A high degree o f
c ompl exat ion is evidenced b y a signi ficant change i n the measured
redox po tential in var ious complexing solut ions .
The Ce ( I V ) / Ce ( I I I )
coupl e exhibits a po tential of 1 . 2 8 V in 2 M HC l ; 1 . 44 V in 1 M H2 S04 ;
1 . 6 1 V in 1 M HN0 3 ; and 1 . 7 V in 1 M HC 104 . 45 , 5 2
The Bk ( IV ) /Bk ( II I )
couple exhib i t s a formal reduc t ion po tent i a l o f 0 . 0 2 2 V i n 2 M K2 C03 ;
0 . 88 V in 7 . 5 M H3 P04 ; 1 . 3 V in 1 M HN03 ; and 1 . 6 V in 1 M
HC104 . 69 , 7 1
The forma l reduc t ion potential o f the Am( IV ) /Am( I I I )
coupl e i s a l s o changed b y compl exing solven t s , a s are the po ten t i a l s
o f many o f the redox coupl e s tha t inc lude the te t ravalent oxid at ion
s tate as one member of the couple .
Th is is due to the fac t tha t the
tetravalent lanthanide and ac tinide ions are the mos t strongl y
c ompl exed of the £- trans i t ion element oxidation state s .
27
The reduc t ion po tentials of higher ox idat ion s t a te couples of some
act inides are presented in Table
These potential value s are based on
v.
exper imental work and theoretical estimates . 4 5 , 5 3 , 69 , 7 2 , 7 3
4.
S o lut ion Absorption Spec tra
The f orbital elec trons of the lan thanide and , to a lesser extent ,
the ac tinide ions are shie lded from the surrounding chemic a l
environment , and thus absorpt ion spec tral bands due t o f- f elec tronic
transit ions are extremely sharp band s and are , for the mos t par t ,
una f fec ted by compl exing ligands .
The absorpt ion spec tra o f the
triva lent l anthanides and of many oxidat ion states o f the ac t inides
typic a l l y exhibit this line- l ike charac ter with mos t ab sorpt ion bands
appearing in the visib l e and near-UV s pec tra l reg ions .
These f- f
t rans i tions are of low probab i l i ty due to the " forbidden" nature o f the
trans i t ion as desc r ibed by quantum mechanical se lec t ion rules .
The
actinide f- f absorption bands are generally an order o f magni tude
greater in intensity than those of the lanthanides . 44
The ab sorpt ion spec tra of the di- and te travalent lanthanide and
certain ac t inide ions exh ibit broad -band trans i t ions ari s ing from f- d
or ligand-metal charge- tran s fer mechanisms .
These "al lowed " trans i t ions
resul t in high mo l ar absorptivities , severa l orders of magni tude grea ter
than those of the f- f trans it ions .
These bands are strongly affec ted by
the surround ing chemical env ironment .
Th is is espec ially true for the
charge- trans fer bands , since the elec tron originates from the l igand
itse l f .
28
V
TABLE
R E DUCTION POTE NT IALS OF HIGHER OXIDATION STATE COUPLES OF
SOME ACT I N IDE ELEMENTSa
(-0.3] b
Po5+
0. 32
I
2+
uo
2
3
Np05
0.582c
-
2
N p0 +
2
0.063
0.938
1.1 37
I
�
PuO -
- 0.9c
0.58
+
uo2
0.739
Npo;
Th4 +
Po 4
-t.9
+
I
· u4 +
N p4 +
- 0.631
0. 1 55
�
Puo +
2
Amo +
2
I
0.677
1.043
0.913
1.6
Pu0+
2
1.17
t.02
1.14
+
A m0
2
t .69
I
Pu4 +
A m4 +
1.74
� alues
u3
+
N p3 +
'I
0.447
I
Th3 +
0.982
2.34
Pu3+
Am3 +
'I
reported i n volts � NHE i n 1 M HCl0 a t 25 ° C , unless
4
o therwise s p e ci fied .
b
Brackets indicate es timated value .
�alues
reported in 1 M hyd roxide s olutions .
CHAPTER II
EXPERIMENTAL DETAILS
A.
1.
Introduc tion
Spec trophotometer&
Solution absorpt ion spec tra and so l id state re flectance spec tra of
nonrad ioac tive sample s were recorded us ing a Cary Model 14 -M
s pec trophotometer .
Solut ion absorption spec tra o f inac tive as we l l as
rad ioac tive mater ials were recorded with a Cary Model 14 -H
s pec trophotometer ( see Figure 3 ) .
So lut ion ab sorption spec tra o f some
inac tive sampl es were recorded us ing a Tektronix si licon-vid icon based
rapid- scan spec trophotometer . 7 4
Solid s tate ab sorpt ion spec tra of
rad ioac t ive , microgram- si zed sampl es sea led in quartz capi l l aries were
recorded with a microscope spec trophotometer/Tektronix 4 05 1 data
acquis i t ion fac i li t y . 75
2.
Vol tamme ter
E lec trochemical measurements were performed wi th an EG&G PARC Mode l
1 7 30 / 1 7 90 /1 7 5 potentiostat/coulometer/ universal programmer
compl emented by an Es ter l ine Angus Model XY53 0 recorder ( see Figure 3 ) .
3.
G l oved Boxes
A bas ic requirement in the hand l ing of rad ioac tive materia ls is
that a l l operat ions be performed in a sui t able enc l os ure tha t provides
necessary containment .
29
30
Fi gure 3 .
Equipment for spe ctroele ctro chemi cal s tudies o f
radi oactive actinide elements . Voltamme ter (A) � s p e c troph o t ome ter (B ) ,
and modi fied g love d b ox (C) .
31
a.
Prepara t ive gloved box .
Chemic a l manipu l a t ions of larger
quantities ( > 10 mg ) of rad ioac tive ac tinides were per formed in a
conven tional three- foo t , standard- flow , air-vent ilated gloved box .
This box was equi pped with a centri fuge , hot plate , and bulk-head
connec tions for the input of var ious gase s and electrical power .
b.
Mod i fied gloved box .
A conventional three- foo t gloved box was
mod i fied wi th an appendage , fit ted with quartz windows , wh ich s l ides
into the sampl e compar tment of the Cary Mode l 14-H spec tropho tometer
( see Figures 3 and 4 ) .
Bul k-head connect ions were made to the
vol tamme ter for per forming simul taneous e lec trochemic a l and
spec troscopic measurement s of rad ioac tiv e ac tinid e s .
c.
Inert atmosphere gloved box .
An inert (hel ium) atmos phere
g loved box was used for preparat ion of anhydrous fluoride s .
An RF-heated
Mone l tube furnace , wh ich could be pres surized with fluor ine , was
ins ta l led in the rear of the box for th i s purpose .
4.
Accessory Equipment
a.
� meter .
A Corning Mode l 1 3 0 pH meter was used in conj unc t ion
with a Fisher Sc ien ti fic Company Mode l E-5A comb inat ion glass
e lec trode with a s i lver/ si lver chloride in ternal re ference elec trod e .
b.
Alpha detec t ion equipment .
Alpha spec tra were taken wi th a
s il icon sur face-barrier de tec tor and a Tracor/Northern Mode l TN-1 7 10 ,
4096-channel ana l yzer .
Gross alpha count ing was performed wi th a
gas- f l ow propor t ional counter .
c.
X-ray spec trometer .
A General E lec tric Mode l XRD-6 X-ray
generator wi th a fine- focus copper X-ray tube was used with a nicke l
fil ter to per form X-ray powder dif frac t ion anal ys i s .
Powder pat terns
32
F igure 4 .
b ox r emove d
S p e c t r oph o t ome t e r / mod i f i e d g l o v e d b o x a s s emb l y .
fr om s p e c t r o ph o t ome t e r
s pe c t r oph o t ome t e r
( l owe r
ph o t o ) .
( u p p e r pho t o )
and
in s t a l l e d
G l ov e d
in
33
were recorded with a Nore lco/Ph i l l ips Debye-Scherrer- type powder
c amera ( 5 7
d.
mm
dia . ) u s ing Kodirex 3 5
Raman spectrophotome ter .
mm
X-ray fi lm .
Raman spec tra were recorded with a
Ramanor Mode l HG 2 S spec trophotometer ( Instrument s SA) .
Thi s ins trumen t
employs a doub le monochromator with curved ho lographic gratings ,
pho toe l ec tr ic detec t ion , and pul se count ing elec tronics .
B.
1.
Electrodes and Ce l l s
Convent iona l Working E l ec trodes
Mic roelec trodes used in this research inc lude : ( a) a hanging
mercury drop e lec trode (HMDE ) ( PARC Mode l 9 3 2 3 ) , ( b ) an 18-gauge
plat inum wire , ( c) an 18-gauge nickel wire , ( d) a 22 -gauge go ld wire ,
and ( e ) a microe l ec trode made of re ticul ated v itreous carbon (RVC)
( Fluoroc arbon , Anaheim , CA) .
Al l of the met a l wir e elec trodes
( Enge lhard Mining and Minerals , Newark , NJ) were made by seal ing the
wire into the end of a glass tube with epoxy , wi th the except ion of
platinum , which was sealed by fus ing the end of the glass tube .
The
RVC microe l ec trode was cons truc ted by seal ing a smal l sec t ion of 10
pores per inch ( ppi ) RVC in the end of a glass tube wi th epoxy.
E lectr ical contac t was made by threading a copper wire through the RVC
ma trix and packing UCAR C- 34 carbon cement ( Union Carb ide Corporation ,
New York , NY) around the contac t , fo l lowed by curing at ll0 °C for 24 hr .
Larger convent ional elec trodes for bulk elec trol ys e s of so lut ions
were made from ( a ) sect ions of 10 ppi RVC , with electrical contact
provided as described above , and (b ) sec t ions of 40-mesh plat inum
screen (Engelhard Mining and Minerals ) , which were soldered to copper
34
wires for electrical contac t .
These electrodes were par t i a l l y submerged
such tha t onl y the e lec trode material was in contac t wi th the solution .
2.
Opt ica l ly Transparent E lec trodes
a.
Reticu l a ted vi treous c arbon opt ica l ly transparent e lec trodes
(RVC-OTE ' s ) .
RVC-OTE ' s were construc ted bas ed on direc t ions by
Norve l l and Mamantov 2 8
but wi th some modificat ions .
S l ices o f
var ious poro s i ty grades ( 10-100 ppi ) of RVC were sec t ioned with a
d iamond saw to var ious th ickne sses from 0 . 6 2 mm for 10 0 ppi to 5 mm
for 10 ppi .
Electrical contac t was made as described above .
The RVC
s lices , wi th contacts in place , were then sandwiched be tween two
quartz microscope s l ides .
The edges were epoxied on three sides , wi th
the RVC pos i tioned a few mm from the open bo ttom of the ce l l .
A sma l l
d iame ter pl astic suc t ion tube was epoxied into the top o f the ce l l .
Epoxy ( W. J . Ruscoe Company , Akron , OH) was extended to coat the
entire external port ion of the RVC , which ex tended beyond the ce l l , as
wel l as the electrical contact , to ensure mechanic a l s t ab i l i t y .
A
c ompl eted RVC-OTE is shown in Figure 5 .
b.
P l a t inum s creen OTE ' s .
Shee ts of 4 0-mesh plat inum sc reen were
trimmed into sec tions approximate ly 2 . 5 x 7 . 5 em .
Elec trical contac t
was made by solder ing a copper wire to the edge of the plat inum
screens .
The e l ec trodes were then sandwiched between two quartz
plates as described above .
c.
Gold screen OTE ' s .
Trimmed sec tions of si lver sc reen ( 4 0 mesh )
were so ldered to copper wire leads and were then elec tropl ated with gold
( Se l-Rex "Orig ina l Br ight Go ld" patented proce s s , Se l-Rex Corp . ,
Nut ley , N. J . ) .
These go ld screens were then sandwiched between quartz
plates as desc ribed above .
35
Fi gure 5 , A r e t i culated vitreous carb on op t i cally t rans p arent
ele ctrode (RVC�OTE ) .
36
d.
Porous metal foam OTE ' s ( PMF-OTE ' s ) . 7 6
Porous metal foam (As tro
Me t Associates , Cinc innati , OH) was ob tained as the copper and nicke l
material ( s eries 280-5 ) in bulk sheets wh ich were sec t ioned into
e lec trodes between 3 and 5
mm
thick .
The PMF was elec tropl ated wi th
Pt , by the Oak Ridge Ga seous Di ffus ion P l an t , us ing a proprie tary
process , or with Au , by ORNL us ing the Se l-Rex proce ss , or was
amalgamated . 27
e.
E l ectrode coverage was checked by vol tammetry.
Amalgam OTE ' s .
Nicke l PMF , si lver sc reen , and gold sc reen
OTE ' s we re pl ated with mercury us ing a procedure described by He ineman
� �. 27 after they were sea led between quar t z pla te s .
This plating
procedure was fo l lowed by dipping the electrodes in c lean , dry mercury .
The excess mercury was dis lodged by vigorous shaking of the OTE ' s .
3.
Re ference E lect rodes
Saturated calomel re ference elec trodes ( S CE ' s ) (Al tex-Beckman
Ins truments , Irvine , CA and Corning Scienti fic Instruments , Med fie ld ,
MA) of the semi-micro type with fiber j unc tions were used for al l
potential measurement s .
4.
Counter E lectrodes
An 18 -gauge plat inum wire (2 em in l ength) , seal ed in a glass
tube , or a 40-mesh plat inum sc reen ( about 1 x 3 em) was used as the
counter elec trode .
5.
Teflon Ce l l Holder
A spec ial ly des igned Te flon ce l l holder was construc ted wh ich
pos i t ions a quart z ce l l ( 0 . 5-cm pa thl ength and 0 . 75 mL volume to
37
2 . 0-cm path l eng th and 1 mL volume ) in the sampl e beam o f the
spec trophotometer .
The ce l l ho lder is fitted wi th a s l id ing lid in
which three e lec trodes are mount ed : a working elec trode ( RVC , PMF , or
me tal sc reen OTE ) , an SCE re ference elec trode , and a plat inum wire
counter el ec trode ( see Figure 6 ) .
When the l id is lowered ( see Figure
7 ) , the three elec trodes are �ersed in the e lec troac t ive sol u tion
for spe ctroel ec trochemica l measurements .
C.
Reagents
The common inorganic ac id s , base s , and sal ts used in thi s work
were of s t andard AC S-cer t i fied reagent grade and were ob tained from
one or more of the fo l l owing : Mal l inckrodt Inc . , Paris , KY ; J . T.
Baker Chemical Company , Fairlawn , NJ ; Fisher Scientific Company ,
Pit tsburgh , PA ; or Allied Chemica ls . Morr is town , NJ .
Uranium sal ts and oxides ( 99 . 0% purity) , ces ium fluor ide ( 99 . 9 % ) ,
ces ium carbonate ( 99 . 9% ) , po tass ium hydroxide (ul tra- pure ) , po tass ium
carbonate (ul tra-pure) , and deuterochl oride and deuterated water were
obtained from Thioko l-Ventron , Al fa Produc ts , Danvers , MA.
Lanthanide s a l t s (99 . 9%) were ob tained from Th iokol-Ventron , Al fa
Produc t s and from Re search Chemic als-NUCOR, Phoenix , AZ .
Europium ,
samarium , and pra s eodymium chlorides were ob tained in high purity
( Cerac-pure ) from Cerac , Inc . , Milwaukee , WI .
Mul t imil l igram quan t i t ie s o f the transuranium e l ements used in
thi s work were ob tained as the chloride sa l ts from the High Flux
Isotope Reac tor/ Transuranium Processing P l ant (HFIR/TRU) complex o f
ORNL under the Depar tment of Energy ' s program of transur anium element
produc t ion and research .
38
Tef lon cell holder with lid in f-'up P p o s i t i on . P latinum
Figure 6 .
s creen working e le c t rode (A) , p latinum counter electrode (B ) , S CE
refe rence electrode ( C ) , and op t i cal p ath through quar t z cell (D) ,
39
Figure 7 .
Tef lon cell holder with lid in 11down u p o s i tion ,
40
S t ock so lut ions were prepared with dis t i l led-deionized water
unless otherwise indicated .
Nitrogen gas ( 9 9 . 9%) was ob tained from Linde/Union Carb ide
Corporation , New York , NY .
Ozone ( 10% by volume ) was prepared by
pass ing oxygen ( 99 . 5% ) (Ho ls ton Oxygen Company , Knoxvi l l e , TN) through
a high-vol tage arc-d ischarge apparatus .
D.
1.
Procedures
General Procedures
Initial electrochemical invest igat ions o f redox couples invo lved
the use of cyc l ic vol tamme try for the de terminat ion of ( a) the proper
potential requ ired to generate produc ts , ( b) the revers ib i l i ty of the
reac tion , and ( c ) the exis tence of pos s ib le interfering side
reac t ions .
In mos t cases the same elec trode material was used for
s pec troelec trochemis try as wa s used for vol tamme try .
Ini tial spec tral invest igations of redox couples were performed
by ( a ) record ing the so lution absorption spec trum of the electroac t ive
spec ies in its ini t ia l oxidat ion s tate , ( b ) oxid izing or reduc ing this
spec ies with chemical reagents to ob tain the des ired produc t ox idat ion
s tate , and ( c ) recording the ab sorption spec trum of thi s produc t
spec ies .
The spec tra were us ed to determine the iden t i ty of the
produc t and the proper moni toring wavel ength ( s ) , corresponding to
absorp t ion maxima , for const ant wavelength spec troe lec trochemis try .
Af ter the init ial experiment s , or in l ieu of the s e , absorb anc e vs
t ime spec troe l ec trochemical experiment s were per formed .
A po tential
s tep was appl ied to the redox system , in equi l ibrium , in order to
41
generate the produc t oxid at ion state .
The generation o f produc t was
monitored by measur ing the ab sorbanc e o f the produc t ( or reactan t ) at
a cons t ant wave length as a func t ion of time or by recording the en tire
spect rum as a func tion of time .
S pectropo tent ios tatic determina tions of thermodynamic parame ters
( forma l reduc tion potential and n-value ) were then app l ied to amenab le
redox systems in the fo l l owing manner .
The ab sorbance of the produc t
( or reactant) oxidat ion state was measured at var ious va l ue s of
a pp l ied potent ial in equi librium.
These data were then us ed to
c ons truc t a po ten t i a l vs log ( [ Ox ] / [ Re d ] ) (Nerns tian ) p l o t .
2.
Lanthanide E l ement s
a.
M ( I I I ) /M ( I I ) redox couple s .
So lut ions o f from lo -1 to lo - 3 M
Sm( I I I ) , Eu( I I I) , and Yb ( I II) were prepared separa t e l y by diss o lving
weighed quantities of the appropriate ch lor ide sa l t s in a 1 M KCl .
The pH o f the resul tant solut ions was measured with a glass electrode
a nd was adj us ted to pH 6 by the add i t ion of KOH .
S ince the lanthanide
chl oride sa l t s were hydrated with an unknown number ( from 6 to 9) of
water mo lecul es , an exac t we ighing to have a known concentrat ion was
not po s s ible ; there fore , the concentrat ions of the lanthan ide
solutions were approximate .
In experiment s where accurately known
concentrat ions of lanthanides were required , the solut ions were
submit ted to ORNL Ana lytical Chemis try Divis ion for quantitative
ana lys is .
Cyc lic vol tammog rams of the Sm( I I I ) , Eu ( I II ) , and Yb ( II I ) ch loride
solut ions in 1 M KC l at pH 6 were recorded wi th an HMDE .
Cyc l ic
42
vol tammograms of the Eu( I II ) solut ion were recorded us ing an RVC-OTE
( 100 ppi , 0 . 6 2 mm pa th length ) .
The so lut ions were purged with
nitrogen gas for 15 min pr ior to vo ltammetric ana lys i s in order to
remove dis s o lved oxygen .
The Eu( I I I ) /Eu( I I ) redox couple was inve s t iga ted us ing RVC and Pt
screen OTE ' s via spec troelectrochemis try.
About 5 mL o f 5 . 0 x lo -3 M
Eu( I I I ) in 1 M KC l at pH 6 wa s placed in a te flon boat .
was pl aced on the OTE to prevent edge ef fec t s . 3 6
An optical mask
The OTE was then
d ipped in the bulk solut ion and , with the use of a hypodermic syringe ,
a port ion of the bulk so lut ion was drawn up into the OTE c e l l .
Thi s
assemb l y was pl aced in the sample beam o f the spe c t rophotometer .
With
the add i t ion of an SCE and a Pt wire elec t rode placed in the bulk
solut ion , be low the OTE , spec troe lectrochemis try wa s per formed .
Spec tropo tent ios tatic experiments were performed to de termine the
formal reduc tion potent ial of the Eu( I II ) /Eu( I I ) red ox coupl e and the
n-va lue in the RVC-OTE .
Equ ilibrium attainment was de termined by
monitoring the absorbance � time curve of the sys tem after each
potent ial step .
Spectra were re corded as a func tion o f time us ing the Cary Mode l
14-H and the si licon-vidicon rapid- scan spec trometers during the
g enerat ion o f Yb ( I I ) by elect ro lysis of Yb ( I I I ) us ing a si lver amalgam
screen OTE .
b.
The reduc t ion of Sm( I II ) wa s also stud ied with this OTE .
M ( IV ) /M( I I I ) redox couple s .
Solut ions o f from lo - 1 to lo - 2 M
in Ce ( I I I ) , Pr ( I I I ) , and Tb ( I II) were prepared separate l y by
dis s o lving the ch l oride sa lts in ( a ) 5 . 5 M K2 C03 , ( b ) 2 M (NH4 ) 2 C0 3 ,
43
( c ) 2 M Na 2 C03 , and (d) 5 M Cs 2 C03 .
In order to e l iminate chlor ide
ion , some sol u t ions of the above-ment ioned lanthanides were prepared
by precipitat ing the lanthanide hydroxide with KOH fo l lowed by
centri fuga tion and wash ing and disso lut ion of the we t hyd roxide in the
var ious carbona te so lut ions .
The hydroxide ion concentrat ion of all
of the carbona te so lutions , wi th the except ion of the ammo nium
c arbonate solution , was adj us ted to about 1 M by the add it ion of KOH.
Ini tial spec tra were recorded in 1-cm quartz cuvets wi th the
appropriate carbonate solut ion placed in the re ference ce l l .
For bulk
e lec tro l ys i s approximate l y 2 0 mL o f the lanthanide carbonate so lution
was pl ac ed in a sma l l beaker , and a large convent ional RVC or Pt
screen working , the re ference , and the counter electrodes were
immersed .
S t irr ing of the solut ion was provided by the bubb l ing o f
nitrogen or by e lec trochemic a l ly generated oxygen .
The solutions were
elec trolyzed for periods of time ranging from 10 min to 3 hr before
absorpt ion spectra were recorded .
For highly ab sorb ing solutions ,
quar tz spacers were used to reduc e the opt ical path length to 3
to 1
mm
or
mm .
The spectropotent ios tatic method was appl ied t o the determinat ions
of the formal reduc tion po tent ial and n-val ue of the Ce ( IV) /Ce ( II I )
redox couple in 5 . 5 M K2 C03 u s ing an RVC-OTE ( 100 ppi , 1 mm
path leng th ) .
A solid oxidat ion produc t , formed at the RVC e l ec t rode during
e le c t ro l ys i s of the Tb ( I II) carbonate solution (5 . 5 M K2 C03 , 1 M
OH- ion ) , was col lec ted , rinsed with alcohol , dried , and inves t igated
by X-ray powder diffrac t ion
and so l id state re flectance and laser Raman
44
spec troscopie s .
The sample was also submitted to ORNL Anal yt ical
Chemis try Division for thermogravimetr ic/mas s spec tral and X-ray
f luorescence ana l yses .
Chemical oxidat ion of the Pr ( I II ) and Tb ( I II ) carbonate sol ut ions
was accompl ished by bubbl ing ozone through the soluti ons for a few
hours to ensure su ffic ient oxidation of the lanthanide spec ies .
3.
Ac t inide E lement s
a.
Uranium.
Solut ions of from 10 -1 to lo -2 M U(IV) and U(VI )
were made separately by dissolving UC l4 and U02 C l 2 in 1 M KCl .
The pH
o f these solutions was adj us ted to desired values by the addit ion of
HCl or KOH.
Solut ions of from 0 . 1 to 0 . 5 M U( VI) were prepared in 1 M
KCl 1n 9 9 . 8% D 2 0 by fir s t dissolving a weighed quantity o f U03 in DCl
fol lowed by evaporat ion to near drynes s and sub sequent d issolut ion o f
the sal t in the KCl /D 20 solut ion .
The pH ( pD) was adj us ted by
add i tion of DC l .
Cyc l ic vol tammograms o f the uranium so l utions at various pH
value s were recorded at an HMDE and a Pt wire microe lectrode .
Ini t ia l ab sorption spec tra o f the U ( IV) and U (VI ) solutions were
recorded pr ior to chemical reduc tion of the uranium spec ies by zinc
amalgam .
The U (V I ) /KC1/D 20 solution was treated wi th Eu( I I ) to reduce
the U(VI ) spec ie s .
Spec tra of the reduced species were then recorded .
Absorbanc e vs t ime spec troe lec trochemic al experiment s were
per formed us ing the U (VI ) solutions at var ious pH value s .
�
Ab sorbance
time curves were recorded during the elec trochemic a l reduc tion of
U ( IV ) to U ( I I I ) in 1 M KCl ( pH 0) in a si lver screen amalgam OTE .
The
45
compl ete spec tra o f these spec ies were also recorded as a func t ion o f
t ime dur ing the e lectrolys i s .
An RVC-OTE wa s used wh i le ab sorbance v s time curves and
compl ete spec tra as a func tion o f time were rec orded dur ing the
e lectrochemical reduc tion o f U ( VI ) at var ious pH value s in 1 M KC l .
b.
Neptunium.
Mul t imi l l igram quanti t ie s of neptun ium- 2 3 7 were
obt ained as the oxide , Np02 , wh ich was dissolved in ni t r ic aid and
hea ted to drynes s .
The resul tant sa l t was fumed to drynes s in
perch l oric ac id , thus oxid izing the Np to the hexav a l ent state .
The
Np02 ( Cl 04 ) 2 s o l id was dissolved in 1 M perch lor ic ac id , resul t ing in a
s tock so lut ion wh ich was about 2 M Np (VI ) .
Al iquo ts o f th is
s olut ion were d i l uted separately wi th 1 M HC l04 , 2 M Na 2C03 , and 5 . 5 M
K2 co 3 to prepare so lut ions between lo - 2 and lo -3 M Np ( VI ) .
A stock so l u t ion o f oxidat ion- s tate pure Np( V ) was prepared 7 7 by
mixing equal quan t i t ie s of Np( IV) and Np(VI) in perch l o r ic ac id .
The
Np( IV) wa s prepared by peroxide reduc tion o f a mixture o f Np ( V ) and
Np( IV) , ob tained by d irect dissolut ion of the mixed oxide in ni tric
acid . Excess peroxide was e l iminated by hea t ing .
Ini t ia l spectra of the Np so lut ions were recorded pr ior to
e lectrochemica l stud ie s .
Cyc l ic vo l tammograms o f the Np so lutions
were recorded wi th the Pt and RVC microelectrodes us ing a 1-mL a l iquo t
o f so l ut ion in a sma l l beaker or in a quartz ce l l in the te flon ce l l
holder .
The absorpt ion spectra o f Np (VI) and Np (V) were recorded in
RVC-OTE as a func tion o f time during the elec t rolytic reduc t ion
o f Np(VI ) in 1 M HC l04 .
Absorbance vs time curve s were al so
an
46
recorded , as we l l as data for the spec tropotent ios t a t ic dete rminat ions
o f E0 ' and n for the Np( V I ) /Np ( V ) redox couple in 1 M HCl 04 .
Np ( VI ) in 2 M Na 2 C03 wa s firs t reduced to Np ( V ) and the spec trum
was recorded .
The Np (VI) s tock so lut ion was then adj us ted to pH 1 3 by
the add i t ion of hyd roxide ion , and the solut ion was invest igated by
cyc lic vol tammetry and by recording complete spectra as a func tion of
time dur ing electrolysis at a large- sur face plat inum screen electrode .
An E0
'
va lue for the Np(VII ) /Np(VI) redox coupl e was der ived from the
vol tammogram .
The Raman spec tra of bo th Np ( V I ) and elec trogenerated
Np( VI I ) in 2 M Na 2 C03 were recorded by remov ing the samples from the
g loved box in sealed capi l l aries by a technique de scribed
e lsewher e . 78
c.
Amer ic ium.
Mul t imi l l igram quantities of ame r ic ium-243 were
pur i fied at TRU/ORNL by chromatographic separat ions desc r ibed
e lsewhere . 7 9 The americ ium was received as an air-dried chl o r ide sal t ,
AmCl 3 . nH 20 , wh ich resul ted from evapora tion of the e l uant from the
final ion-exchange column .
K2 C03 , and 5 M Cs 2 C03 ] .
This chl oride salt was disso lved direc t l y
In order to e l iminate chl or id e ion , some
solutions were prepared by dissolving the AmCl 3 . nH 20 in water and by
prec ipitat ing Am( OH) 3 by the add it ion of hydroxide ion .
After
c entri fugat ion and washing with di lute base and then wi th water , the
Am( OH ) 3 wa s dissolved in the various carbonate solut ions .
The
hydroxide conc entrations of the carbonate so lut ions cont aining Am( I I I )
were measured wi th a pH glass elec trode which was cal ibrated wi th
solutions of known NaOH concentration .
47
Chemica l oxidat ion o f the Am( I I I ) carbonate solutions was
accompl ished by us ing peroxydisul fate- si lver ( I ) oxide . 5 3
Spec tra of
the in itial solut ions and the oxid ized so lut ions were recorded .
Cyc lic vo l t ammograms of the Am( I I I ) carbonate solut ions were recorded
at var ious pH value s .
A l i quo ts of from 0 . 7 5 to 1 . 5 mL o f the Am( I I I ) carbona te
solutions , which were from 1 . 2 x 10 - 2 to 4 . 0 x lo -3 M Am ( I II ) , were
placed in quartz ce l l s in the te flon ce l l ho lder .
In i t i a l spec tra
were rec orded and then al iquo t s of the solut ions wer e removed for
a l pha count ing .
The electrodes (RVC or Pt OTE ' s ) were immersed in the
solutions , and an in i t i a l potential appl ied to the ce l l .
The
potentia l was made more pos itive unt i l a change in the spec t rum was
noted or un t i l visual ob serva tion confirmed an oxid at ion reac t ion .
Depend ing on the pH o f the so lut ion and the type o f OTE used , some
spectra were recorded on ly after a po tent ial was no longer imposed or
a fter the e lec trodes were compl etely removed from the solut ion .
The prec ipitate formed by the elec trochemica l oxidat ion of
Am( I I I ) in po ta s s ium carbonate so lut ion was col l ec ted , washed , and
centr i fuged .
The so l id was then disso lved in 1 M perchloric ac id , and
the so l ut ion spec trum was recorded .
The so l id was also analyzed by
X-ray powder d i f frac t ion .
Add it ional charac teriza tion o f the oxid ized Am spec ies in
carbonate solut ions inc luded spectropho tometric and poten tiometric
t itrat ions wi th K4Fe C CN) 6 in 2 M Na 2C03 .
48
So l id anhydrous AmF4 and Cs 3AmF 7 were prepared and d i s s o lved in
concentrated HF so lution sa turated with Cs F.
Spec tra were recorded
both pr ior to and fo l lowing bul k solut ion electrolys i s .
d.
Curium.
Mu l t im i l l igram quantities of cur ium-248 were
pur i fied at TRU/ORNL as de scr ibed el sewhere . 79
Received as the so l id
CmCl 3 . nH 2 0 s a l t , it was dissolved in water , and the cur ium wa s
prec ipitated by the add i t ion of KOH . The resu l t ing prec ipitate was
washed wi th di lute KOH and was centri fuged and dissolved separa tely in
Initia l spec tra o f Om( I I I ) in the carbona te med ia were recorded
prior to e lect rochemical oxidat ion with large- sur face RVC and Pt
e lec trodes .
The spec tra of the re sul tant so lut ions were also
recorded .
4.
Data Treatment
Da ta obta ined from the spec tropo ten tios tatic experiments fo r the
.
.
d eterm1nat 1ons of E
0 .
and n for redox couples were ana lyzed by a
l inear leas t squares program us ing a Tektronix 405 1 mic rocompu ter . The
program is de scr ibed in the Tektronix "PLOT 50" 4050Al0 Software
Package , Vo lume 4 , Pr ogram 9 .
This program , us ing the least squares
method , fi ts a user-de f ined func t ion ( the Nernst equa tion ) , where
optima l parame ter values are found by minimizing the sum of the
squares of the dif ferences between the observed and calculated values .
The Ne rns t ian plot and at tendant parameters were displayed
on the Tektronix 405 1 CRT display and pr inted by a Tek tronix 46 3 1 Hard
Copy Un i t .
Drawings were then made from the hard copy output .
49
Error limits reported in th is work are based on one standard
devia t ion and do no t inc lude es timate s of sys tematic errors .
The
e rrors associated wi th the de termination of formal reduc t ion
potent i a l s by vo l tammetric measurements depend upon ( a ) the acc ur acy
and stab i l ity o f the re ference elec trode , ( b ) the amb ient temperature
var iat ion in the l aborat ory , ( c ) the proper cal ibration of the
potentiostat/recorder , and ( d ) the reproduc ib i l ity o f the
vol tamme tric response of the redox system.
The error l imits reported for the spec tropo tentios tatic
determinations o f the formal reduc tion poten tials re flec t a
c onservative est imate of the acc uracy of the determinat ion and not
necesar i l y the ful l extent of the precis ion ind icated by the leas t
squares program .
CHAPTER III
RESULTS AND DISCUSS ION
A.
1.
Lanthanide Element s
M ( I II ) /M( I I ) Redox Couples
The onl y divalent lanthanide ions suf ficiently stab l e to exi s t in
aqueous so lut ion are those of samarium , europium , and yt terbium , and
therefore , the se M( I I I ) /M( I I ) redox couples are amenab le to
spec troelec trochemical inves t igation in aqueous solut ion .
The pH
range in which these redox couples can be studied is l imited .
Above
pH 7 the hydrolys i s of the M( I I I ) ions proceeds and prec ipi tation o f
the ions occurs .
A t lower pH values interference from hydrogen gas
evo lut ion , at po tent ials required to generate the M( I I ) ions , is
subs tantia l .
Thus pH 6 was se lec ted as the optimum val ue for
inves t igation of these redox couples . B O
a.
V o l t amme try .
Ini tial elec trochemic al inve s t igat ions o f the
M ( I I I ) /M( I I ) redox couples were per formed by record ing cyc l ic
vol tammograms of the sys tems at an HMDE .
Cyc l ic vol tamme try is an
e ffec t ive technique for ini tial electrochemic al study of new sys tems .
A great deal of information about fairly compl icated sys tems can be
der ived in a sing le experiment . B l
The HMDE was se lec ted because of
the cathodic po tent ials required to generate lanthanide M( I I )
oxidat ion states .
The HMDE provides a cathodic range of from abou t 0
to - 1 . 8 V in 1 M KCl at pH 6.
[All po tent ial measurements in th is
50
51
document are reported versus the NHE un less otherwise ind icated : SCE
0 . 24 15 V � NHE . ]
=
At potent ials more negat ive than - 1 . 8 V , the
evolut ion of hydrogen gas and/or the reduc tion o f po tas s ium ions
o ccur s at the electrode sur face .
Cyc lic vol tammograms of the Eu ( I II ) /Eu( I I ) , Yb ( I II ) /Yb ( I I ) , and
Sm( I II ) / Sm( I I ) redox couples in 1 M KCl at pH 6 are displayed in
Figure 8 .
The l ack o f compl ete elec trochemic a l revers ib i l ity o f the
Eu ( I II ) /Eu( I I ) redox coupl e was ev idenced by the large d i f ference
between the anodic and cathodic peak po tentia l s .
In add ition , the
recording of consecu t ive cyc l ic potential sweeps a l so ind icated the
l ack of compl ete reversib i l ity in that the peak po tential values
changed wi th each sc an .
potential sc an .
The peak separat ion grew larger with each
The Eu redox couple is not completely irreversib l e ,
however , because the anodic peak current is approximate l y equal to the
c athod ic peak current .
This behavior of the Eu red ox sys tem has been
noted by others . 82
The formal reduc t ion potent ial , Eo ' , ob ta ined fr om the cyc l ic
voltammogram wa s -0 . 3 4 � 0 . 0 1
v.
This approximate value wa s
determined by calculating the mid-point between the anodic and
cathod ic peak potent ials . 1 3 , 8 3
The cyc l ic vo l tammogram of the Eu redox
coup l e in an RVC-OTE in 1 M KCl at pH 6 is shown in Figure 9 .
The
sweep rate used was qui te slow ( 5 mV/ s ) and was nec ess ary because o f
the re lative ly high internal re sis tanc e o f the RVC electrode .
Es s en t ia l ly the same behavior o f the Eu couple was ob served at the RVC
e lect rode as was observed at the HMDE .
The di fference be tween the two
was , however , a sh i ft in the Eo ' value ; a t the RVC e le c t rode , it was
- 0 . 43
+
0. 0 1
v.
The lack o f to tal rever s ibil ity o f the Eu ( I II ) /Eu ( I I )
52
O R N L- DWG 80- 1 5439
u
I
0
0
:I:
20
fL A
�
t- u
z
w
a:: O
a::
--
=> u
U 0
0
z
<t
t
0
- 0. 5
- 1 .0
- 1 .5
P OT E N T I A L ( vo l t s v s SC E )
- 2. 0
Figure 8 . Cyc l i c vo ltammograms o f M( I I I ) s pecie s o f Eu (A) , Yb ( B ) ,
and Sm ( C ) i n ' 1 M KCl . pH 6 , HMDE , swe ep ra te : 2 00 mV/ s , ( M ( I II ) J =
1 . 0 X 10 - 2 M .
53
t
u
c
0
J:
SCAN RATE :
5 mV/sec
RVC-OTE
�
u
t­
z
w
�0
::::)
u
I l O O p.A
u
c
0
z
<
+
0.5
0.0
- 0.5
- 1 .0
- 1.5
POTENTIAL {VOLTS SCE)
vs
Figure 9 . Cyc l ic vo l tammog ram of Eu ( I I I ) in 1 M KC l in an RVC-OTE .
pH 6 , RVC-OTE : 100 p p i , 0 . 6 2 mm pa th length , [ Eu ( I I I)] = 1 . 0 x to - 4 M .
54
redox couple did no t prec lude the spec troe lec trochemic al inve s t igation
o f th is couple .
The cyc l ic vol tammogram of the Yb( I II ) /Yb ( II ) redox couple in 1 M
KCl at pH 6 (see Figure 8 , page 52 ) displ ayed e l ec trochemic a l
reversibil ity in that the peak po tential separat ion for the anod ic and
cathodic peaks was 65 mV , much less than that for the Eu case .
In
addi tion , the anod ic peak current for the Yb sys tem was equal to the
cathod ic peak current .
in peak current
values .
The E 0
Consecut ive po tent ial sc ans showed a reduc tion
but compl ete reproduc ib i l ity in the peak po tent ial
1
value determined for this coupl e in the stated
med ium , derived from the cyc l ic vol tammogram , was - 1 . 18
+
0 . 01 V.
Compl ete cyc l ic vol tammograms o f the Yb sys tem a t the RVC
e lectrode were not po s s ib l e due to the limited cathodi c range of RVC .
The RVC elec t rode exhibi ted a potent ial range o f from + 1 . 4 t o - 1 . 0 V
in 1 M KC l at pH 6 .
Cyc l ic vol tammograms of the Yb sys tem at a nicke l
amalgam wi re microe lec trode were pos s ib le , but the vo l tammograms
exhibited a shape ind icat ive of a less- than-rever sib l e redox couple .
This wa s shown by the fac t that the anodic and cathodic peak
potentials were great l y separated .
The shape o f the vol tammogram was
similar to that o f the Eu sys tem at the HMDE .
The cyc l ic vol tammogram of the Sm( I II ) / Sm( I I ) redox couple in 1 M
KC l at pH 6 is shown in Figure 8 , page 5 2 .
The cathodic peak is clearly
vis ib le , ind icat ing the reduc tion of Sm( II I ) , but the anodic peak is
55
absent , wh ich ind icates an irreversible redox coup l e at the e lectrode .
The reasons for th is irreversib i l ity are as fo l lows .
The reduc t ion of
the Sm( III ) s pec ies occurs at a po tential at wh ich hydrogen gas is
e volved at the HMDE .
The H 2 gas probabl y carries the reduced species
away from the e lec trode surface ; thus , Sm( I I ) may no longer be present
at the electrode to be oxid ized during the anodic sweep.
Also the
Sm( I I ) s pecies reac t s rather qu ic kly with the solvent to form H 2 and
Sm( III ) , and there fore , no Sm( I I ) is present at the e lectrode to be
oxid ized and give rise to an anodic peak .
The EO
I
value for the
Sm( I I I ) / Sm( I I ) redox couple in the s tated medium wa s e s t imated from
the cyc l ic vol tammogram to be - 1 . 50 + 0 . 0 1 V .
This value was
calculated by us ing the boundary condit ion equations for a to ta l l y
irreversib l e wave , a s described by Bard and Faul kner . S l
The E 0
I
values de termined in these three cases are in good
agreement wi th the va lues of the standard reduc tion po tent ials
reported by Nugent � �. 69 o f -0 . 3 5 V for Eu ( I II ) /Eu ( I I ) , - 1 . 1 5 V for
Yb( III ) /Yb ( I I ) , and - 1 . 5 5 V for Sm( III ) / Sm( I I ) .
b.
Spectrophotometry.
The solution ab sorpt ion spec tra of the tri-
and divalent ions of europium , yt terb ium , and samarium are displayed in
Figures 10 , 1 1 , and 12 , respec tively.
These spec tra are adapted from
tho se of Carnal 1 84 and are o f the spec ies in 1 M HC l04 .
The spec tra of
the lanthanide spec ies in 1 M KC l are essent ial l y ident ical to those in
Figures 10 , 1 1 , and 12 .
The diva l ent spec ies were prepared chemic a l l y by zinc amalgam
reduc t ion o f the trivalent species in deoxygena ted so lvent , wi th the
56
ORNL-DWG 78- 49977
WAVELENGTH
300
250
400
350
( nm)
500
420
600
iOOO
750
2
r=>
4t
39
37
35
33
t=
�
0
29
27
25
WAVENUMBER
�
Cl)
m
�
a:
3t
WAVELENGTH
250
24
22
( x ta5 m:.. t )
300
( nm)
400
20
48
46
t4
42
iO
8
500
2000
:I: t600
EUROPIUM
(U )
t200
800
400
Q L-�--����--L-�--�----�
27
25
43
4t
39
37
35
33
3t
29
23
2i
49
47
t5
t3
u
WAVENUMBER
Figure 10 .
1 M HC l04 .
h to!5 m-4 )
So lut ion ab sorpt ion spect r a o f Eu ( II I ) and Eu ( I I ) in
57
ORNL-OWG 80·1 5789
225
250
300
WAV E L E N G T H ( n m )
350
4 00
500
3
Y T T E RBIUM
2
>1>
1a.
a:
0
(/)
m
<l
a:
<l
...J
0
�
45
41
37
33
29
WAV E N U M B E R
25
600
750
1 000
1m )
21
( x 10 5 m- 1 )
17
13
9
WAV E L E NGTH (om)
300
400
500
2 50
2000 ------�----�--.--r---,
1 600
1 200
Y T T E RBIUM ( TI )
BOO
400
0 L-�����-L����-L�--��
19
15
23
35
34
39
27
43
5
WAV E N U M B E R ( x 10 m-1 )
Figure 1 1 .
1 M HCl 04 .
S o l u t ion abs orpt ion spec tra of Yb ( I I I ) and Yb ( I I ) in
58
ORNL-DWG 80· 1 5790
4
3
>1-
5
1-
a.
a:
WAVELENGTH ( nm )
300
2 50
600
..
750
1
0
38
42
WAV ELENGTH
300
2 50
800
18
22
26
1
5
WAVENU MBER ( x 10 m- }
30
34
ct
0
�
500
400
SAMARIUM ( ID )
2
0
(/)
m
0:
ct
.J
350
(nm}
4 00
44
500
600
4 00
200
0
1
43
Figur e 1 2 .
M HC l 04 .
39
35
31
WAVE NUMBER
27
23
( x ta5 m-1 )
19
15
So lut ion absorpt ion spec t ra o f Sm( I I I ) and Sm( I I ) in
59
except ion of Sm( I I ) , which wa s generated by dissolut ion o f samar ium
metal in di lute HCl .
It was not pos s ible to record a complete
spec trum of the blood-red solution o f Sm( II ) due to the inter ference
from hydrogen gas evolution .
c.
Spec troe lec trochemis try.
The Eu ( I II ) / Eu ( II ) redox coupl e in
1 M KC l at pH 6 was s tud ied us ing P t sc reen and RVC op tical ly
transparent elec trodes .
Al though the Pt sc reen OTE al lowed for
e lec trochemical generat ion and spec tral iden t i fic a t ion of Eu( I I ) , it
d id no t prov ide a sufficient cathodic potential range for complete
s pec tropotent iost a t ic study of this coup le .
The RVC-OTE was there fore
used to per form thi s s tud y.
The ab sorbanc e vs time curve for a se lec ted potential-s tep
e lec tro l ys i s for the reduc tion o f Eu ( II I ) in an RVC-OTE is shown in
Figure 1 3 .
The wave leng th monitored was 320 nm , corres ponding to an
absorpt ion maximum in the Eu ( II ) spec trum .
The ab sorb anc e vs time
curve is quite sim i l ar in shape to a current � time curve for the
reac t ion .
Both absorb ance and current are direc t measurements of the
quan t i t y of elec troac t ive spec ies reac ted .
Equ i l ibr ium is attained ,
throughout the total pa th leng th o f the OTE illuminated by the sample
beam , when there is no change in the absorb ance with time ( at 3 0 min
for the curve in Figure 13 ) .
Such absorbanc e vs time curve s were
recorded for al l po tential-step elec tro l yses to ensure the attainment
o f equil ibrium prior to recording the comple te spe c t rum.
Absorpt ion spec tra of Eu ( I I I ) and Eu ( II ) at various value s o f
appl ied po tent i a l , in equi l ibrium , are shown in Figure 14 .
Due t o the
l arge difference between the mo lar ab sorptiv it ie s o f the Eu( I II ) and
60
ORNL- DWG
78 - 19975
0.5
0.4
w
u
z
<X
�
0
(f)
0.3
�
STEP:
-0 . 41 V to -0 . 46 V/NHE
WAVELENGTH : 320 nm ( Eu2•)
0.2
RVC - OTE
0.1
o L-------�---L--�--��0
5
10
15
20
25
30
35
TIME (min)
Figure 1 3 . Abs orbance vs time curve for the po tent ial-step
e lectrolysis o f Eu ( I II ) in a;-RVC-OTE . RVC : 100 ppi , 1 mm pa th length ,
[ Eu( I I I ) J = 1 . 0 x lo -2 M.
61
ORN L- OWG 80- 9658
50
45
WAVE N U M BER (
40
35
x
10 5 m- 1 )
30
25
t
w
u
z
<l
m
0::
0
(J)
m
<l
200
2 50
3 00
3 50
4 00
W AV E L E N GTH ( n m )
Figure 14 .
So lut ion absorpt ion spec tra o f Eu ( I I I ) and Eu ( I I ) at
var ious va lues o f appl ied po tent ial in an RVC-OTE . 0 . 242 V (A) ,
-0 . 3 09 V ( B ) , - 0 . 3 34 V ( C ) , -0 . 3 59 V ( D ) , -0 . 3 84 V ( E ) , a n d -0 . 409 V
( F ) . RVC-OTE : 100 p p i , 0 . 6 2 mm pa thl ength , [ Eu ( I I I ) ] = 5 . 0 x 10 - 3 M
�n 1 M KC l at pH 6 .
62
Eu ( I I ) s pec ies ( s ee Figure 12 , page 5 6 ) , i t wa s necessary to se lect an
ini tial concentra t ion o f Eu( I II ) which was too low to observe any
absorpt ion peaks due to Eu ( I I I ) ( s pec trum A , Figure 14 ) .
As more
negat ive po tent ials we re appl ied to the RVC elec trode , absorbanc e
p eaks due to Eu ( I I ) grew in ( s pectra B through F) .
The potent ia l
range se lec ted for the exper iment was based o n the value o f
E0 1 obta ined from the cyc l ic vo l tammogram o f the Eu( I I I ) / Eu( I I ) redox
coupl e .
The po ten t i a l s were chosen to provide both ful l y reduced and
ful ly oxid ize d cond i t ions so that suf ficient da ta were co l l ec ted to
generate an accurate Ne rns tian plot . l 3
A tabul at ion of the data for the spectropotentios tatic
determ inat ion of thermod ynamic parameters (E 0
I
and n) for the
Eu( I I I ) /Eu( I I ) redox couple in 1 M KC l is given in Tab le VI .
The data
were derived from the ab sorb ance of Eu ( I I ) , at 32 0 nm , in equ i l ibrium ,
a t the various va lues of appl ied potent i a l .
Tab le VI i s defined in Chapter I , page 7 .
The no tat ion us ed in
The Nern s t ian plot ,
obta ined by p l o t t ing the appl ied potent ial versus the log of
absorbanc e ra t io , is shown in Figure 15 .
The data points fa l l on a
s traight line , as required by the Nern s t equa tion .
The slope of the
l ine yields an n-va lue of 1 . 007 (corresponding to a one-e lec tron
reduc t ion) , and the y- intercept yields an E 0 1 value of -0 . 3 9 1 + 0 . 005 V .
Th i s va lue is i n reasonab le agreement wi th the value s reported by
Morss and Haug85 ( -0 . 3 5 + 0. 0 3 V) and Biedermann and S i lber8 6 ( -0 . 3 79
� 0 . 0 0 1 V) .
The spec troe lectrochemical behavior of the Yb ( I II ) /Yb ( I I ) redox
c oupl e wa s st ud ied in 1 M KC l at pH 6 u s i ng a si lver amalgam sc reen
63
TABLE VI
DATA FOR SPECTROPOTENTIOSTATIC DETERMINATION OF THERMODYNAMIC
PARAMETERS FOR THE Eu( I I I ) /Eu ( I I ) REDOX COUPLE
Potential
( volts vs SCE )
0
Ab sorbance
A2
A2-A 1
A3 -A2
A2 -A1 /A3 -A2
1og( ArA 1 / A3-A 2 )
(0 . 098 )=A 1
-0 . 550
0 . 15 0
0. 05 2
1 . 2 17
0 . 0 43
-1 . 3 69
-0 . 575
0 . 2 15
0 . 117
1 . 15 2
0 . 10 2
-0 . 993
-0. 600
0 . 388
0 . 290
0. 979
0 . 2 96
-0 . 5 2 8
-0 . 62 5
0 . 650
0 . 5 52
0. 7 17
o . 770
-0 . 1 1 4
-0 . 650
0 . 950
0 . 85 2
0 . 4 17
2 . 043
0 . 3 10
-0. 675
1 . 16 5
1 . 067
0. 2 02
5 . 282
0. 7 23
-0. 700
1 . 2 78
1 . 180
0 . 08 9
- 1 . 000
( 1 . 3 67 )=A 3
13 . 2 6
1 . 1 23
64
ORNL-OWG 8 0 - 9 6 5 5
w
(.)
(/)
ui
- 0.55
>
(/)
�
0
>
...J
<(
Iz
w
I0
a..
0
w
...J
a..
a..
<(
- 0.60
E0
= -0.63V
-0.65
-0.70
-1.2
KCl .
'
Figure 1 5 .
pH 6 .
-0.8
-0.4
0
0.4
log{(Eu(III)] I [ un]}
Eu
0. 8
f. 2
Nern s t ian p l o t for the Eu ( I I I ) / Eu ( I I ) redox coupl e
1n
1 M
65
OTE .
It was necessary t o use th is elec trode because the reduc tion
potent ial of the Yb couple exceeds the cathodic potential range o f
both the P t sc reen and RVC OTE ' s .
A Tektronix s i l icon-vid icon based
rapid-scan spec tropho tometer was us ed to monitor the si lver amalgam
OTE dur ing the po tent ial- step electro l ysis of Yb ( I I I ) .
�
time curve for thi s reac tion , monitored at 3 5 3
Figure 16 .
nm ,
The absorb ance
is reproduced in
No te that equ i l ibrium occurs in less than three minutes .
Th i s time to equ i l ibrium was much shorter than tha t for the Eu couple
( see Figure 13 , page 5 9 ) .
The reasons for this were : ( a ) the
concentrat ion of Yb ( I I I ) was a fac tor of ten les s than tha t o f
Eu( III ) , ( b) the pa th length of the Ag amalgam sc reen OTE was 0 . 6 2
�
the 1
mm
mm
path leng th of the RVC-OTE , ( c ) the si lver amalgam sc reen
has a lowe r e lec trical re s i s t ance than that of RVC , and ( d ) the
potent ial step appl ied to the Yb system (-0 . 76 to - 1 . 5 6 V) was much
greater than the sma l l steps (0 . 05 V) used to s tudy the Eu system .
The rather large po tentia l- s tep was an effort to generate quickl y the
Yb ( I I ) be fore in ter fe renc e from gross hyd rogen gas evolut ion was
encountered .
The cathodic po tential range o f the si lver amalgam
screen OTE d id no t al low for the complete spec tropotent ios t atic study
o f this redox coupl e .
Due to the requirement tha t equil ibrium be
attained at more negative po tent ials than that for the evolut ion of
hydrogen gas at the Ag amalgam OTE ( corre s pond ing to a po tential where
Yb ( I II ) is compl etely reduced ) , no measurement of the formal reduc t ion
potential via spec t roelec trochemistry was per formed .
The absorpt ion spec tra o f Yb ( I II ) and Yb ( I I ) as a func t ion of
t ime dur ing the po tential- step elec tro lys is of Yb ( I I I ) are shown in
66
0.4
LL.J
u
:z:
�
0.2
e::::
__,___
_
._-
_,,_,--·
0
V)
co
c:(
0 �--------���--�--�--_.--�
_.__�
__
0
1
2
T I ME
3
(mi n )
Figure 16 . Ab s orbance vs time curve for the po tential- s tep
e lec trolys i s of Yb ( I I I ) usinS:a s i lver ama lgam sc reen OTE .
Ag( Hg ) -OTE : 0 . 6 2 mm path length , wave l ength cons tant : 3 5 3 nm ,
poten t i a l -s te p : -0 . 7 6 V t o -1 . 5 6 V .
67
Figure 17 .
The charac ter is t ic Yb ( I I ) ab sorpt ion peak , at about 3 5 3
nm, was ob served to increase with the time o f electrolys i s .
The other
maj or peak in the Yb ( I I ) ab sorption spec trum , at 240 nm , is not
c lear l y de fined , in Figure 17 , due to the insuffic ient light output of
the UV s ource l amp us ed with the rapid- scan spec tropho tome ter .
Figures 16 and 17 are hand-drawn copies of the pho tographs taken of
the CRT output of the rapid- scan spec trophotome ter .
Add i t ional exper iments wi th more conc entrated solut ions o f
Yb( II I ) i n the s t a ted medium were conduc ted us ing the Cary Mode l 14 -H
spectrophotometer .
The in i t ia l colorl e s s solut ion o f Yb ( I II ) was
reduced in a si lver amalgam sc reen OTE , and a lemon-ye l low color was
observed in the ce l l .
The spec trum of th is solut ion ver i fied the
pre senc e of Yb ( I I ) .
Spectroe lec trochemical inve s t igat ions of the Sm( I II ) / Sm( I I ) redox
c oupl e in 1 M KC l at pH 6 were not succe s s ful us ing any of the OTE ' s .
The reduc t ion po tential for the redox coupl e was beyond the cathod ic
l �itat ions of the e lectrodes .
No indication of the elec trochemic al
generation of divalen t samarium was noted v isua l l y , and hydrogen
e volution at the electrode prevented spec tral ana l ys i s .
2.
M ( IV) /M( I I I ) Redox Couples
a.
Cer ium.
The only te travalent lanthanide ion suf f ic ient l y
s tab l e to exi s t in aqueous solution i s cerium( IV) .
The Ce ( IV ) /Ce ( II I )
redox coup l e i s therefore amenabl e to spec troelec trochemica l study in
thi s medium.
The formal reduc t ion potential o f the Ce ( IV ) / Ce ( II I )
redox coupl e varie s cons iderab l y in different mineral ac id so lutions
68
0. 4
LIJ
u
�
�0.2
0
V)
co
ex:
•
•• ••••
•
• •• •
•
• •• •• •••
• ••
•• .
•
•;i •,
• • •• • •• •••• ••
•
•
•
••• • •• •• • • ••
•• • • • •
•
• ••
...
g • •• • •• • • • • • •• •
• ••
• •• • • •••
• •• • • •• • • •• •• •• • • ••• ••• ••f• •• •• • • • • • •• •
• • ••• •••
•
••
•
•
••• •
•
•
•
•
•
••
•
• •
•
• • • • • • •••• ••••• •
•
•
•
.
• • • •.,• . • .
.•
•.
e
•
•
· ·· ·
.
·
·
··
.
.
·
.
····
··
.
.
·
.
• • • ••
..
· � ···
•
••
•. .•
•
d
•
• ••
·· · ·· · .••
•
•
• • • • • • •• . . . .
..
• •• •• ••
· ··
• ••
•
•
•
••
. . .. .... .
•
c.
•••
•
•
•
•
•
•
•
. •• •• •• •
•• •
•
••
••• •
•
•
•
•
b
•
•
• ·
·• •·· •· · · · · ·· ·• • · · •
· · .. . • .. · • · · · ··• • ·
····
•
•
••• • • •• ••
• •.
a
· · · · •·· . . . ·· ·· ·· · .• •
. . .. . . .. · · ··· · ·
.
.
.
• .•
•
.. •
.·
•
.
0
220
260
.
. . . . . . . . .. . .. . . . . . . . . . .. . . . . .
300
WAVELENGTH
340
380
420
(nm)
Fig ure 17 . S o l u t ion abs orpt ion spec tra o f Yb ( I I I ) and Yb ( I I ) as a
func t ion of time dur ing th e po tent ial-s tep elec tro lys i s o f Yb ( I I I ) in 1 M
KC l . 1 sec ( a ) , 15 s ec ( b ) , 3 0 s ec ( c ) , 60 sec ( d ) , 90 s ec ( e ) , 1 2 0 s e c
( f) , and 1 5 0 s ec ( g) · Ag (Hg ) -OTE : 0 . 6 2 mm path length , potent ial-s tep :
-0 . 7 6 to - 1 . 5 6 V.
69
and is dependent upon the degree of complexat ion o f the cer ium ions by
the anions of the various ac ids .
A complexing agent prov iding a more
stab le Ce ( IV) compl ex than that of Ce ( I II ) enab les the oxidat ion
reac t ion to occur at a more favorabl e ( less nega t ive ) potent ial .
Var ious compl exing aqueous so lut ions were inve s t igated us ing the
Ce ( IV) /Ce ( I II ) couple in an effort to find a medium in wh ich Ce ( IV )
could be generated mos t eas i l y , such that other less s tab l e M( IV)
oxidat ion states of the l anthanide and ac tinide elements might also be
generated in this same medium. The solub i lity of trivalen t cer ium wa s
inves t ig a ted in conc en trated aqueous solutions of ces ium fluoride ,
pho sphoric ac id , alka l i metal oxalate s , and alka l i metal carbonate s .
Although the M( IV) s tates o f the ac t inide elements were reported to be
solub le and quite s t ab l e in concentrated fluoride med ia , 6 1 , 6 4 the
trivalent states are qui t e insoluble , as was c l ear l y demonstra ted in
experiment s with Ce ( I I I ) sal t s .
Solut ions o f concentrated phosphor ic
acid dissolved Ce ( I II ) to a large extent , as did the alka l i met a l
carbonates .
The alka li metal oxalates themselves bad lim i ted
s olub i l i ty in water , and it was not pos s ib l e to ob tain a so lut ion of
oxalate ion much grea ter than 1 or 2 M.
Spectroelec trochemical experiments with Ce ( I II ) chl oride
dissolved in about 15 M phosphoric ac id were per formed to determine
the formal reduc t ion potential of the Ce ( IV ) /Ce ( I II ) redox coup l e .
The Ce ( II I ) solution was placed in a RVC-OTE and a gol d screen OTE and
increas ing l y more pos i t ive potentials were app l ied , unt i l a change in
the absorption spec trum indicated the appearance o f Ce ( IV ) .
The
absorpt ion spec tra of Ce ( I I I ) and Ce ( IV) are shown in Figure 18 .
The
70
O R N L- OW G
>-
t::
>
�
a..
a:
0
en
a:l
<(
a:
<(
_J
0
:::!:
225
800
et - 4 4 2 1
WAVELENGTH ( n m )
250
300
CERIUM (III )
600
400
200
0
WAVE NUMBER ( X 10
40
6000
>1>
t=
n.
0:::
35
30
WAVE NUMBER
25
5
0
m- t )
( X 1 05 m- 1 )
20
5000
4000
C ERI U M
( IV)
0
(/) 3000
m
<{
0:::
<{ 2000
_J
0
::E
1 000
0
300
400
500
WAVELENGTH ( n m )
600
1000
Figure 18 . Solut ion absorpt ion spectra o f Ce ( II I ) in 1 M HC l04 and
Ce ( IV ) in 1 M H2 S 04 .
71
Ce ( I I I ) s pec trum wa s adapted from the work o f Carnal t 8 4 and was
obtained in 1 M HCl 04 .
The Ce ( IV) spec trum wa s recorded wi th the Cary
Mode l 14 -H spec tropho tome ter from an aqueous solut ion o f eerie
ammonium sul fate in 1 M H2 S04 .
I t was determined that concentrated phosphoric ac id provided a
potential shi ft of onl y about 1 V from the reduc tion po tential o f the
Ce ( IV) / Ce ( I II ) redox coupl e in perchloric ac id ( 1 . 7 V ) .
Th is is an
insuffic ient shi ft in potential to al low for the generat ion of any
o ther l anth anide M( IV) s tates .
As ide from this re sul t , bo th the RVC
and go ld sc reen OTE ' s were damaged at high anodic potentials in H3 P04 .
Phosphoric ac id studies were discont inued .
Rather surpr is ing resu l t s were ob tained from the fir s t so l ub i l i ty
experiments wi th Ce ( I I I ) ch loride in 5 . 5 M K2 C03 .
The colorless sa l t
was dissolved in the carbonate solut ion b y v igorous shaking o f the
conta iner , fo l l owed by mild he at ing .
The Ce ( I II ) sal t appeared to
change , wh ile s t i l l in the solid form , from a cl ear sand- l ike material
to a more opaque s o l id , fo l lowed by complete disso lution .
This new
sol id material wa s probab l y cerium hydroxid e , or hydrated oxide , wh ich
then dis solved .
The pH o f pure 5 . 5 M K2 C03 wa s measured at a glass
e lec trode to be about 1 2 . 5 .
The mos t intere s t ing resul t of these
d is so lu tion exper iments was that , upon st anding for a few minutes ,
fresh l y prepared , colorless solut ions o f Ce ( I I I ) in the carbonate
s olut ion became tinted ye l l ow at the solut ion surfac e .
The spectrum
confirmed the presence of Ce ( IV) , wh ich mus t have been the re sul t o f
72
oxidation by atmospheric oxygen .
This sugge sted a subs tantial sh i f t
in the value o f the formal reduc tion potential in concentrated
c arbona te solut ion .
The use o f vol tammet ry in concentrated carbonate med ia was
s ever e l y limited by the poor vol tammograms ob tained with bo th the Pt
and RVC electrodes .
It was not pos s ib le to observe reproduc ible
current-vol tage curves at tributed to the cerium coupl e .
This was in
part due to the lim ited anodic potential range of these elec t rode s .
The oxygen evolution wave occurred a t a low anodic po tential (+ 1 . 0 V
for Pt and +0 . 7 for RVC) because of the high hydroxide ion
concentration in the solut ions .
Onl y an es t imate of the value o f
'
E0 (-0 . 1 3 V) was derived from the vol tammogram.
The spec tropo tentiostatic method was appl ied to the cerium redox
s ys tem in 5 . 5 M po tass ium carbonate to de termine the formal reduc t ion
potential o f the coup l e .
used for this exper iment .
An RVC-OTE ( 100 ppi , 1
mm
path leng th ) was
The ab sorption spec tra of Ce ( IV ) and
Ce ( I II ) at various value s of appl ied potential in 5 . 5 M K2 C0 3
shown in Figure 19 .
are
S pec trum A wa s recorded at a potential where
cerium was compl etely in the reduced form .
As more pos i t ive
potent ials were app l i ed ( B through D) absorpt ion due to the oxid ized
form wa s obs erved to inc rease .
At the potential at wh ich spec trum
E was recorded , cerium wa s compl etely in the oxidi zed form.
The
p otential range se l ec ted for th is exper iment was based on the
E o ' value obtat' ned from th e eye 1 1c
' vo 1 tammogram s .
Th e data from the spec tropotentiostatic experiment are col l ected
in Tab le VII .
The absorbance values , recorded at 3 04
nm ,
are val ue s
73
ORNL-DWG 80-81 31
t.O
0.8
11.1
(.)
z
�
a::
0
(/)
m
�
0.6
0. 4
0.2
0
300
400
500
WAVELENGTH (nm)
Figure 19 . Solut ion absorpt ion spec tra o f Ce ( IV ) and Ce ( I II) at
var iou s va lues o f appl ied po tential in 5 . 5 � K2C 0 3 . -0 . 2 6 V (A) , 0 . 0 0 2 V
( B ) , 0 . 0 4 V ( C) , 0 . 0 6 V ( D ) , 0 . 44 V ( E ) . RVC-OTE : 100 p p i , 1 mm pa th
length , [ Ce ( I I I ) J = 1 . 0 x lo - 3 M .
74
TABLE VII
DATA FOR SPECTROPOTENTIOSTATIC DETERMINATION OF THERMODYNAMIC
PARAMETERS FOR THE Ce ( IV) /Ce ( I II ) REDOX COUPLE
Potential
( volts vs SCE )
Absorb ance
A2
Data Set A
- 1 . 00
( 0 . 0 528 )=Al
-0. 28
0 . 0750
0 . 0 222
0. 982 4
0. 0226
- 1 . 6 459
-0. 24
0 . 16 90
0 . 1 162
0 . 888 4
0. 1 3 08
-0 . 8834
-0 . 2 0
0 . 4 500
0 . 3 97 2
0 . 607 4
0. 6 5 3 9
-0 . 18 4 5
-0. 18
0 . 67 18
0 . 6 190
0. 3856
1 . 6053
0. 2 05 6
0 . 20
( l . 0 57 4 ) =A 3
Data Se t B
- 1 . 00
( 0 . 006 l ) =A 1
-0. 24
0 . 17 80
0 . 17 20
0 . 9 22 0
0. 18 66
-0 . 7 2 9 2
-0. 2 0
0 . 4 70 2
0. 4 64 0
0 . 6 30 1
0 . 7365
-0 . 13 28
-0. 16
0 . 84 50
0. 8 3 90
0 . 2 550
3 . 2 900
0. 5172
-0 . 1 2
1 . 04 50
1 . 0 3 90
0. 0610
0 . 20
( 1 . 1000 )=A3
17 . 0 32
1 . 23 1 3
75
from two separate experiments .
The Nernstian plot derived from these
data in Tab le VII i s shown in Figure 2 0 .
The s lope yie lded an n-va lue
o f 0 . 95 , corres pond ing to a one-e lec tron exchange .
It wa s found 8 7 that the formal reduc tion potent i a l of the
Ce ( IV) / Ce ( I II ) redox coup l e was 0 . 05 1 � 0. 005 V in 5 . 5 M po tass ium
carbonate sol ution .
This is a signi ficant sh i ft from the value of 1 . 7
V reported in noncompl exing , perchl oric ac id solut ion . 69
Assuming a
s imilar po ten t i a l difference for other M( IV) /M( I I I ) redox coup l es of
the lanthanide e l ements , on ly praseodymium and terb ium would have
EO ' values in the range to provide stab i l ity for the M( IV) s pec ies in
concentrated carbonate solut ion ( see Figure 2 , page 2 5 ) .
b.
Praseodymium.
Trival ent praseodymium , as the ch l oride or
hydroxid e compound , was disso lved in 5 . 5 M po tass ium carbonate .
The
spectrum of the re sul t ing l ight green solut ion is shown in Figure 2 1 .
This spectrum is typical o f the trivalent lan thanides , cons i s t ing of
sharp f- f tran s i t ions o f low mol ar absorptivity.
The Pr ( I I I )
carbonate so lut ion wa s placed in an electro l ys i s ce l l .
Increas ing ly
more po s i t ive po tentials were appl ied to a Pt- screen anode un t i l a
change wa s noted visua l l y , or spec tra l l y , as a resu l t of trans ferr ing
the so l ution to a quartz cuvet and recording the absorpt ion spec trum .
At fir s t the resul t s of the e l ec trolys i s experiment s were no t
reprod uc ib l e .
c ri t ic a l .
It was then de termined that the so l ution pH was
It wa s fi rst discovered by working with the terb ium sys tem
( see be low) tha t pH 1 4 ( 1 M hydroxide ion ) was necessary for bulk
s tab i l ity of the M( IV) s pecie s .
Thus after proper adj us tment of the
on- ion conc entrat ion and at a po tent ial o f 1 .4 V ( at a plat inum
76
ORNL- DWG 8 0 - 8 1 37
-O.t2
w
u
�
-0.1 6
>
en
-
0
>
E
_J
�
z
w
-0.20
'
0
- O. f 9 V
=
�--- - - - - - - - - - - - -
b
a..
0
� - 0.24
_I
a..
a..
<t
. - 0.28
- 1 .8
M
F ig ure 20.
K2 C03 .
- 1.2
- 0.6
log
0
0.6
( [ce (IV)]/(Ce(IIU] )
1.2
Nerns t ian plot for the Ce ( IV ) / Ce ( I I I ) redox coup le in 5 . 5
77
ORN L-DWG 80-8630
WAVE NUMBER l • tO!l m- 1 1
50
40
30
25
10
15
20
Pr (Ilr)
1
w
u
z
<I
m
!r
0
V)
m
<I
200
300
4 00
500
600
700
800
900
1000
WAVELENGTH l nm )
WAVE NUMBER l x 10 !l m - 1 )
50
�
5
�
40
25
20
tO
15
Pr C m )
0.6
Q.
Q:
�
m
0.4
<I
Q:
<I
....J
�
0.2
WAV ELENGTH l n m )
Fig ure 2 1 .
5 . 5 M K2 C03 .
S o l u t ion abs orpt ion spec tra o f Pr ( I I I ) and P r ( IV ) in
78
s creen anode ) , Pr ( I II ) wa s oxidized to Pr ( IV) .
The re sul t ing ye l l ow
solut ion of Pr ( IV ) exhib i ted the spectrum shown in Figure 2 1 .
The
Pr ( IV) spectrum is characterized by a broad-b and ab sorp t ion ( f-d or
charge- trans fer) with a maximum at 2 8 3
nm.
The mol ar absorpt ivity of
this maximum wa s es timated to be greater than 1000 .
This was
determined by corre lat ing , in various part i a l l y oxid ized so lut ions , a
measured decrease in ab sorbance of a Pr ( I I I ) absorpt ion peak of known
molar ab sorp t ivity wi th the measured increase in ab sorb ance of the 2 8 3
nm peak of Pr ( IV) .
Spectroelec trochemic a l experimen ts were pe rformed us ing the
rapid- scan spectropho tome ter to fur ther inve s t igate the Pr ( IV) /Pr ( I I I )
redox coupl e in carbonate solut ion us ing the RVC and P t sc reen OTE ' s .
The re sul t s of po tenti al-step electrolysis and simu l t aneous recording
o f compl ete spec tra , as a func tion of time , ver i fied the previous
resul t s .
It wa s also de termined that carbonate solutions of Pr ( IV)
were susceptib l e to photolytic reduc tion by the sourc e lamp of the
rapid- scan spectropho tometer .
c.
Terb ium.
Tr iva lent terbium , as the ch loride or hydroxide
compound wa s disso lved in 5 . 5 M K2 C03 .
The co lorless so lut ion wh ich
resulted exh ibited the spec trum shown in Figure 22 .
Thi s so lut ion was
p laced in an electro lysis cell and increasingly more pos i t ive po tentials
were appl ied to a plat inum sc reen anode unt i l a change was no ted .
At a
potentia l of 1 . 3 V a golden-brown color appeared at the sur face of the
Pt sc reen.
This color diss ipated into the bulk of the solut ion .
It
was found that th is co lor would remain throughout the bulk of the
solut ion if the OH - ion concentrat ion were adj usted to about 1 M.
The
79
ORN L-DWG 80-8629
50
30
40
25
WAVE NUMBER I • 10!1 m" 1)
20
15
tO
Tb UlZ: )
t
300
200
1.2
50
30
40
400
500
25
20
600
700
WAVELENGTH l nm)
800
900
1000
WAVE NUMBER I X IcT m-1}
.
15
1.0
Tb ( ID I
>
1-
� 0.8
�
1-
�
a::
�
g
0.6
0.4
0.2
0
200
309
Figur e 22 .
5 . 5 M K2 C03 .
400
500
600
WAVELENGTH lnml
700
800
900
1000
So lut ion absorption spec tra o f Tb ( I I I ) and Tb ( IV) in
80
resu l t ing red-brown Tb ( IV) solut ion exh ibited the spectrum shown in
Figure 22 .
It is charac ter ized by a broad-band tran s i t ion wi th a
maximum at 3 65 nm and with a mol ar absorpt iv ity greater than 1000 .
The mol ar absorptiv i ty was estimated by the same method as that
described for Pr ( IV) .
The colors of solut ions of Pr ( III ) , Pr ( IV ) ,
Tb ( I II ) , and Tb ( IV) in 5 . 5 M K2 C03 are displayed in Figure 23 .
Spec troelectrochemical experiments at the RVC-OTE were no t
reproduc ible for reasons other than pH.
RVC was no t iner t at
potent i a l s higher than about 1 V in concentrated carbonate media .
RVC
was apparen t l y oxidi zed to produce a ye l low col oration o f the solut ion .
Th i s color gave a featureless spec trum start ing at about 4 00
extended as a shoulder into the UV spectral region .
nm
and
Pt elec trodes
were therefore used for al l sub sequent elec trochemis try in concentrated
carbonate media When anodic potentials exceeding 1 V were required .
Both Pr ( IV) and Tb ( IV) were elec trochemic a l l y generated in 5 M
Cs 2 C0 3 and 2 M Na 2C03 a t pH 1 4 .
The spec tra recorded in the se
s olutions were identical to those in potassium carbonate sol ut ion .
Pr ( IV) and Tb ( IV) were also bo th chemical ly generated in po tass ium
carbonate by ozone oxidat ion .
The solutions were adj us ted to the
p roper pH pr ior to bubbl ing of the ozone through the solut ions for
three hour s .
The re sul tant spec tra were the same as those recorded
after elec trochemical oxid ation .
A l iterature val ue re ported for the
ozone redox coup l e in basic media is about 1 . 24 v . 88
This is the
approximate po tential Wh ich was appl ied to the anode to ob tain these
M ( IV ) s ta tes .
Chemical oxidat ion o f tr ivalent praseodymium8 9 and terbium90 in
aqueous so lut ions of po tass ium phosphotung state has been previou s l y
81
.
A
.
.. ... .
.i
-�
B
Figure 2 3 .
S o lutions of P r ( I I I )
Tb (IV) (D) in 5 . 5 � K2 C0 3 .
D
c
(A) , P r ( IV)
(B) , Tb ( I I I )
( C ) , and
82
reported .
Props t 9 1 reported the prepara tion o f a so lid Tb ( IV)
compound by elec tro lytic depo s i t ion .
However , the electrochemic a l
generat ion o f solub le Pr ( IV) and Tb ( IV) s pe c ies in aqueous med ia has
not , to the knowledge of th is author , been prev ious ly reported .
A sol id compound was prepared by the elec trochemic a l oxidat ion o f
a Tb ( II I ) carbonate solution a t an RVC elec trode .
The red-brown ,
p owder- l ike solid exhibited the solid- s tate re flectance spectrum shown
in Fig ure 24 .
For compar ison , the re flec tanc e spec tra o f so lid
Tb ( I I I ) carbona te and Tb 7 0 12 are al so displayed in Figure 24 .
The
arrow in th is figure indicates the maximum ab sorbance o f the
Tb ( IV)- carbonate so lution spectrum.
The data suggest tha t th is
red-brown so lid cons is t s of Tb in an oxida t ion state grea ter than
M( I I I ) .
X-ray di ffract ion pa t terns o f the red-brown so l id were made but
the re sul t s were no t par t icu l ar l y conc lusive due to the nonc rys t a l l ine
nature of the so l id .
The X-ray data do sugge s t , however , that the
solid is prob ab ly a mixed-va l enc e state compound cons i s t ing of bo th
Tb ( I I I ) and Tb ( IV ) .
Thermograv ime tric-mass spec tra ana lysis (TGA-MS ) indicated at
leas t two carbonate ions were as soc ia ted with a terb ium atom . 9 2
Two "water" peaks at d i fferent temperatures in the TGA-MS data
indic ated wa ters of hyd rat ion in an outer sphere conf igurat ion and an
inner sphere moiety , probab ly hydroxide ions .
S imilar data were found
for clos e ly re lated compounds by Sas try � �. 9 3
The so lid compound seems to be a mixed-valence compound of Tb ( II I )
and Tb ( IV) with carbona te and hydroxide ions as par t o f the struc ture .
83
O R N L- OWG 80 - t 9 2 6 6
\
\
\
\
\
\
\
t
lLJ
u
z
�
CD
0:
0
(/)
CD
�
'
'
,
'
'
'
'\
'
'
\
'
\
\
\
\
\
\
\
' c
'
'
\
200
4 00
600
800
1 000
WAV E L E N GTH ( n m )
' ...
'\
\
'
.... ,
'
t 200
Figure 24 . Sol id- s tate re flec tanc e spectra of Tb 2 C C03 ) 3 ( A ) ,
unknown s o l id ( B ) , and Tb 7 0 1 2 ( C) . Arrow ind icates ab sorpt ion
maximum of Tb ( IV) in carbona te so lu tion.
84
The signi ficance of the preparat ion of such a compound by electro­
chemical oxidation is that perhaps compounds of the ac t in ides may be
prepared in a s im i l a r manner .
B.
1.
Ac tinide El ements
Uran ium
The chemis try of uranium in aqueous solut ion is we l l known .
The
relative l y low- l evel radioac t ivity o f natur a l l y occurring uranium-2 38
and the varie ty of oxidat ion states exhibi ted by th is element ( see
Tab le IV , page 17 ) make it an exce l lent sub s t i tute for the heav ier ,
more rad ioac t ive ac t inides in pre l im inary electrochemical and
spectroscopic stud ies .
a.
U (VI ) /U (V ) /U ( IV ) redox couples .
The known oxida t ion state s
o f uranium are we l l charac terized , wi th the except ion o f U(V ) .
The
U(V) s pe c ies uo 2+ is uns tab le in aqueous solut ion and tends to
d isproportionate .
It is also easi l y oxidized to the very stab l e U (VI )
specie s , uo 2 2 + . 66
It wa s expec ted that spec troe lec trochemical
techniques could be used to generate electrochemic a l l y and iden t i fy
s pec tral ly the U (V ) oxidat ion state be fore it could disproport ionate .
Th e redox couples involving U (VI ) , U(V ) , and U ( IV) were s tud ied
2
by the reduc tion o f the ye l low uo 2 + ion in 1 M KC l at var ious pH
value s .
KC l wa s se lected for the extended potential range of
e lec trode s in this so lvent .
Vo l tammograms o f the uo2 2 + solut ion were recorded at an HMDE and
p lat inum microe lectrode , at var ious pH values , in 1 M KC l .
These
85
vol tammograms are shown in Figure 2 5 .
Vo l t ammogram A wa s rec orded at
an HMDE at pH 0 and displays an irreversible reac tion curve .
A peak
was observed for the reduc t ion of U(VI ) to U(V) , 94 but no peak was
noted dur ing the anodic sc an .
This same solut ion was stud ied at a
p la tinum wire electrode (vol tammogram B) to de termine if an oxidat ion
wave occurred at po tent ials more pos i t ive than those attainab le at an
a�DE .
The reduc tion wave wa s less pronounc ed , because o f the hydrogen
wave at pl a tinum at pH 0, but st i l l no oxidat ion wave was no ted .
At
pH 0 the U02 + s pecies is uns t ab l e ; 66 immed iate ly after it forms by the
reduc tion of uo 2 2 + , it disproportionate& be fore it can be oxid ized at
the elec trode surface .
It should be mentioned tha t vo ltammograms A
and B in Figure 2 5 are the th ird scans o f mul tiple scans .
That is ,
due to the 11cata l ytic current wave11 of the U (VI ) reduc tion
(disproport ionat ion and regenerat ion o f the reac tant ) , it was
necessary to take several scans be fore reproduc ibi l ity was attained .
The fi rst sc ans had very large current val ues , unre lated to the
conc entrat ion of uranium.
Vo l tammogram C in Figure 2 5 is of the
uo 2 2+ so lut ion at pH 3 at a platinum elec trode .
An oxidat ion wave was
noted in th is case , indicating a certain stab i l i ty o f the
U02 + s pec ies at pH 3.
This is in agreement wi th li terature data
which indicate that U (V ) is somewhat stab l e at pH va lues be tween 2 and
4 in aqueous media . 94
Solut ion ab sorpt ion spec tra of U(VI ) , U( V ) , U( IV) , and U ( II I ) are
shown in Figure 2 6 .
source s 95 , 96
These spec tra are adapted from literature
and are indi s t inguishab l e from those spec tra recorded in
86
O R N L- DWG 80- t 92 6 4
I200 p.A
t
0
0
0
J:
�
1- u
z
w
0::
0::
:::>
u
B
u
0
0
z
<{
l
0. 5
0
- 0.5
POT ENTIAL ( volts vs SCE )
F igure 2 5 . Cyc l ic vo l tammog rams
HMDE ( A ) , pH 0 at Pt ( B ) , and pH 3 at
[ u < v r > J = 1 . 0 x to- 2 M .
of U ( V I ) in 1 H KC l . pH 0 at
Pt ( C ) . S c an ra te : 200 mV/ s ,
87
U (IV)
f
t.O
400 .
600
BOO
1 000
!200
0.8
>1-
� 6
�
0:
g
m
<l
0:
4
:3 2
0
�
0 �------�--_j,
450
400
340
WAVELENGTH (nm)
Figure 2 6 . So lut ion ab sorption spec tra o f U ( V I , V , IV , and II I ) .
In 1 M HC l04 exc ept U ( I I I ) in DCl04 a t pH ( pD) 3 . 0 9 .
thi s work .
88
The spec t ra of the U ( VI ) , U(V) , and U ( IV) ions were
obtained from so lut ions in which the uranium spec ies was generated by
chemical reac tions rather than by electrochemic a l reac t ions .
The
U(V I ) ( ye l low) so lut ion spec trum was recorded , and then the pH o f the
solut ion adj usted to 0.
Zinc amalgam was placed in the solution , a
green color appeared , and the re sul tant U( IV) spec trum was ob tained .
In order to record the spec trum of colorless U (V ) , it wa s nec es s ary to
adj ust the U (VI ) solut ion to pH 3 and add Eu( I I ) [which was prepared
by zinc amalgam reduc t ion of Eu( I II ) } .
The reduc tant Eu( I I ) so lut ion
was also ad j us ted to pH 3 p rior to mixing with the U(VI ) .
Attempts were made to record the Raman spec trum of U ( V ) as the
U02 + ion in 1 M KC l .
A spec trum of the U ( V I ) species was ob tained ,
but no signal at tributed to U ( V ) wa s observed .
It was determined that
the beam of the argon- ion laser was hea t ing the solution and increas ing
the speed of the disproportionation reaction .
A dark green sol id was
noted in the bot tom of the Raman sample tube .
Thi s was U( IV) oxide
which resul t s from the disproportionation at pH 3 .
Spec troelec trochemical inves t igat ion o f the U(VI ) /U ( V ) / U ( IV)
redox couples was per formed at pH 0 and pH 3 .
The U(VI )-KCl so lut ion
was adj usted to pH 0 and pl aced in the RVC-OTE ( 100 ppi , 1
beam of the Cary Model 14-H spec trophotometer .
mm )
in the
A po tential- s tep was
appl ied wh ich was much greater than the cathodic peak potential in the
U(V I ) sol ution vol tammogram .
This was from 0 . 24 V t o -0 . 80
v.
Complete absorpt ion spec tra were recorded a s a func t ion o f time during
the po tentia l-step electrolysis and are displayed in Figure 2 7 .
Spec trum A i s due to the starting material UOz 2 + .
As the elec trolys i s
proceeded ( s pectra B and C) , the ye l low co lor disa ppeared and a green
25
WAVE N U M B E R I x 1 05
20
89
m- t )
15
t
w
u
z
<l
m
Q:
0
(/)
m
<l
400
700
600
500
WAV E L E N G T H I n m )
800
Figure 2 7 . So lu t ion absorpt ion spec tra o f U ( VI ) and U ( IV ) as a
func� ion o f time dur ing po tential- s tep elec tro l y s i s o f U ( VI ) . In i t i a l
spec trum (A) , 1 0 min (B) , 3 0 min ( C ) , and 50 min ( D ) . RVC-OTE : 100
ppi , l mm path length , po tentia l-s tep : 0 . 24 V to - 0 . 8 V , s pec tra
o f f- s e t for c l ar ity .
90
color wa s observed , wi th the concomitant appearanc e of spec tral peaks
attributed to U ( IV) .
to the produc t U ( IV ) .
S pec trum D is the result of compl e te reduc tion
The absorbance o f the U(VI ) and U ( IV ) peaks
accounted for 100% of the uranium present as calcu l a ted by Beer ' s law
and thus no measurab le U ( V ) wa s formed .
The RVC-OTE wa s f i l led with a so lut ion of U ( V I ) in 1 M KC l/DC l at
pH ( pD ) 3 .
A po tential of -0. 2 6 V wa s appl ied to the ce l l and the
spec trum of U ( V ) shown in Figure 2 8 , wa s recorded before inter ference
c aused by the formation of so lid U ( IV) oxide occurred .
It was
necess ary to us e a re lative l y high concentrat ion ( grea ter than 0 . 1 M)
of U (V I ) in the star t ing so lution due to the short path length o f the
OTE and the l ow mo l ar absorp tivity of uo 2+ •
b.
U ( IV ) /U( I I I ) redox coup�.
Th is coupl e was studied in 1 M
KCl , at var ious pH value s , by reduc tion of U( IV) s tock so lut ions .
U ( I I I ) is un s tab le in aqueous so lution and is eas i ly oxidized by wa ter
and air . 45
Cyc l ic vol tammograms of the U( IV) /U( I I I ) redox couple , recorded
at an HMDE , are shown in Figure 29 .
Vo ltammogram A displays a
revers ible wave from which an E o ' va lue of -0 . 65 + 0 . 0 1 V wa s derived
( pH 0) .
This value is in good agreement with the value repor ted by
Nugen t � a l . 69 of -0 . 6 3 1 v .
At pH 1 (vo l tammogram B) the cathodic
peak po tential is more negative than tha t of A and is a less revers ib le
wave .
At pH 2 (vo l tammogram C) the redox couple is even less
reversible , as indi cated by the radical change in peak potential
value s and the l ower current value s .
The cathodic peak current does
not equa l the anodi c peak current in C , also ind icat ing a le s s than
revers ible elec trode reac tion .
91
0.3
O R N L- D WG 8 0 - 1 9 2 6 3
r-------�--�--�
� 0.2
z
<{
CD
a:
0
en
�
0.1
8
A
0 --------�--�--._--�
1 20 0
1 3 00
1 500
1 6 00
1400
WA V E L E N G T H
( n m)
Figure 2 8 . S o l u t ion absorpt ion spec tra of U ( VI ) and U ( V ) in an
RVC-OTE . U (VI ) in i t ial solution (A) and e lec t rogenera ted U ( V ) ( B ) .
RVC-OTE : 100 ppi , 1 mm pa th length , 1 M KC L/0 20 at pH ( pD) 3 , adj us ted
with DC l , [ U (VI ) ) = 5 . 0 x 10 - 1 M .
92
I 2 0 0 p. A
c
l
(J
0
0
I
1<[
(J
-
lz
w
a::
0::
=>
(J
(J
0
0
z
<[
-
l
- 0. 5
P OT E N T I A L
- LO
( vol t s v s S C E )
Figure 29 .
Cy clic voltammograms of U (IV) in 1 M KC l a t an �IDE .
pH 0 (A) , pH 1 (B) , and pH 2 (C ) .
[U (IV) ]
1 , 0 x lo-1 M , sweep rate :
200 mV/s .
�
93
So lut ion absorpt ion spectra o f U( IV) and U ( I II ) were ob tained by
record ing the spectrum o f the U( IV) s tock so lut ion in a 1-cm quartz
cuvet fo l l owe d by reduc t ion wi th zinc ana lgam in a deoxygenated
solution to obtain U ( I II ) .
The initial green co lor of U( IV ) gave way
to a much darker green or nearly black solut ion , wh ich yielded the
charac ter i s tic U ( I I I ) spectrum.
Spec troelec trochemis try o f the U ( IV) /U ( I I I ) redox couple in 1 M
KC l at pH 0 wa s performed in a si lver amalgam sc reen OTE and a porous
nicke l me t a l amalgam foam OTE .
These elec trodes were us ed bec ause o f
the requirement t o at tain a sub s tantial l y cathodic po tential in mol ar
acid solution.
These elec trodes combine the character i s t ics of
mercury wi th the op t i c a l l y transparent qual i ty o f the sub s trate metal
materia l s .
Absorpt ion spec tra of U( IV) and U ( I I I ) as a func t ion o f
time during po tential- s tep elec trolysis in the si lver amalgam sc reen
OTE are shown in Figure 30 .
Spec trum A is tha t of U( IV) on ly.
As the
e lec trol ysis proceeded , absorption peaks due to U ( I I I ) were ob served
to appear ( s pec trum B ) .
Spec trum C is exc lus ive l y tha t of U( I I I )
indicating complete reduc t ion in the OTE .
no po tential appl ied to - 1 V � SCE .
the E o
'
The po tential-step was from
It was no t pos s ib le to de termine
va lue of th is couple by spec tropotent ios tatic methods due to
the inter ference from hydrogen gas evolut ion at the potential
corresponding to compl e te reduc tion at equil ibrium cond i t ions .
2.
Neptunium
Neptunium- 2 3 7 has suf f icient radioac t ivity assoc iated with
q uant it ies greater than a few mil ligrams to require that al l
manipul a t ions be performed in containment gloved box fac i l i t ie s .
The
94
t
400
500
600
700
WAVE LENGTH
800
900
1 000
( nm)
F igure 3 0 . So lu t ion absorp t ion spec tra o f U ( IV) and U ( I I I ) a s a
func t ion of time dur ing the po ten tial- s tep electrolysis o f U( IV) .
0 min (A) , 10 min ( B) , 2 0 min ( C ) .
S i lver ama lgam sc reen OTE : 0 . 6 2 mm
path length , po tential-s tep : 0 . 24 V t o -0 . 7 6 v [ U ( IV ) ] = 1 . 0 x to- 2 M
in 1 M KC l at pH 0.
95
variety of oxidat ion states ac cessible within a narrow po tential range
in aqueous sol u tion and the charac ter i s t i c ab sorpt ion spec t rum
associated wi th each oxidation state make ne ptunium an exce l lent
cand idate for spec troe lectrochemical stud ies .
In addit ion , the
heptavalent oxidat ion state is access ible in compl exing aqueous
s ol ut ions .
a.
Np(VI ) /Np( V ) redox couple .
The stab le Np ( V ) s pec ie s , Npo 2 + '
can be oxid ized to reasonabl y stab l e Np( VI ) , Np02 2 + , in aqueous
solut ion .
This redox coupl e was inves t igated in 1 M HC l04 , a
noncompl exing medium.
Vo l tammograms of the Np( V I ) /Np( V ) couple in 1 M HC l 04 were
recorded at platinum wire and RVC mic roelec trode s .
vol tammogram is shown in Figure 3 1 .
A representative
The Eo ' value ob tained from thi s
vol tammogram was 1 . 1 1 � 0. 0 1 V , in good agreement wi th the literature
value of 1 . 13 7 v. 6 9
Solution absorption spec tra o f emerald green Np( V ) and red-purple
Np( VI ) in 1 M HCl 04 are shown in Figure 32 .
These spec tra were
adapted from literature spec tra ; 9 7 , 98 s pec tra ob tained in thi s work
were identica l .
The spec tra of Np( V I ) and Np( V ) were ob tained from
the corresponding stock so lut ions in 0 . 5-cm quar tz cuvet s containing 1
ml of solution .
Spectropo tent ios tatic de terminat ions of Eo ' and n for the
Np(VI ) /Np(V) redox couple in 1 M HC l04 were per formed us ing an RVC-OTE .
Current vs time curves were recorded simul taneous l y wi th absorbance �
t ime curves during the potent ial- step elec trolys i s of Np(VI ) .
After
attainment of equi l ibrium , compl ete spec tra were recorded at various
96
......
c
0
I 10�A
�
t- u
:z
L&.J
�
�
:::>
(..)
u
......
c
0
:z
<C
t
1 .4
1 .2
1 .0
o.s
0.6
POTENTI AL ( vo l ts vs SCE }
0.4
F igure 3 1 . Cyc l ic vol tammog ram o f Np( V ) in 1 M HC l04 .
e lec trode , swee p ra te : 100 mV/ s , [ Np ( V ) ] = 3 . 3 x 10=2 M.
Pt
97
>.....
400 !>
i=
a.
a:::
0
(Jl
CD 200 1<{
a:::
<{
...J
0
:E
0
I
I
-
Np (V)
-
1--.1\.
..,..
600
1000
WAVELENGTH (nm)
>1-
>
i=
a.
a:::
0
(Jl
m
<{
a:::
<{
..J
0
:E
1400
40
Np (VI)
20
0
600
moo
WAVELENGTH ( nm)
1400
t 3 00
Np (VII)
600
0
600
800
400
WAVELENGTH ( n m )
Figure 3 2 . S o l u t ion absorpt ion spec tra o f Np ( V ) and Np ( V I ) in 1 M
HCl04 and Np(V I I ) in 1 M LiOH.
98
values of appl ied po ten t ia l .
These spec tra are shown in Figure 33 .
Spectrum A is that of the ini t ia l Np( VI) solution with its main
absorbance peak at about 12 10 nm .
As reduc tion proceeded , Np( V )
absorpt ion peaks (main absorbanc e peak at 980 nm ) appeared as shown in
spectra B through E.
Fo llowing compl ete reduc t ion the produc t was
exc lusive l y Np( V ) , as determined from spec trum F .
The data derived
f rom the peak at 9 80 nm for the spec tropotentios tatic de terminat ions
o f E0
I
and n are co ll ec ted in Table VII I .
The Nernst ian plot for the Np( VI) /Np (V) redox coupl e in 1 M
HCl 04 is shown in Figure 34.
The E 0
I
value was determined to be 1 . 140
� 0. 005 V (n = 0 . 9 3 ) in exc e l lent agreement with the 1 . 13 7 V value
from the li terature . 6 9
This resul t veri fies the us e fulne s s of the
spectropotentios t a t ic method with the RVC-OTE in a gloved box
experiment .
The stock solut ion o f ye l l ow-green Np( VI ) in 2 M Na2 C03 was
reduc ed at a pl a tinum sc reen large sur face elec trode to Np( V ) at -0. 2
V.
The so lut ion absorpt ion spec tra o f the starting solut ion and of
the produc t Np( V) were recorded .
Na 2 C03
is shown in Figure 3 5 .
The spec trum of green Np (V) in 2 M
The compl etely d i f ferent appearance of
the spec trum of Np( V ) in 2 M Na 2C03 as compared to tha t in 1 M
HCl04 ( see Figure 3 2 ) indicates strong complexat ion o f the Np( V ) by
the carbonate ion .
b.
Np (VI I ) /Np(VI ) redox couple .
Neptunium has been repor ted to
be stable in the heptava lent state in carbonate solution . 99
The
Np(VII ) /Np( VI ) redox couple was stud ied in 2 M Na 2C03 by elec t rolys is
o f the Np( VI ) s tock solut ion .
99
t
w
0
z
<
m
a:
0
en
m
<
800
1 20 0
1 000
WAV E L E N G T H
(n m )
Figure 3 3 . Solu t ion ab sorpt ion spec tra o f Np (VI ) and Np( V ) at
var iou s va lues of appl ied po ten t i a l in 1 M HC l04 . 1 . 34 V (A) , 1 . 18 V
( B ) , 1 . 16 V ( C ) , 1 . 14 V (D ) , 1 . 1 2 V (E ) , and 1 . 04 V ( F) , RVC-OTE : 100
ppi , 1 mm pa th length , [ Np ( VI ) ] = 2 . 0 x lo - 2 M .
100
TABLE VIII
DATA FOR SPE CTROPOTENTIOSTATIC DETERMINATION OF THERMODYNAMIC
PARAMETERS FOR THE Np( VI ) /Np( V ) REDOX COUPLE
Potent ia l
( vo l t s .!.! SCE )
Abs orbanc e
A2
1 . 10
( 0 . 0060 )=A 1
A2-A l
A3 -A2
A2 -A 1 /A3 -A2
log (A2 -A 1 /A3 -A2 )
1 . 00
0. 0 280
0. 0 22 0
1 . 5 22 0
0 . 0 14 5
-1 . 8 3 9
0 . 98
0. 0575
0 . 0 5 15
1 . 492 5
0 . 0 345
- 1 . 462
0 . 96
0 . 1 160
0. 1 100
1 . 4340
0 . 0767 1
-1 . 115
0 . 94
0. 242 0
0 . 2360
1 . 3 080
0 . 1804
-0 . 7438
0 . 92
0 . 4 650
0 . 4590
1 . 0850
0 . 423 0
-0 . 3 7 3 7
0 . 90
0 . 770 1
0. 764 1
0. 7 799
0. 9 7 9 7
-0 . 08907
0 . 88
1 . 088 3
1 . 082 3
0 . 4 6 17
2 . 344
0 . 3 700
0 . 86
1. 2 85 2
1 . 2 79 2
0 . 2 648
4. 8 3 1
0 . 6840
0 . 84
1 . 4280
1 . 4220
0 . 1 2 20
1 1 . 66
1 . 0667
0 . 82
1 . 494 1
1 . 488 1
0 . 0559
26 . 6
1 . 42 5
0 . 80
( 1 . 5 500 )=A3
101
ORNL- DWG
8 4 - 5 52 8
0. 9 8
w
u
(/)
en
>
en
�
0
>
0. 9 4
-.J
<l
�
0. 90
w
b
00 . 86
- t .5
F igure 34 .
1 M HC l04 .
- 0 .9
- 0. 3
0.3
log((NpO�]/(Npo�+J )
0.9
I .5
Nerns t ian plot for the Np (VI ) /Np ( V ) red ox coup l e in
102
ORNL- DWG 8 1 - 5529
40
�
> 30
i=
II.
It:
0
20
::l
4
It:
5
�
10
0
�----L-----L---�--._--�--�
300
1 300
500
1 100
700
900
WAVELENGTH (nml
F igure 3 5 .
S o l u t ion absorpt ion spectrum o f Np ( V ) i n 2 M Na 2 C03 .
10 3
Vol t ammograms of Np (VI ) in 2 M Na 2 C03 a t pH 1 3 were recorded at
a pl atinum wire microe lec trode .
shown in Figure 3 6 .
A representative vo l tammogram is
The vo l tammogram indicates a reversib le reac tion
for the Np( VII ) /Np( V I ) redox couple .
Suc c e s s ive potential sweeps were
reproduc ible , and an Eo ' value was determined from the vol tammogram to
be 0 . 46
+
0. 0 1 V .
Bulk so lution elec tro l ys i s at 0. 80 V o f the ye l low-green Np( V I )
specie s in 2 M Na 2C03 a t pH 1 3 resul t s in the formation of a dark
green solut ion of Np( VI I ) .
The solut ion ab sorpt ion spec tra of Np( VI )
and Np( V I I ) in th is medium are shown in Figure 3 7 .
The Np( VI )
spec trum exhib i t s no features other than an UV cut- off at about 400
nm .
The Np( V I I ) spec trum is near ly identical to tha t in 1 M LiOH ( see
Figure 3 2 , page 9 7 ) .
The Raman spec tra of the Np( V I ) and electrogenerated Np( V I I )
species in carbona te so lut ion were recorded in
charac terize these ions .
an
effor t to fur ther
The 5 14 . 5 nm line of the argon ion laser was
used at 100 mW ou tput for Raman exc itat ion .
The Raman spec tra of
Np (VI ) and Np( V I I ) in 2 M Na 2C03 are shown in Figure 3 8 .
The ob served
vibrat ion frequenc ies of the Np(VI ) and Np (VII ) ions in this medium
are in c l ose agreement with those reported by Bas i l e � a 1 . 1 00 , 10 1
which sugge s t s that the struc ture of the Np( VII ) s pec ies is at leas t
very sim i l ar to that wh ich it exhib its in mol ar hydroxide solut ions ,
where it is in the l inear Npo2 3 + form .
The fac t tha t the same number
of oxygen atoms surround the Np(VI) and Np( V I I ) ions in this medium is
a l so ev idenced by the sim i l ar ity of their ab sorption spec tra and the
reversib i l ity of the Np( VI I ) /Np( V I ) redox coup l e .
104
r�
0.6
0.4
0.2
0
POTENTIAL ( vo l ts vs SCE)
-0 . 2
Figure 3 6 . Cyc l ic vol tammog ram o f Np ( V I ) in 2 M Na 2 C0 3 a t pH 1 3 .
P t e lec trode , sweep rate : 5 0 mV/ s , [ Np(VI ) ] = 10 - 3 M.
105
t
LLJ
<....)
�
ca
0::
0
Vl
ca
ct:
300
500
�IAVELENGTH
700
( nm}
9 00
Figure 3 7 . So lut ion absorp tion spectra o f Np( V I ) ( A ) and Np ( V I I )
( B ) i n 2 M Na 2 C0 3 a t pH 1 3 .
106
E
t
8
>trn
z
w
tz
D
A
600
c
800
tl
WAVENU M B E R
1 000
( cm-1 )
1 200
Figure 3 8 . Raman s � ec tra o f Np ( VI ) and Np ( V I I ) i n 2 M Na 2 C0 3 a t
p H 13 . Npo2 3 + ( A ) , Np02 + , ( B) , Cl04 - ( C) , c omp l e xed co3 2 - ( D ) , and
free C0 3 2 - ( E ) .
1 07
3.
Amer i c ium
The oxidat ion states of americ ium are reasonab ly stab le in aqueous
med ia , wi th the excep t ion of Am( IV) , wh ich tends to dispropo rt ionate .
It is pos sib le to stab i lize Am( IV) in st rong ly compl exing aqueous
solut ions .
Previous work , presented ear l ier in th is report ,
concerning the s tab il i za t ion of Pr ( IV) and Tb ( IV ) in concentra ted
carbonate so lut ion sugges ted the pos s ib i l ity of generat ing and
s tabi lizi ng Am( IV) in the same med ium. 8 7
The stud y of Am( IV) i n concentrated fluor ide so lut ions i s al so of
intere s t bec ause of the stab i l i ty prov ided by fluor ide ion compl exat ion6 1
and the po s s ib i l ity of ataining higher oxid ation states in th is medium
by
e lectrochemic a l means .
a.
Am( IV ) /Am( I I I ) redox couple .
The ox idation o f Am( I I I ) was
s tud ied in 2 M ( NH4 ) 2 COJ , 2 M Na 2C03 , 5 . 5 M K2 C03 , and 5 M Cs 2 C03 .
The pH o f these solut ions was ad j us ted to various value s , wi th the
except ion of the ammonium carbonate solut ion , wh ich is se l f- buf fered at
pH 9 . 5 .
Cyc lic vo l t ammograms o f the Am( IV) /Am( I I I ) redox couple were
recorded in 2 M Na 2 C03 and 5 . 5 M K2 C03 at var ious pH value s .
Cyc l ic
vol tammograms re corded in the sod ium carbona te so lution ar e shown in
Figure 3 9 .
Vo l tammogram A was recorded at pH 1 2 . 5 in 2 M Na 2 C03 ; the
oxidation wave is par t i a l ly ob scured by the oxygen gas discharge wave .
There is no evidence for a corresponding reduc t ion wave , so the
e lec trochemic a l reac t ion is irreversib l e under these cond i t ions .
In
an attempt to separate the two oxid ation wave s , the pH o f the
carbonate so lut ion was lowe red by bubb l ing C0 2 gas through it .
This
generated a mixture of carbonate-bicarbonate spec ie s in the so lut ion .
108
A
t
u
-
c
0
:E:
...
... u
<t
z
w
Cll:
Cll:
B
::::» _
u
u
c
0
z
<t
�
JJA
s
I
c
1 .0
0. 8
0. 6
POTEN T I A L ( V
0.4
vs
0.2
SCE )
Fig ure 3 9 . Cyc l i c vo l tammograms o f Am( I I I ) in 2 M Na 2 C03 at
var ious pH value s . pH 12 . 5 (A) , pH 10 . 2 ( B ) , and pH 9 . 7 ( C ) , Pt
e le c t rode , sweep ra te : 50 mV/ s , [ Am( I I I ) ] = 1 .0 x lo - 3 M.
109
The vol tammogram ( B in Figure 3 9 ) recorded at pH 10 . 2 fo l l owing the
C02 t reatment exhibited the Am oxidat ion wave more c l ear l y because of
the shi ft of the oxygen di scharge wave to a more anodic potentia l .
add i t ion , a reduc t ion wave was also vis ible .
In
Fol lowing a second
C02 t reatment , yie lding a so lut ion of pH 9. 7 , the vol tammogram ( C in
Figure 3 9 ) indicated that the Am( IV ) /Am ( I I I ) redox coupl e became more
revers ibl e .
The l imit of th is experimental approach was reached at pH
8 where Am prec ipitated as Am( OH ) J •
A formal reduc t ion po tential for
the Am( IV) /Am( I I I ) redox couple was es timated from the reve r s ib l e
cyc lic vol t ammogram t o be 0 . 9 2 � 0 . 0 1 v .
As suming the same sh i f t in
potential for the Am( IV) /Am( I I I ) couple as was ob served for the
Ce ( IV ) / Ce ( III ) redox couple in th is med ium , a standard reduc t ion
potential ( E 0 ) for the Am coupl e was es t imated to be 2 . 6 2
+
0.01 v.
Cyc lic vol tammograms o f the Am( I I I ) spec ies in 5 . 5 M K2 C03 ( pH
12 . 7 ) were not very use ful because the oxygen gas discharge wave
obscured the Am( I I I ) oxida tion wave .
The K2 C0 3 s o lution was ad j us ted
to pH 14 in an effort to reproduc e the cond it ions under wh ich Pr ( IV )
and Tb ( IV) were generated .
S ince cyc l ic vol tammetry wa s no t us e ful at
this high pH , bul k so lution elec tro l ys i s fo l lowed by spectropho tomet ry
was pe rformed at cons tant potential in a Te flon spectroe lec trochem ical
c el l hol der .
The convent iona l three elec trode sys tem wa s us ed without
separate compartments for the counter and re ference electrode s .
This
was necessary because of the limited quantity of Am wh ich was
avai lab l e for the fi rst experiment s .
the working anode .
A plat inum sc reen was us ed as
RVC was us ed for some exper iments where the
poten t ial at wh ich RVC is des t royed was no t approached .
[ RVC can be
1 10
oxidized at po tent ials in exc ess of 1 V in s trong carbonat e sol u tions
resul t ing in a ye l low discolorat ion of the solut ion . ]
Solut ion absorpt ion spectra were recorded prior to and after bul k
e lectro l ysis .
The so lution ab sorpt ion spec tra o f Am( I II , IV , V , and
V I ) are shown in Figure 4 0 .
These spec tra were adapted from the work
of Schul z l0 2 and were taken in 1 M HCl 04 , wi th the exception o f Am( IV )
which wa s in 10 M H3 P04 .
The solution ab sorpt ion spec trum of Am( I I I )
in 5 . 5 M K2 C03 , prior to electrolys i s , i s shown in Figure 4 1 .
A
potential was appl ied to the ce l l and was increased to more pos i t ive
value s unt i l a change was no ted visua l l y or in the rec orded spec trum.
At 0. 94 V the init i a l l y pink- tan so lution of Am( I II ) became
golden-brown and exhibi ted the spec trum shown in Figure 4 1 .
Since
oxygen gas wa s generated simul taneous l y at the platinum working anode ,
the oxidation o f Am( I I I ) wa s slow ; only par t ia l oxidation to Am( IV ) is
indic ated by the spec trum in Figure 4 1 .
A value for the formal reduc t ion po tential o f the Am( IV) /Am( I I I )
redox couple wa s approximated from the poten t ia l ( 0 . 9 4 V ) at which
Am( IV) was first observed .
Assuming the shi ft in po ten t i a l provided
by carbonate ion compl exat ion to be 1 . 7 V, a value for E 0 of the
Am( IV) /Am( I I I ) couple of 2 . 6 5 + 0 . 0 5 V was calcul ated . l0 3
Thi s va lue
agrees we l l wi th that derived from the resul ts of cyc l ic vol tammetry
d iscussed above and wi th the 2 . 6 2 + 0 . 05 V val ue reported by Mor s s and
Fuger . l0 4
Bulk so lut ion electro l ys i s o f Am( I II ) in 2 M Na 2C03 was per formed
using larger volumes of so lution and separate (Vyc or fr i t s ) ce l l
compartments for the re ference and counter elec trode s .
A po tential
111
ORNL- DWG 60- t734�
400
I
>15 300
i=
I
I
-
Am ( I I I )
Q.
0:::
0
Ul
CD
<(
0:::
<(
..J
0
�
200
-
100
-
0
200
400
�
CD
<(
0::
<(
..J
0
�
20
0
600
BOO
700
WAVEL ENGTH
142
>1-
�
0::
<( 20
0::
<(
..J
0
�
700
WAVELENGTH
900
(nm)
1 100
moo
100
5 H7 1-
75
92
50
i=
Q.
0::
0
1/)
CD
<(
0::
<(
..J
0
:::;:
0
U)
m
500
900
(nm)
>1-
Am( V )
40
0
300
tOOO
10
60
5
(nm)
Am ( IV )
Q.
�
A
BOO
600
WAVELENGTH
>1> 30
0:::
I
-
67 1-
42
300
25
0
500
700
WAVELENGTH
(nm )
900
F igure 4 0 . S o l u t ion absorpt ion spec tra of Am ( I II , IV , V , and V I ) .
Am( IV) in 10 M HJP04 , others in 1 M HC l04 .
uoo
112
ORNL- DWG
30
40
25
20
l
5
WAVE NUM B E R ( x t 0 m- )
8 1 - 4 564
10
15
160
>-
1 40
s
120
�
100
243Am ! m l
1-
0
(/)
m
<l
a::
<l
....1
0
�
80
--
60
-
40
20
300
400
500
700
600
800
900
1000
1100
WAVELENGTH ( nm l
40
30
25
20
WAVE NUM B E R ( K t05 m- 1 )
10
15
243Am !IDl
f
+
UJ
u
z
<l
m
a:
0
(/)
m
<l
243
Am ( N)
300
400
500
600
700
800
900
1000
1100
WAV E L E NGTH ( n m )
Figure 4 1 . Solut ion absorpt ion spectra o f Am( I I I ) and Am( I I I )
p lus Am( IV ) i n 5 . 5 M K2 C03 .
113
o f 1 V was appl ied to the cel l , correspond ing to a po tential jus t
beyond the anodic peak po tential observed in the cyc l i c vol tammogram .
St irring was provided by bubb l ing N 2 gas through the solut ion .
Wi thin
a few minutes , the pink-tan so lut ion turned golden ye l l ow and then
dark orange-brown .
The spec tra of the initial Am( III ) solution and the
Am( IV) oxidat ion produc t in 2 M Na 2C03 are shown in Figure 42 .
The
absorpt ion peak of Am( IV) at ab out 3 60 nm wa s de termined to have a
molar absorp t ivity near 2 2 00 .
This value was based on a determinat ion
o f the Am concen trat ion by alpha counting technique s .
Am( III ) was also oxidi zed at 1 . 0 V a t a P t sc reen anode in 2 M
CNH4 ) 2 co3 and in 5 M Cs 2C03 wi th the same resul t s as described above .
A di fference was noted , however , in the solub i l ity o f the Am chl oride
and hyd roxide so l i d s in the various carbonate so lutions .
The
solub i l ity wa s good and dissolut ion was rapid in the 5 . 5 M K2 C03 and
in the 5 M Cs 2 C03 .
CNH4 ) 2 C03 .
The so l ub i l ity was reduced in 2 M Na 2 C03 and 2 M
It is evident that the so lub i l ity of the Am compounds is a
func tion of the pH o f the carbona te solution .
The Am( IV) /Am( I II ) redox couple was also studied in conc entrated
f luor ide so lut ion .
So l id AmF4 and Cs 3AmF 7 ( 2 to 3 mg) , prepared under
anhydrous condi tions , were dissolved in concentrated HF saturated wi th
Cs F ( about 1 2 M HF and 15 M CsF) .
temperatur e .
The so l id s disso lved read i l y at room
The resul tan t so lution was placed in a plastic cuve t , and
the absorption spec trum was recorded .
The spect rum , shown in Figure
4 3 , exhibited ab sorbances due on ly to Am( IV) and was nearly id entical
to that of Am( IV) in the so l id state (as AmF4 ) lOS and in 1 3 M NH4 F
solution . l06
Wi th time , the Am( IV) solut ion was reduc ed to Am( III ) by
114
1
w
(.)
z
<{
m
a:
0
en
m
<{
500
90 0
700
WAV E L E N G T H
1 100
( n m)
Figure 4 2 . So lut ion ab sorpt ion spec tra of Am( I I I ) ( A ) and Am( I V )
{ B ) i n 2 M Na z COJ • pH 9. 5 .
115
w
�
i
0
1/)
Ill
<I
300
500
700
900
1 1 00
1 300
WAVELENGTH (nm)
B
w
I
Ill
<I
300
50 0
700
900
1 1 00
WAVELENGTH (nm)
Figure 4 3 . So lut ion ab sorption spec tra o f Am( IV) (A) and Am( IV)
p l us Am( I I I ) (B) in concentrated HF sa tura ted wi th Cs F .
116
radiolysis produc t s .
Am( III ) was , for the mos t par t , insolub le in
HF-CsF solution ; h owever , a su f ficient quant i ty was so lub le to exhibit
the charac teris tic Am( I I I ) abs orp tion peak at 500 nm .
The spectrum
o f th is mix ture of Am( IV) and reduc tion produc t Am( I I I ) in HF-Cs F is
shown in Figure 43 .
Vo ltammetric stud y of the Am( IV) spec ie s in HF-Cs F so lution was
n ot use fu l bec ause of the large background current s at the RVC and Pt
working electrode s .
There fore , bul k solut ion electro l ys i s was
per formed in an effort to oxid ize the Am( IV) spec ies to higher
oxidat ion states .
E l ectro l ysis exper iments were conduc ted from 3 0 min
to 1 hr from 0 . 95 to 2 . 34 V at RVC and Pt elec trode s .
The ab sorption
spec tra recorded after e lectrolysis show a signi ficant decrease in the
Am( IV) p re sent and an increase in the ab sorpt ion peak of Am( I I I ) at
500 nm; in add i t ion , more so l id AmF3 wa s formed than tha t present at
the start of electrolys i s which indicated a reduc t ion rather than an
oxid a t ion.
At the potential appl ied Am( IV) was reduc ed to Am( III ) .
There fore , the E 0 1 for the Am( IV ) /Am( I I I ) redox coup l e in thi s medium
1s probabl y more po s i t ive than the anodic poten t i a l lim i t of RVC and
Pt in HF-CsF so lution , i . e . , more pos i t ive than 2 . 34 V � NHE .
Ch emical oxida t ion o f Am( IV) i n HF-Cs F so lut ion wa s per formed
with ozone and peroxyd i sul fate- si lver ( I ) oxide .
The Am( IV) was
oxid ized quan t it a t ive l y , but the initial so l id AmF 3 present in the
solut ion wa s una ffec ted .
The oxidation of Am( IV) to Am ( V I ) has been
reported in 1 3 M NH 4F , lOS in which the spec trum of Am(VI) wa s
obtained .
No spec trum attributable to Am(VI ) wa s observed in the
present work ; however , add i t ional so l id may have been formed ,
ref l e c t ing a reduc ed solub i l ity of Am(VI ) in HF-Cs F so lut ion as
compared to that in 1 3 M NH4F .
1 17
b.
Am( V ) /Am( IV ) /Am ( I I I ) redox couple s .
As previousl y stated ,
Am( I I I ) is oxid ized to so lub le Am( IV) at a po tential of 0 . 94 V in 5 . 5
M K2 C03 .
Increas ing the po tential app l i ed to 1 . 10 V resu l t ed in the
format ion of a dark tan prec ipitate .
A por tion of th is prec ipitate
was col lec ted and disso lved in 1 M HC l04 .
The spec trum of the
resu l t ant so lution was that of Am( V ) , ident ical to the spec t rum shown
in Figure 40 , page 1 1 1 .
A port ion of the dark tan sol i d was
col lec ted , rinsed with alcohol , dried in air , and placed in a
c apil lary tube for X-ray powder di ffraction and microscope
s pec tropho tometric analyses .
The compound was ident i fied as a
potass ium Am02+ doub le-carbonate sa lt [ KJAm0 2 ( C0 3 ) 2 or
K5 Am02 ( C0 3 ) 3 depending on the pH of the carbonate so lut ion and the
t ime of standing of the prec ipitate in contac t with the so lution] .
Sim i l ar re su l t s were ob tained in 5 M Cs 2 C03 a t potential s exceeding
1 . 10 V except th at the so l id wa s a ces ium Amo2 + doub le-c arbona te sa l t .
No so lid format ion was de tec ted when the oxid a t ion was per formed in 2
M Na 2 C03 solution .
At po tent ials higher than 1 . 10 V the Am( V ) was
generated as a solub le spec ies in 2 M Na 2 C03 .
No charac ter i s t i c Am(V)
s pec trum wa s observed in the Na 2 C03 solut ion .
However , upon add i t ion
o f KOH to a mixture of Am( IV) and Am(V) in carbonate so lution , the
orange-brown color , due to Am( IV) , disappeared , and a c o lorless
solution re sul ted .
The spec trum of th is co lor less solution is shown
in Fig ure 44 , spec trum A .
The solut ion was ac idified with HN03 and
spec trum B in Figure 44 was recorded .
those o f Am(V ) .
Both spec tra are probab l y
The sh i ft s in the ab sorbance peaks of Am(V ) in the
carbonate so l ut ion compared to those in the ac id med ium can be
118
t
w
0
z
�
m
([
0
(/)
m
�
A
8
30 0
500
700
WAVE LENGTH
(nm)
9 00
Figure 44. S o l u t ion absorption spec tra o f Am ( V ) in 2 M
Na 2 COJ-KOH (A) and in 1 M HN03 ( B) .
119
attributed to st rong compl exation by carbonate and/or hydroxide ions .
Although the so lut ion yielding spec trum B was more di lute in Am( V ) ,
the peaks are more in tense due to the higher mol ar ab sorptivity of
Am( V ) in ac id so lut ion .
It is no t c lear if Am ( V ) was produc ed by direc t ox ida tion of
Am( I I I ) or by the oxidation of s tab le Am( IV) in carbonate media .
The
potential at Which Am(V) was generated was we l l beyond the us e ful range
o f the RVC or Pt elec t rode .
There fore , no vol tammet r ic measurements
coul d be made to aid in the eluc idat ion o f the redox reac t ion
mechanism.
It is obvious , however , tha t Am(V ) produc t ion in carbonate
media was no t th e re sul t of the disproportionation of Am( IV) , since no
Am( V ) was produc ed at po ten t ials le ss than 1 . 0 V .
c.
Am(VI ) /Am(V) /Am( IV ) /Am( I I I ) redox couples .
In an effort to
fur ther character ize the oxidation produc ts of americ ium in carbonate
solut ions , and in par t icular to sub s t ant iate the ident i ty o f the
s tab l e Am( IV) s pec ie s , chemical oxidat ions o f the Am( I I I ) solut ions
were per formed .
Am( I I I ) solutions in 2 M Na 2 C03 were oxid ized to
Am( VI ) by sod ium pe roxydisul fate- si lver ( I ) oxide , as de scr ibed
e lsewhere . l0 5 The re sul t ing red-brown solution c losel y re semb led (by
eye ) the colorat ion of the Am( IV) solut ion wh ich was electrochemical l y
generated .
The spec trum o f the Am(V I ) solution was recorded and
compared direc t l y to that of Am( IV ) in the same med ium.
The spectra
exhibi ted on ly ins igni f ic ant differences in the pos i t ion and s lope of
the UV cut-off shoulders .
1 20
A s tock so lution o f Am( I I I ) in 2 M Na 2 C03 was div ided in ha l f .
One ha l f was oxid ized e lec trochemic al ly at 0 . 98 V for 2 t o 3 hours to
g enerate Am( IV) . The other ha l f was oxid ized by peroxydisul fate­
s ilver ( I ) oxide to Am( V I ) and the two solut ions , equ a l in Am
conc entrat ion , were compared .
The Am(VI ) solut ion was darker than the
Am( IV) so lut ion , and there was a sub t l e difference in the shades o f
the red-orange t o red-brown so lutions .
The spec tra o f the se two
solut ions were qu ite s imilar except for the low-intens ity ab sorpt ion
peak of Am( VI ) at about 1000 nm .
S ince spec tral ev idence was no t par ticul ar l y conc l us ive in
d is t inguishing be tween Am( IV) and Am( VI ) in carbonate solut ions , other
chemic al methods were ut i l ized .
The two prev ious l y mentioned
solutions wi th equal conc entrations of Am( IV) and Am(V I ) were both
acid i fied with HN0 3 , and the ir ab sorption spec tra recorded .
The
solut ion containing Am(VI ) in Na 2C03 yie lded onl y the spec trum of
Am( VI ) in ac id medium ( see Figure 40 , page 11 1 ) .
Upon ac id i ficat ion
o f the Am( IV) spec ies in Na 2 C0 3 medium , no spec trum of ac id ic Am( IV )
was ev ident , but equ a l quan t i t ies o f Am( I I I ) and Am( V ) were ob served .
These spec ies re sul t from the disproportionation o f Am( IV) .
Add i t ional charac terization o f Am( IV) in carbonate solution
inc lud ed spec trophotometric and potent iometric t i t rat ions wi th
K4Fe ( CN) 6 in 2 M Na 2C03 .
The [ Fe ( CN) 6 ] 3 -/ [ Fe ( CN ) 6 ] 4- redox coupl e was
determined l07 to be rever sible in 2 M Na 2 C03 and have an E01 va lue of
0 . 50
+
0 . 01 v .
S tock solut ions o f Am( IV) in 2 M Na 2 C03 a t pH 9 . 7 were
prepared by bulk so lution elec trolysis at a constan t po tential o f
12 1
0 . 95 v.
The elec trolys i s was cont inued un til the current was cons tant .
Aliquo t s of these solutions were removed and diluted for alpha
coun t ing anal ys i s .
A known vo lume o f the Am( IV ) s tock so l u t ion was
placed in a quartz cuvet , and an initial spec trum wa s record ed .
The
spec tr a of Am( IV) and Am( I I I ) as a func tion of titrant added are
d ispl ayed in Figure 4 5 .
Spec trum A is exc lus ive l y that of Am( IV) .
As
t it rant was added ( s pec tra B through E) , additional Am( I I I ) was
produc ed un t i l al l of the Am( IV) was reduc ed to Am( I I I ) ( spec t rum F) .
The data ob ta ined from the spec tropho tome tric titrat ion are lis ted in
Tab le IX.
The absorbance of the 508 nm peak of Am( I II ) wa s us ed and a
d ilution factor was accounted for during the titrat ion .
A p l o t of the
Am( I I I ) absorbanc e vs volume of titrant added is g iven in Figure 4 6 .
It was found from the da ta ob tained in the spectrophotome t r ic
t it rat ion th at 0 . 88 equivalent of Fe ( CN) 6 4- was required to reduc e one
equiva lent of the higher- oxidat ion- state Am spec ies to Am( I II ) .
Although th is de terminat ion yie lded a value less than the one
equiva lent required to reduc e Am( IV) to Am( I II ) , i t does support a
one-elec tron reduc t ion ra ther than the two- or three-e lectron
reduc t ion required for Am ( V ) and Am(VI ) , re spe c t ivel y .
The re su l t s of
the spec trophotome tric t i t rat ion were checked and veri fied by alph a
c ounting the samples again and submitt ing the ferrocyanide so lution to
ORNL Analytical Chemis try Divis ion for quan t i tat ive ana lys is .
Potentiome tric ti trat ions o f the Am( IV )-Na 2 COJ s o lut ion were
a lso per formed to ob tain additional ver i fic at ion o f the id ent ity of
the higher oxid a t ion s t ate of amer icium .
The resul t s of two separate
122
t
w
0
z
ct
al
a:
0
CJ)
al
<
450
490
WAVELENGTH
(nm)
F igure 4 5. S o lu t ion abs o rp t ion spec tra o f Am( IV) and Am( III) as
a func t ion of ti trant added dur ing a spec t ropho tome t r ic titration of
Am( IV ) wi th ferrocya n i de solu t ion . 0 lJL ( A) , 50 ll L (B) , 100 ll L ( C) ,
1 50 JJ L ( D ) , 17 5 pL ( E ) , and 2 00 J.l L ( F) .
123
TABLE IX
DATA FROM THE SPECTROPHOTOMETRIC TITRATION OF Am( IV) WITH
FERROCYANIDE SOLUTIONa
Volume of ti tran t
( uL )
Am( I I I )
absorbance
Dilut ion
fac tor
Am( I I I ) absorbance
( correc ted)
0
0 . 00 7 5
1 . 000
0 . 0075
50
0 . 02 80
1 . 100
0 . 03 08
100
0 . 0825
1 . 2 00
0 . 0990
150
0 . 1 2 50
1 . 3 00
0 . 16 2 5
175
0 . 1 5 75
1 . 3 50
0 . 2 130
2 00
0 . 1600
1 . 400
0 . 2240
22 5
0 . 1600
1 . 4 50
0 . 232 0
2 50
0 . 15 4 0
1 . 500
o . 23 10
2 75
0 . 1500
1 . 550
0 . 2325
3 25
0 . 14 70
1 . 650
0 . 242 6
aconcentrat ion o f ti trant : 5 . 34 x lo -3 M. Vo l ume o f titrant
required : 20 3 � L. Ini t ial volume of Am : 500 �L. Conc entrat ion of Am :
2 . 46 x 10 -3 M ( from al pha counting ) . Equival ents o f titrant per
equiva lent of Am : 0 . 88 .
12 4
. 24
0
. 20
w
(.)
z
<(
m
a:
0
.1 6
� .1 2
<(
. 08
.04
o ����-----L--�--_.--�
0
100
200
VO L U M E OF TI T RAN T
(ILL)
300
F igure 4 6 . Abs orbance vs vo lume o f ti trant added in the
s pec troph otome t r ic ti trat ion-of Am( IV) with ferrocyanide so lution.
Absorbance cons tant at 5 0 8 nm .
1 25
experiments yie lded 0 . 94 and 0 . 6 2 equivalents o f ferrocyanide to reduc e
o ne equiva lent of the Am to Am( I II ) .
The data for one potent iometric
t itrat ion are pres ented in Tab le X , and the titration curve is shown
in Fig ure 4 7 .
Both ti tra t ion experimen ts sugges t a one-elec t ron
reac t ion , indicating reduc tion of Am( IV) to Am( I II ) .
The reason why
a l l the determina t ions yie lded a lower number of equiva l ents than
expec ted for a one-electron reduc tion is not immediately clear .
No
compl ications were indicated by the re sul ts of the cyc l ic vo l tammetry
experiments wi thin the po tential range of the elec t rode s in th is
solut ion .
The revers ib i l ity of the vol tammetric wave indicates that
only the Am( IV) /Am( I I I ) redox couple is involved , s ince the higher
oxida t ion states of Am involve the dioxo-cation spec ies Amo2 + and
Amo2 2+ .
The exchange of oxygen is required in any red ox couple
involving Am ( I I I ) and Am(V) or Am(VI ) , and these react ions are no t
reversible at an elec trode .
The spec trosc opic and electrochemic a l re sul t s comb ined wi th those
of the chemic al te s t s and the spec trophotome tric and po tent iome tric
t itra tions support the evidence for a rever s ib le Am( IV) /Am( I I I ) redox
coupl e in concentrated carbonate solut ion .
The stable Am( IV) spec ies
may be oxidized elec trochemic al l y to Am(V) or to Am(VI ) .
The
exis tence of stab l e Am( IV) in conc entrated carbonate so lut ion has not ,
to the knowledge of th is author , been re ported previous l y .
4.
Curium
Because of its lower spec i fic ac tivity and its cur rent
avai labil ity in mil ligram quant ities , cur ium- 248 is the Cm isotope
126
TABLE X
DATA FROM THE POTENTIOMETRIC TITRATION OF Am ( I V ) WITH
FERROCYANIDE SOLUTION8
Volume of ti trant
(ll L)
Po tential
(mV � SCE )
Vo lume o f tit rant
(JJ L)
Potent ial
(mV � SCE )
0
69 1
700
311
50
676
750
2 95
100
668
800
2 85
150
658
85 0
2 78
2 00
650
900
273
2 50
64 1
950
268
3 00
633
1000
264
3 50
624
1050
264
4 00
6 15
1 100
26 1
4 50
60 2
1 1 50
258
500
59 2
1 2 00
2 55
5 50
578
1 2 50
25 3
600
548
13 00
25 1
6 50
3 56
8Concentrat ion of ti trant : 5 . 34 x lo 3 M. Vo lume o f ti trant
required : 6 50 � L. Ini t ia l volume of Am : 1 500 �L. Concentration of
Am: 2 . 46 x lo -3 M ( from alpha counting) . Equ ivalents o f ti trant per
equiva lent of Am: 0 . 94 .
-
127
.60
....--
w
0
(J)
.50
g!l
rJ)
-
0
�
_,
<
1-
z
w
1-
.4 0
0
Q.
. 30
•
•
•
•
•
•
•
•
•
•
•
. 20 �--�---L--_.----�
0.8
1 .6
1.2
0 .4
0
V O L U ME O F T I T RA N T
( m L)
Figur e 4 7 . Po ten t i a l v s volume o f titrant added for the
potent iome t ric titration o f� ( IV ) w i th ferrocyanide so lution
•
1 28
bes t su ited for elec trochem ic al and spec troscopic stud ie s .
Curium is
known in two oxidation states , Cm( I I I ) and Cm( IV) , in aqueous solution
as wel l as in the so l id state .
Al though Peretrukhin � �. 108
reported the exis tence of Cm( VI ) , based on re sul ts o f be ta-decay o f a
short - l ived isotope of Am as Am0 2+ , th is work has no t been con f irmed .
Aqueous solutions of Cm( I I I ) are very stab l e .
So lutions o f Cm( IV) ,
uns tab le in noncomplexing aqueous med ia , can be prepared in
concentrated fluor ide solution by dissolution of CmF4 or Cs 3 CmF 7 .
No
one has ye t succ eeded in preparing Cm( IV ) in sol ut ion by direc t
oxidat ion o f Cm( I II ) . 5 3
Based on an es t imated s tandard reduc t ion
potent ial of the Cm( IV) / Cm( I I I ) redox couple , 69 it was expec ted that
Cm( IV) could be pre pared by oxidat ion of Cm( I I I ) in concentrated
aqueous carbonate so lution .
So lut ion ab sorpt ion spec tra of Cm( I I I ) in 1 M HC l04 and Cm( IV ) in
15 M CsF at 10 . 5 °C , adapted from the literature , 5 3 are displayed in
Figure 4 8 .
Mul t imil l igram quantitie s o f sol id ye l low-green CmCl 3 . nHzO
were converted to light green Cm(OH) 3 and disso lved separate l y in 5 . 5
M K2 C03 and 5 M Cs zC03 •
In itial spec tra were recorded prior to
e lec trochemic al oxidation .
The solut ion spec trum of colorless Cm( I I I )
i n 5 . 5 M KzC0 3 is shown in Figure 4 9 .
The similar i ty o f the trivalent
cur ium spec ies in HC l04 to th at in KzC03 is ind icated by the similar ity
in their so lut ion ab sorpt ion spec tra ; however , minor differences
between the two spec tra ind icate a cer tain degree of complexat ion
provided by the carbonate ions .
129
ORNI.- OWG 80- 17346
tOO .-------�--...
Cm (III)
>­
...
>
i=
11.
0:
0
fit
CD
�
0:
�
.J
0
�
50
0
200
400
WAVEL ENGTH (nm)
600
t50
>1-
>
i=
8:
100
5
50
0
1/)
m
�
0:
�
�
300
500
700
900
WAVELENGTH (nm)
Figure 4 8 . So lution ab sorpt ion spec tra o f Cm( I I I ) i n 1 M
HCl04 and Cm( IV) in 15 M Cs F at 10 . 5 °C .
130
ORNL-DWG 80- 9659
4 5 40
>I-
>
1-a..
a:
0
CJ)
Ill
35
30
WAV E N U M B E R ( x 1 0 5
25
20
m- 1 )
15
24Sc m ( I I I )
30
20
<(
a:
<(
..J
0
10
�
0
3 00
Figure 4 9 .
K 2 C03 .
4 00
5 00
WAV EL E N G T H
(nm)
600
So lu t ion ab sorpt ion spec trum o f Cm( I I I ) i n 5 . 5 M
7 00
131
The On( I II )-K 2 COJ solut ion was pl aced in a quartz cuve t in a
Te flon ce l l ho lder . Cyc lic vo ltammetry at Pt and RVC working
e lec trodes indic ated no oxidat ion wave wi thin the potential range of
e ither of the e lec trode s .
per formed by p l a c ing
Bu l k solut ion elec tro l ysi s was then
RVC working , Pt counter , and SCE re ference
e lec trodes direc t l y in contac t with the so lution in the cuve t .
Inc reasing l y more pos i t ive po tentials were appl ied to the ce l l unt i l a
change wa s noted visua l l y or by examinat ion o f the recorded so lut ion
absorpt ion spec tra .
l igh t ye l l ow.
At 1 . 1 V the in i t i a l l y color less so lution turned
The solut ion absorpt ion spec trum showed onl y
charac ter i s t i c Cm( I I I ) peaks with an addit ional broad-b and absorp t ion
s tart ing at about 400 nm and inc reas ing to an UV cut-o ff .
The same
exper iment wa s repeated us ing a pl atinum sc reen working elec t rode .
At
potent ia l va lues exceeding 2 . 0 V , no discolorat ion of the so lution was
noted ; only oxygen gas wa s evolved from the anode .
I t wa s la ter
d etermined tha t the RVC elec trode material was being oxidized in th is
carbonate medium and tha t it was th is oxid ized elec trode material
which gave rise to the ye l l ow co lor of the so lut ion .
Exper iments per formed with Om( III ) in both K2 C0 3 and Cs 2 C03
indicate no evidence for oxidation o f Om( I I I ) to higher oxidat ion
s tates .
An es t imated E 0 of the Cm( IV) /Cm( I II ) redox coupl e has been
reported as 3. 1 V which is the same as the value s reported for the
M( IV) /M( I I I ) redox couples of both Pr and Tb , i . e . , 3 . 2 and 3 . 1 V ,
respec t ive ly.
The te travalent states o f both Pr and Tb have been
prepared succes s ful l y in concentrated aqueous carbonate so lution , as
reported prev ious l y in th is dissertat ion .
It is not present l y known
132
why the above-mentioned experiments did no t resul t in the ox idation
o f Cm( III ) to Cm( IV) , unless the E0 of the Cm( IV) /Cm( III ) redox couple
is more pos i t ive than the e s t imated 3 . 1 v . 69
CHAPTER IV
S UMMARY AND CONCLUS IONS
A.
Opt i c a l l y Transparent Elec trodes
Of the var ious types of opt ical ly trans parent el ec trodes ut i l ized
in th is re search proj ec t , the RVC and PMF OTE 1 s were found to be
superior to the sc reen type OTE 1 s for spec troe l ec trochemic a l
inve s tigat ion o f the l anthanides and ac tinides .
[ This document
c ons t i tutes the fir s t report of the app l ication of porous metal foam
for use as an opt i c a l l y trans parent electrode . 76 J
path length () 1
mm )
The re l a t ive l y long
and large elec trode surface area combined wi th a
rel a t ive ly sma l l elec troac tive sampl e volume ( < 1 mL) provid ed by the
RVC and PMF OTE ' s made these elec trodes part icularly we l l su i ted to
the s tud y of the transplutonium elements , many of wh i ch are avai lab le
only in limi ted quan t i t ie s and exhib it re la tive l y low mol ar
abs orp t ivitie s .
The RVC-OTE was found to exhibit an ex tensive potential range in
aqueous so lut ion of from 1 . 4 to - 1 . 0 V at pH 6 .
Thi s po tential range
could be exc eeded for appl icat ions other than vol tamme try to the
l imits where gross interference from evolut ion of hydrogen gas
( cathod ic ) or oxygen gas ( anod ic ) occurred .
The RVC-OTE was found to
exhibit good optical trans parency ( 5 2 % T for 0 . 5 mm and 2 7% T for 1 . 0
mm
of 100 ppi RVC) throughout the UV-VIS-near IR spec tra l region .
was found to be iner t in mos t aqueous so lven ts stud ied .
133
On l y in
RVC
1 34
concentrated phosphoric ac id and concentra ted carbona te solut ions , at
potent ials exceed ing 1 V , was it noted tha t RVC was des troyed by
oxida t ion , imparting a ye l lowish colorat ion to the solut ion .
RVC
seemed to be una ffec ted by contact with rad ioac tive ac t inide
solut ions .
I f ad sorpt ion of the rad ioac tive ac tinides on the RVC
e lec trode occurred , it did no t affec t the resul ts of vol tamme tric or
spec troe lec trochemic a l experiments with these solut ions .
PMF is a three-dimens ional network of me t a l st rut s very similar
in struc ture and appearanc e to RVC .
The PMF-OTE has many of the
advantages of the RVC-OTE inc lud ing good opt ical transparenc y and
allowing for a reasonab ly long opt ical pa th leng th wh i l e mainta ining a
sma l l sampl e volume .
The PMF-OTE also has a large elec trode sur face
area wi thin a sma l l vo lume of the materia l , and since it is nicke l or
copper , it ha s a much higher elec trical conduc tiv ity than does the RVC
g las sy carbon .
Add it iona l l y , the e ffec tive potential range of PMF is
only l imi ted by the e lec trode materia l which is pl ated on the foam
sub s t rate .
Platinum wa s pl ated on nickel foam sub s trate for anodic
use , and mercury was deposi ted on the sub strate for extended cathodic
appl ications .
The mercury coated PMF d id no t have the same cathodic
range as pure mercury , but an intermed iate range between tha t of pure
nicke l and pure mercury (0 to - 1 . 6 V i f used immed iately after
e lec trodepo s i tion) .
Another cons iderat ion in direc t comparison be tween the RVC and
PMF OIE ' s is the cos t .
For example , an RVC-OTE was made wi th glass
microscope sl ides ; the total cost was j us t under one dol lar .
A
comparab le PMF-OTE was made us ing nickel foam ; the to tal co s t was
135
about ten dol lars .
The expense o f plat ing a nob l e met a l on the PMF
sub s trate should be added to the cost given above .
B.
1.
Lanthanide Element s
M ( III ) /M ( I I ) Redox Couples
The M( I I I ) /M ( I I ) redox couples o f Eu , Yb , and Sm , the onl y
lanthanide elemen ts known t o exist in the divalent state in aqueous
s olution , were invest igated by cyc l ic vol tamme try , solut ion ab sorption
spec trophotome try , and spec troe lec trochemis try .
The E0 1 val ue s
obta ined from cyc l ic vol tammetry [ -0 . 34 � 0 . 0 1 V for Eu ( II I ) /Eu( I I ) ,
-1 . 18 � 0 . 0 1 V for Yb ( II I ) /Yb ( I I ) , and - 1 . 50 � 0 . 0 1 V for Sm( I II ) / Sm( II ) ]
were found to agree wi th li terature values . 69
The spec tropotentio­
s ta t ic method for de terminat ions of E0 1 and n was appl ied to the
Eu( III ) /Eu ( I I ) redox couple us ing an RVC-OTE .
determined [-0 . 3 9 1 � 0 . 005 V for n
=
The E0 1 val ue so
1 . 007 ] was s l ight ly higher than
tha t derived from vol tammetry bu t st i l l in agreement wi th reported
values . 85 , 86
The M( I I I ) /M( I I ) redox couples of Yb and Sm were no t amenab l e to
s pec tropo tentio s t a t ic ana lys is because of the ins t ab i l i ty of the ir
d ivalent ions and the l imited cathodic range of the OTE ' s in aqueous
s olution .
Yb ( I I ) was elec trochemic a l l y generated and spec tral ly
identi fied in a si lver amalgam sc reen OTE , but Sm( I I ) could no t be
prepared us ing any OTE but wa s generated by dissolution of Sm met a l in
d ilute HCl .
2.
M ( IV ) /M( III ) Redox Couples
S ince Ce ( IV) was the only te travalent lanthanide ion found to be
suf f ic ientl y stable to ex ist in aqueous solut ion prior to thi s work , the
1 36
Ce ( IV) /Ce ( III ) redox couple was used as a re ference coup l e to study the
shi ft in reduc tion po tential prov ided by var ious compl exing aqueous
s olut ions .
A complexing medium which could prov ide a subs t an t ia l l y
less po s i t ive sh i ft in the reduc tion po tential of the cerium couple
may a l so be used to generate and stab i l ize other less s tab le M( IV)
sta tes o f the lanthanides and ac tinides .
I t was found that in 15 M H3 P04 the sh i ft in the po tent ial for
the cerium coup l e was insuf f i c ient to al low for generat ion of any
other lanthanide M( IV) spec ie s .
Dif ficul ty with the solub i l ity o f
M ( I I I ) ions of the lanthanides was experienced in concentrated Cs F
so lut ion and the insolub i l ity of alkali metal oxalates prevented
preparat ion of complexing solut ions in concentrat ions greater than 2 M.
Ce ( II I ) was found to be qu ite soluble in 5 . 5 M K2 C03 and was
oxidized to Ce ( IV ) by oxygen in the atmosphere .
Since vol tammetric
methods were no t par ticular l y use ful bec ause of high background
currents in concentrated carbona te so lut ion , spec tropo tent iome t r ic
determinations of E0 1 and n were appl ied to the Ce ( IV) /Ce ( I I I ) redox
couple in order to quant i fy the sh i ft in the reduc tion potential
provided by carbona te ion complexation .
0 . 05 1 � 0 . 005 V ( n
=
The E0 1 val ue was found to be
0 . 95 ) re flec ting a sub stan t i a l shi ft in the
potential from the 1 . 7 V value repor ted in HCl04 . 69
Assum ing a
s imilar shi ft in po tential ac ross the lanthanide serie s , it was
expec ted tha t both Pr ( IV) and Tb ( IV) may al so be generated and
s tab i l ized in thi s same med ium.
Pr ( I I I ) and Tb ( I I I ) sal ts were separate ly d i s s o lved in 5 . 5 M
K2 C03 and elec tro l yzed at Pt and RVC elec trodes .
The appearance o f
new spec tral bands and change s in the colors of the carbonate solut ions
1 37
confirmed the generat ion of Pr ( IV) and Tb ( IV ) .
These re sul t s were
reproduc ed in 2 M Na 2 C03 and 5 M Cs 2C03 ; however , Pr( IV) and Tb ( IV)
coul d no t be stab i l i zed in bulk so lut ion in 2 M (NH4 ) 2 C03 s ince pH 14
was required ; ( NH4 ) 2 C03 is sel f-buf fered at pH 9 .
Pr( IV ) and Tb ( IV)
were al so prepared by oxidat ion by ozone in 5 . 5 M K2 C03 adj us ted to pH
14 .
Assignment of oxidat ion s ta te IV to the oxid ized Pr and Tb
spec ies was based on ( a) corre lat ions o f the known M( I V ) /M( I I I ) redox
potentials in noncomplexing med ia 69 wi th the significantly less
pos i t ive formal reduc tion po tent ial of the Ce ( IV ) /Ce ( III ) redox couple
in concentrated carbonate solution and (b) pred i c t ion o f the
approximate pos i t ion and intensity of the elec tron- tran s fer bands of
these M( IV) s pecies in chloride ion compl exes . l09
The electrochemical generat ion o f solub l e te travalent spec ie s o f
Pr and Tb in aqueous solution has no t , to the knowledge o f thi s
author , been reported prev ious l y .
A sol id compound prepared b y electrochemical oxid at ion of
c arbonate solut ions of Tb ( I I I ) has been analyzed by TGA-MS , X-ray
powder diffrac tion , and X-ray fluorescence and re flec tanc e
spec troscopies and was shown to be a compound consi s t ing of mixed
Tb ( I I I )-Tb ( IV) oxidation state s .
The preparat ion o f th is so l id
compound sugge s t s the pos s ib i l ity of generat ing sim i l ar compound s of
the ac tinides , which may have impor tant app l ications in the recovery
a nd reprocessing of the transuranium elemen t s .
138
C.
1.
Ac tinide E lemen ts
Uranium
U ranium redox coup les were used a s sub st itutes for the heav ier ,
more radioac tive ac tinides in prel iminary experiments bec ause of the
we l l-known reduc tion po tentials for uranium redox couples and the wel l­
charac terized ab sorpt ion spec tra of their ions in solut ion .
The U (VI ) /U (V ) /U ( IV) redox couples were stud ied in 1 M KC l at
different pH values .
Cyc l ic vol tammetry and spec troe lec trochemis t ry
were empl oyed to study the stabil ity of U(V ) in aqueous so lution .
U(V I ) was reduced to U ( V ) in a pH range between 2 and 4 .
Be low pH 2
U(VI ) was reduc ed to uns tab le U ( V ) which disproportionated to U( IV)
and U(VI ) ; the net effec t was reduc tion to U( IV) .
U(V) was generated
elec trochemic a l l y in KCl-0 20 at -0. 26 V whi l e the ab sorpt ion spec trum
was recorded in an RVC-OTE .
The U ( IV) /U ( I I I ) redox coupl e was stud ied in 1 M KCl at
d ifferent pH value s . The couple was found to be rever sib l e in
deoxygena ted solven t at pH 0 wi th an E0 1 value of -0 . 65 � 0 . 0 1 V , in
g ood agreemen t wi th the reported - 0 . 6 3 1 V value . 69
U( III ) was
generated elec trochemic ally us ing si lver amalgam screen and porous
nicke l amalgam foam OTE ' s and ab sorpt ion spec tra were rec orded as a
func tion of time .
The simul taneous elec trochemical generation and spectral
charac terization of U(V) and U ( II I ) in optic al ly transparent
e lec trodes has demons trated the us e fulness of spec troe l ec trochemical
technique s in the study of less stable oxidation states of the
act inide elements .
1 39
2.
Neptunium
E lec trochemical and spec troscopic s tud ie s o f nep tunium- 2 3 7 as
wel l as heav ier ac tinide s were carr ied out in containment gloved box
fac i li t ie s .
The Np (VI ) /Np(V) redox couple was stud ied in 1 M HC l04 and 2 M
Na 2C03 .
Cyc l ic vol tamrne try of the Np (VI )-HC104 solut ion showed the
Np (VI ) /Np(V) couple to be revers ib l e with an E0 1 value of 1 . 1 1
V as compared to the 1 . 1 3 7 V li terature value . 6 9
+
0.01
The so lut ion
absorpt ion spec tra were recorded during the spec tropotentios tatic
determinations of E0 1 and n in an RVC-OTE .
1 . 140
+
0 . 005 V and n
reported values .
=
The values so der ived [ E 0 1
0 . 9 3 ] were in excel lent agreement wi th
A solution of Np( VI ) in 2 M Na 2 C03 was reduc ed to
Np(V ) , and the spec trum of Np( V ) was recorded in this med ium.
The Np(VI I ) /Np ( V I ) redox couple was inves t igated in 2 M Na 2 C03
at pH 1 3 .
Cyc l ic vo l tammograms at a Pt elec trode showed a reversib le
couple wi th an E 0 1 value of 0 . 46 + 0 . 0 1 V as compared to the reported
0 . 58 2 V in 1 M OH- solution . 4 5 , 5 3
Bu lk quan t i t ie s o f Np( VII ) were
prepared by electrolysis of Np(VI) in 2 M Na 2 C03 at pH 1 3 .
Solut ion
absorption and Raman spec tra of the ini tial Np( V I ) solut ion Md ilie
resul tant dark green Np (VII ) solution were recorded .
The Raman
s pec trum of Np ( VI I ) in carbonate med ia was qui te simi l ar to tha t
r epor ted in 1 M LiOH solut ion 100 , 10 1 which sugge s t s tha t the
s truc ture o f Np (VII ) is at least similar in both solut ions .
To the
knowl edge o f this author , this consti tutes the fir s t repor t o f the
Raman spec trum of Np ( VI I ) in carbona te solut ion .
=
140
The above-mentioned exper imen ts us ing neptunium-23 7 demonstrate
the use fulness of spec troe lec trochemis try at opt ic al ly trans parent
e lec trodes in conta inment gloved box fac il ities and also sugges t the
use o f laser Raman spec troscopy-spec troe lec trochemis try to study
rad ioac tive ac tinide elemen ts .
3.
Americ ium
A lthough Am( IV) is reported to be uns tab le wi th re spec t to
d is proport iona tion in mineral ac id so lut ions , it was expec ted that ,
l ike Pr ( IV) and Tb ( IV ) , it could be generated and stab i l ized in
concentrated aqueous carbonate solut ion .
The oxidat ion of 243 Am( I I I ) was s tudied in (NH4 ) 2 C03 , Na 2 C03 ,
K2 C0 3 , and Cs 2 C03 .
Cyc l ic vo l tammetry showed the reve r s ib i l i ty o f the
Am( IV) /Am( I I I ) redox couple to be dependent on pH.
A revers ib l e
v ol tammogram at p H 9 . 7 in 2 M Na 2 C03 yie lded an E 0 1 value of 0 . 9 2
0 . 01 v .
+
This va lue was used to calculate a pred ic ted E 0 va lue in
noncomplexing media by assuming the same sh i ft in the po tential as
that observed for the Ce ( IV ) /Ce ( I I I ) couple .
The value so der ived wa s
2 . 6 2 + 0 . 0 1 V in excel lent agreement with the work of Mors s and
Fuger . l04
Bulk so lution elec trolys i s in Te flon spec troel ec trochemic al ce l l
holders [ s pec i a l l y des igned for this work) us ing RVC and P t sc reen
OTE ' s a l l owed for compl e te elec trolys i s of Am( I I I ) to stab l e Am( IV) .
The so lut ion absorpt ion spec trum of Am( IV ) in carbonate solut ion is
reported here for the first time .
An approximate E 0 val ue calcul ated
f rom resul t s of bulk so lution elec trolysis of Am( I I I ) to Am( IV) was
reported to be 2 . 65 + 0 . 0 1 v . l0 3
Fuger in their recent work . l04 ]
[ This value was c i ted by Mor s s and
141
The ass ignment of oxid at ion st ate IV to the oxid ized amer ic ium
s pec ies was based on ( a ) corre lat ion of the known Am( IV) /Am( I I I ) redox
potential in nonc ompl exing media 69 , 10 2 , 104 with the mea sured sh i f t in
the Ce ( IV ) / Ce ( I I I ) reduc t ion po tential in concentra ted carbonate
solution versus tha t in nonc omplexing med ia , ( b ) predic t ion of the
approximate pos i t ion and inten s i ty of the elec tron- transfer band of
Am( IV) in chl oride ion complexe s , l09 ( c ) direc t spec tropho tomet r ic
compar ison of Am( IV) wi th known Am( VI) carbonate solut ions , ( d )
g enera t ion o f Am ( V ) solutions and compound s a t po tentials exceeding
that for generat ion of Am( IV) , and ( e ) spec tropho tome t r ic and
potent iomet ric ti trat ions wh ich showed a one-elec tron exchange upon
reduc t ion of Am( IV) to Am( I II ) .
The cond i t ions under wh ich Am( IV) was generated in thi s work
[ 0 . 92 V , pH 9- 14 ] correlate we l l wi th a recently prepared po tential vs
pH d iagram for the pred ic ted oxidat ion state spec ies o f Am in aqueous
s olution ( see Figure 50 ) .
Th is diagram , wh ich is based on repor ted
l iterature data , and was prepared by Al l ard , 1 10 pred icts a stable Am( IV )
species in a po tent ial range be tween 0 . 4 and 0 . 75 V and a pH range be tween
9 . 5 and 14 .
Thi s dissertation research cons t i tutes the fir s t repor t of the
generation and stab i l i za t ion of Am( IV) in aqueous carbonate so lut ion
inc lud ing it s solut ion absorpt ion spectrum and revers ib l e cyc l ic
vol tammograms and formal reduc tion po tential of the Am( IV) /Am( I I I )
redox couple . 10 3
14 2
O R N L - DWG 7 9 - 1 6 1 1 0
1.2
1 .0
0. 8
0.6
0.4
>
-
w
0. 2
III
0
-- 0.2
- 0.4
- 0.6
- 0. 8
0
2
4
6
pH
8
10
12
14
oxid a t ion
Fig ur e 5 0 . Po ten t i a l v s pH d iagram for the predi c ted
NHE .
vs
V
s tate spec ies o f Am in aqueo us so lutio n. Po tenti a l in
1 43
4.
Curium
Based on an estimated standard reduc tion po tential for the
Cm( IV) /Cm( III ) redox couple of 3 . 1 v , 69 it was expec ted tha t Om( IV)
could be prepared by oxidation o f Om( III) in concentrated aqueous
carbonate media .
K2 C0 3
The so lution ab sorpt ion spec trum of Om( I I I ) in 5 . 5 M
was recorded prior to e lec trochemic a l oxidat ion in a spec t ro­
e lec trochemic al ce l l at RVC and Pt sc reen e lec trodes .
Inc reasing the
e lec trode po tential to the anodic limit in this sol ut ion did no t cause
the oxidat ion of Om( III ) , since no new spec trum was ob served .
It was
there fore determined that the estimated E0 value for the
Cm( IV) / Cm( I I I ) redox couple is probab ly too low , based on the behav ior
of the Tb ( IV) /Tb ( I I I ) redox couple ; Tb ( IV) can be prepared in
conc entrated aqueous carbonate solut ion , and the Tb couple has the
same E 0 value as that estimated for the Om coupl e .
Add i t ional exper iments with Om( I I I ) in carbonate so l ut ion at
d if ferent pH values and at below room temperature are needed to
c onfirm the above-ment ioned conc lus ions .
Due to the lack o f
add i t iona l quantities of pur ified curium-248 , it was no t po s s ib le to
pur sue th is direc tion in the pre sent work.
D.
Sugges ted Future Re search
The use of compl exing aqueous solut ions as a means to stab i l ize
less s tab le oxidat ion states should be fur ther expl oi ted .
It is
expec ted , for example , that conc entra ted aqueous carbonate so lution
can provide stab i l ity to Cf ( IV) and po ssib l y a l l ow for the oxidat ion
to C f ( V ) .
Concentrated fluoride solut ions , known to stab i l ize Cm( IV) ,
144
may permit oxidation to Cm{V) .
The alka l i meta l oxalates and oxa l ic
acid should be s tud ied as a complexing aqueous solut ion to find a way
to inc rease the oxa late ion concentrat ion .
The elec trochemic a l and spec troscopic s tudy o f less stab le
oxidat ion states in aqueous so lut ion is , o f course , l imited by the
redox and spec tral proper tie s of water .
The use of nonaqueous organic
and mol ten s a l t so lven t sys tems , wi th extended po tential limit s and
spec tral transparenc ies , may provide a sub stantial advantage in the
s tudy of less stable and unusua l oxidat ion states of the lanthanide
and ac tinide elements .
Dime thylsul foxide (DMSO) , acetoni trile , and
d ime thyl formamide ( DMF) are a few nonaqueous organic solven t s wh ich
should be cons idered , since they are common ly used so lvents for
e lec trochemic a l stud ie s .
Some mol ten sa l t sys tems o f promise for the s t ab i l iza t ion o f
h igher oxidat ion states o f the lanthanides and ac t inides are Cs OH-RbOH
eutec tic s , mol ten alka l i me t a l carbonates and low mel t ing ox ide
eutec tic s .
I t is the use of such nonaqueous solvent sys tems in conj unc tion
with compat ib le spec troe lec trochemic al cel l s and elec trode materials
in a gloved box environment tha t should re sul t in the charac ter izat ion
o f some new oxidat ion s t a tes like Cm(V) , Bk ( I I ) , Cf (V ) , and E s ( IV) .
I t is antic ipated that the continued devel opment of improved cel l
des igns and new elec trode materials wi l l enhance the app l ications of
spec troe lec trochemis try to the investigat ion o f the oxidat ion s tate
behavior of the transuranium elements .
LIST OF REFERENCES
' .·
.
LI ST OF REFERENCE S
1.
K. W . Bagna l l , "Lanthanides and Ac tinid es , " Inorganic Chemis try ,
Series I , Vol . 7 , K. w. Bagnal l , Ed . , Univer s i ty Park Pres s ,
Bal t imore , MD , 19 7 2 .
2.
D . T. Sawyer and J . L. Roberts , Jr . , "Experimental Elec trochemis try
for Chem i s t s , " John Wi ley and Sons , New York , NY , 1974 , p. 2 .
3.
T. Kuwana , R. K. Dar l ington , and D . w. Leedy , Ana l . Chem. ,
( 19 64 ) .
4.
T . Kuwana and w. R. Heineman , Ace . � Chern. Res . , 2_, 24 1 ( 19 76 ) , and
reference s there in .
5.
N. Winograd and T . Kuwana , in "Elec troana l ytical Chemistry , " Vo l . 7 ,
A . J . Bard , Ed . , Marce l-Dekker , New York, NY , 19 74 .
6.
w.
7.
H. N. Blount , N. w. Winograd , and T. Kuwana , J . Phys . Chern. ,
( 19 70 ) .
8.
N. lolinograd and T . Kuwana , Ana l . Chem. , 4 3 , 2 5 2 ( 19 7 1 )
9.
c.
10.
w.
11.
w.
1 2.
T. Kuwana , Ber . Bunsenges . Phys . Chern . ,
there in .
13.
T . P . DeAngel is and w. R. Heineman , :!• Chem. E d . ,
14 .
D . E. Alber tson , H. N. Blount , and F. M. Hawkr idge , Ana l . Chern. ,
556 ( 19 7 6 ) .
15.
M . Petek , T. E . Neal , and w. R. Murray , Ana l . Chern. ,
R. He ineman , Ana l . Chem. ,
1Q_,
�'
2 023
3 90A ( 1978 ) .
�'
323 1
E. Baumgar tner , G. T. Marks , D . A. Aikens , and H . H . Richto l , Ana l .
Chem. , g, 2 6 7 ( 19 80 ) .
N. Han sen , R. A. Os teryoung , and T . Kuwana , J . Am. Chem . Soc . ,
1062 ( 19 66 ) .
�,
N. Hansen , T. Kuwana , and R. A. Os teryoung , Ana l . Chem. , 38 , 1910
( 19 66 ) .
146
12,
8 5 8 ( 197 3 ) and re ferences
�,
5 94 ( 1976 ) .
�,
1!,
10 69 ( 197 1 ) .
14 7
2!,
16 .
E. A. Blubaugh , A. M. Yacynych , and w. R. He ineman , Ana l . Chem. ,
5 6 1 ( 19 7 9 ) .
17.
B . S. Pons , J . S . Mat t son , L. o. Winstrom , and H. B. Mark , Jr . , Ana l .
�· , �. 685 ( 19 6 7 ) .
18.
A. Yi lding , P. T. Ki s s inger , and C. N. Re i l ley , Ana l . Chem . , 40 , 1018
( 19 68 ) .
19.
R. Cie s l inski and N. R. Armstrong , Anal . Chem . ,
2 0.
T. P. DeAnge l i s , R. w. Hur s t , A. M. Yacynych , H. B. Mark , J r • , w. R.
He ineman , and J . s . Mat t son , Anal . Chem. , 49 , 1 3 95 ( 19 7 7 ) .
21.
J . s . Ma ttson and c . A. Smi th , Ana l . Chem . ,
22.
w.
23.
R. W. Murray , w. R. He ineman , and G. w. O ' Dom , Ana l . Chem . ,
( 19 6 7 ) .
24 .
D . Lexa , J . M. Savean t , and J . Zickler , l.. Am . Chem. Soc . ,
( 19 7 7 ) .
25 .
w.
26 .
M . L. Meyer , T. P. De Ange l i s , and W. R. He ineman , Anal . Chem. , 49 , 60 2
( 19 7 7 ) .
27.
w.
28 .
V . E. Norve l l and G. Mamantov , Ana l . Chem. , 49 , 14 70 ( 1977 ) .
2 9.
v. s.
3 0.
w.
31 .
I . Pi l j ac , M. Tkalcec , and B. Grabar ic , Ana l . Chem . ,
32 .
R. M. Wightman , J . R. Cockre l l , R. w. Murray , J . N. Burne t t , and S. B.
Jone s , l.. Am. Chem. Soc . , 2!, 2 562 ( 19 7 6 ) .
33 .
w.
34 .
F . M. Hawkr idge and B. Ke , Ana l . Biochem. ,
£,
J . Blaedel and G. A. Mabbot t , Anal . Chem . ,
2!,
5 65 ( 1979 ) .
1 1 2 2 ( 1975 ) .
1Q_,
9 33 ( 1978 ) .
�.
_2!,
1666
2 786
R. He ineman and T. Kuwana , Ana l . Chem . , 43 , 1075 ( 197 1 ) .
R. He ineman , T. P. DeAnge l i s , and J . F. Goel z , Ana l . Chem. , 47 ,
364
( 19 7 5 ) .
1
Srinivasan and C. F. Anson , l.• E lectrochem . Soc . , 1 20 , 1 3 59
( 19 7 3 ) .
R. He ineman , J . N. Burnett , and R. W. Murray , Ana l . Chem. ,
( 19 68 ) .
£,
!!.Q_,
1974
13 69 ( 197 5 ) .
R. He ineman , J . N. Burne t t , and R. w. Murray , Ana l . Chem. , 40 , 1970
( 19 68 ) .
.z!,
76 ( 1977 ) .
148
35 .
D . F. Rohrb ach , E. Deutsch , and w. R. He ineman , in "Charac terization
of So lutes in Nonaqueous Solven t s , " G . Mamantov , Ed . , Pl enum Press ,
New York , NY , 19 7 8 .
36.
W . R . Heineman , B . J . Norr is , and J . F . Goe l z , Anal . Chem. ,
( 19 75 ) .
37.
D . F . Rohrbach , E. Deutsch , w. R. He ineman , and R. F. Pas ternac k ,
Inorg. Chern . , �, 2 650 ( 19 77 ) .
38.
W. R. He ineman and P. T. Ki ss inger , Ana l . Chern . ,
39 .
D . Cohen and W. T. Carnal l , ..:!.• Phys . Chern. , 64 , 19 33 ( 1960 ) .
4 0.
B . F. Myasoedov , V . M. Mikhailov , I. A. Lebedev , 0. E. Ko iro , and
V . Ya . Frenke l , Rad iochem. Radioana l . Let ters , �, 17 ( 19 7 3 ) .
41 .
E . Yanir , M. Givon , and Y . Marcus , Inorg . Nuc l . Chern . Let ters ,
( 19 70 ) .
42 .
R . D . Bayb ar z , J . R. Stoke l y , and J . R. Pe ter son , J . Inorg. Nuc l .
Chem. , 34 , 7 3 9 ( 19 7 2 ) .
43.
J . P. Young , G. Mamantov , and F. L. Wh i t ing , ..:!. • Phys . Chern . , ..?1.. , 78 2
( 19 6 7 ) .
44.
T . Moe l ler , "The Chemis try of the Lanthanides , " Reinho ld Pub l . , New
York , NY , 196 3 .
45 .
F . A. Cot ton and G . Wi l kinson , "Advanc ed Inorganic Chemistry , " 4th
ed . , In tersc ience , New York , NY, 1980 , Chap . 24 , and re ferences
therein .
46 .
N . B. Mikeev , V . I. Spi t syn , A. N. Kamenskaya , I. A. Rumer , B. A.
Gvozdev , N. A. Rozenkev ich , and L . N. Auerman , Dok l . Akad . Nauk SSSR,
2 08 , 1146 ( 19 7 3 ) .
47 .
E . K. Hul e t , R. W. Lougheed , P. A. Baisden , J . H. Landrum , J . F. Wi ld ,
and R . F. Lundqvist , .:!• Inorg . Nuc l . Chem . , !!!_ , 174 3 ( 19 7 9 ) .
48.
K. Samhoun , F. David , R. L. Hahn , G . D. O ' Ke l ley , J . R. Tarrant , and
D. E. Hobar t , J . Inorg. Nuc l . Chem. , !!!_ , 1749 ( 19 7 9 ) .
49 .
F . Dav id , K . Samhoun , E. K. Hulet , P. A. Bai sden , R. W. Lougheed ,
J . H. Landrum , G. D. O ' Ke l ley , and J . F. Wi ld , ..:!.• Inorg. Nuc l .
�· ( in pres s ) .
50.
K. Samhoun and F . David , in 11Transpl utonium E l ements , " W. Mu l ler and
R. Lindner , Eds . , Nor th-Ho l l and , Amsterdam , 1976 , P • 2 97 ; K. Samhoun
and F . Dav id , ..:!. • Inorg . Nuc l . Chem . , !!!_ , 3 57 ( 19 7 9 ) .
1Q_,
!i!._,
79
166 R ( 197 8 ) .
�,
415
1 49
51.
F . David , K. Samhoun , and R. Gui l l aumont , Rev . Chim. Minera1e , 14 , 199
( 19 7 7 ) .
---
--
52 .
T . Moe l ler , "The Chemis try of the Lanthanides , " Reinhold Pub l isher s ,
New York , NY ( 19 6 3 ) .
53.
C . Ke l l er , "The Chemis try of the Transuran ium E l emen t s , " Ver lag
Chemie , GmbH, Ge rmany , 19 7 1 and re ferences therein .
54 .
R. D. Baybarz , J . Inorg. Nuc l . Chern . ,
55.
R. D. Bayb ar z , L. B . As prey , C. E. Strouse , and E . Fukush ima , J . Inorg.
Nuc l . Chem . , 1!_, 342 7 ( 197 2 ) .
56.
L . J . Mu l l ins , A. J . Beamon t , and J . A. Leary , J . Inorg . Nuc l . Chem . ,
3 0 , 14 7 ( 19 68 ) .
57.
v . Scherrer , F . We ige l , and M. Van Ghemen , Inorg . Nuc l . Chem. Letters ,
1_, 589 ( 19 6 7 ) .
58.
L . B. Asprey and B. B . Cunningham , in "Progress in Inorganic
Chemis try , " F . A. Cotton , Ed . , Vo l . 2 , Intersc ienc e , NY , 1960 , P • 2 70 .
59.
R. Hoppe , I srae l .:!.• Chern. , Q, 48 ( 19 7 8 ) .
60.
G . Brauer and H . Kr is ten , !• Anorg . A l l g . Chem . , 456 , 4 1 ( 19 7 9 ) .
61.
L. B. Asprey and R. A. Penneman ,
62.
E . Yanir , M. Givon , and Y . Marcus , Inorg. Nuc l . Chem . Letters , 2_, 369
( 19 69 ) .
63 .
J . R. Stoke l y , Ana l ytical Chern . Div . Annual Prog . Report for period
ending Sept . 30 , 19 7 1 , ORNL-4749 , P • 7 .
64.
T . K . Ke enan ,
65 .
0. L. Ke l ler , Rad ioch imia Ac ta , !2_, 2 1 1 ( 19 78 ) .
66.
J . Se lbin and J . D. Or tega , Chem. Rev . ,
therein .
67.
N. N. Krot and A. D. Ge lman , Dok l . Akad . Nauk . SSSR, 1 7 7 , 124 ( 1967 ) .
68.
N . N. Krot , v . P. Sh i lov , v. B. Nikolaevskii , A. K. Nikaev , A. D.
Ge lman , and V. I. Spit syn , Dok1 . Akad . Nauk . SSSR, 2 17 , 589 ( 1974 ) ;
USAEC Report ORNL-tr- 2 8 2 8 ( 1974 ) .--
..:!.•
Am. Chern. Soc . ,
..:!.•
_§l,
11_,
483 ( 197 3 ) .
Am. Chem . Soc . , 8 3 , 22 00 ( 19 6 1 ) .
3 7 19 ( 19 6 1 ) .
_§!,
6 5 7 ( 19 69 ) , and re ferences
--
--
69.
--
L. J . Nugent , R. D. Baybarz , J. L. Burne t t , and J . L. Ryan , J . Phys . Chem. ,
15 28 ( 19 73 ) .
1]_,
150
70.
C. K. J�rgensen , Molec . Phys . , 1, 2 7 1 ( 1962 ) .
71.
J . R. Stoke l y , R. D. Bayb arz , and J . R. Pe terson , J . Inorg . Nuc l .
Chem. , �, 3 92 ( 197 2 ) .
72.
L. J . Nugent , in "Lan thanides and Ac tinides , " Inorganic Chemistry ,
Series II , K. w. Bagna l l , Ed . , Univ . Park Press , Bal t imore , MD , 1975 ,
Vo l . 7 , P • 19 5 .
73.
J . Fuger , Univ . o f Liege , Radiochem . Lab . , personal communica t ion ,
1980 .
74 .
L. N. Klat t , J . Chromatogr . Sc i . , Jl, 22 5 ( 19 7 9 ) .
75 .
J . P. Young , R. G. Haire , R. L. Fe l l ows , and J . R. Pe terson , J .
Rad ioana l . Chem. , 43 , 479 ( 19 78 ) .
76.
D . E. Hobar t , V . E . Norve l l , and S . A. Morr is , unpub l i shed work .
17.
B . Gu i ll aume , CEN-FAR, Fontenay-aux-Roses , France , pr ivate
c ommunic ation , 19 80 .
78.
B . Gui l laume , G. M. Begun , and R. L. Hahn , " Raman Spec t rophotometric
Stud ie s of ' Cat ion-Cation' Complexes of Pentavalent Ac t inides in
Aqueous So lut ions , " to be pub l i shed .
79.
R. D. Bayb arz , J . B. Knauer , and P . B. Orr , Oak Ridge National
Laboratory Report ORNL-4672 ( 19 7 3 ) .
80 .
z.
81.
A. J . Bard and L. R. Faulkner , "Elec trochemic a l Methods : Fundamentals
and App l ic a t ions , " John Wiley and Sons , New York, NY, 1980 .
82 .
H. B . Herman and J . R. Rairden in "The Encyc l oped ia of
E lectrochemis try o f the Element s , " A. J . Bard , Ed . , Marcel-Dekker , New
York , NY , 19 7 3 , Chapter VI-2 , and referenc es therein .
83.
R. N. Adam s , " E l ec trochemis try at Sol id E l ec trodes , " Mar c e l Dekker ,
New York , NY , 19 69 , Chapter 3 -6 .
84.
W. T. Carna l l , in "Handbook on the Phys ics and Chemistry of Rare
Ear ths , " K . A. Gs chneidner , Jr . and L. Eyring , Eds . , Nor th ­
Hol land , Ams terdam , 19 79 , Vo l . 3 , Chap . 24 , p . 17 1 .
85 .
L. R. Mors s and H . Haug ,
86 .
G . Biedermann and H. B. Silber , Ac ta Chem. Scand . , J:]_, 3 7 6 1 ( 197 3 ) .
87.
D. E. Hobart , K . Samhoun , J . P. Young , v. E . Norve l l , G. Mamantov , and
J . R. Pe terson , Inorg . Nuc l . Chem. Letters , ..!§_, 32 1 ( 19 80 ) .
Borkowska and H . El zanowska ,
l•
l·
E lectroana l . Chem . ,
�'
28 7 ( 1977 ) .
Chem. Thermo . , 1, 5 1 3 ( 197 3 ) .
151
88.
W . M. Lat imer , "Oxidat ion Potentials , " 2 nd Ed i t ion , Prentice Hal l , New
York , NY , 19 5 2 .
89.
B . F . Myasoedov and I . A. Lebedev , Radiokhimiya ,
90.
A. S. Sapryk in , v . P. Shi 1ov , V. I.
Akad . Nauk S S SR , 2 2 6 , 85 3 ( 19 7 6 ) .
91.
R. C. Prop s t ,
92.
R. R. Rickard , Analytical Chemistry Divis ion , ORNL , pr ivate
c ommunicat ion , 1980 .
93 .
R. L. N. Sas try , S. R. Yoganaras imhan , P . N. Mehrota , and C. N. R.
Rao , ..:!.• I nor g. Nuc l . Chem . , !!, 1 1 6 5 ( 1966 ) .
94.
K . A. Kraus , F. Ne l son , and G . L. Johnson , J . Am. Chem. Soc . ,
( 1 949 ) .
95.
D . M. Gruen and R. L. McBe th , J . Inorg . Nuc l . Chern. ,
96.
c.
97 .
P . G. Hagan and J . M. Cleveland , .:!• Inorg . Nuc l . Chem . , 2 8 , 2 905 ( 1966 ) .
98.
c . Ke l l er , J . Gros s , and w. Bacher , Ins t itut fur Rad iochemie ,
Kernforschung szentrum und Univers itat , Kar l sruh e , Germany,
unpub l ished work .
99.
V . I. Dzyubenko , s . A. Karavaev , V. F . Pere trukh in , and N . N. Krot ,
Dokl . Akad . Nauk S S SR , 248 , 13 62 ( 1979 ) .
..:!.•
J!,
Spit syn , and
87 ( 1977 ) .
N. N.
Kro t , Dokl .
Inorg. Nuc l . Chem. , 1§_, 1085 ( 19 74 ) .
.2_,
ll•
2 5 10
2 90 ( 19 5 9 ) .
Musikas , Thes i s , Univers ity of Par i s , Or say , France , 1977 .
100 . L. J. Bas i l e , J. R. Ferraro , M. L. Mitche l l , and J . C. Sul l ivan ,
!.EE.!. • Spec tros c . , l.!, 5 3 5 ( 19 78 ) .
10 1 . L. J . Bas ile , J . C . Sul l ivan , J . R. Ferraro , and P . LaBonv i l le ,
Spectrosc . , !!, 142 ( 19 74 ) .
�·
10 2 . W. w. Schul z , "The Chemis try of Amer ic ium , " Technical In format ion
Center , u. s . Department of Energy , TID-26 97 1 ( 19 7 6 ) , and referenc es
therein.
10 3 . D. E. Hobart , K. Samhoun , R. G. Haire , J. P. Young , M. Jaber , and J .
R . Pe ter son , abs tract o f paper presented at lOth Journee s des
Ac t inide s , Stockho lm , Sweden , May 2 7- 2 8 , 1980 .
104. L. R. Mors s and J . Fuger ,
.:!·
Inorg . Nuc l . Chern. ( in pre s s ) .
105 . L. B . Asprey and T . K. Keenan ,
.:!•
Inorg . Nuc l . Chern . , 1_, 2 7 ( 19 58 ) .
15 2
106 . L. B . Asprey and R. A. Penneman , Inorg . Chem . , !, 134 ( 19 6 2 ) .
e
107 . J . Y. Bourges , CEN-FAR, Fontenay-aux-Roses , Franc e , private
communic ation , 1980 .
1 0 8 . v . F. Peretrukh in , E. A. Erin , v . I. Dzyubenko , v . v . Kopytov , V .
G. Po lyukov , V . Ya . Va si l ' ev , G . A . Timofeev , A . G . Rykov ,
N. N. Kro t , and V . I. Spi tsyn , Dokl . Akad . Nauk SSSR 242 , 1 3 59
( 19 7 8 ) .
109 . L. J . Nugent , R. D. Bayb ar z , J . L. Burnett , and J . L. Ryan , �·
Inorg . Nuc l . Chem. , �, 2 50 3 ( 19 7 1 ) .
1 10 . Reproduc ed here wi th the permiss ion of B. Al l ard , Cha lmers Univers ity
o f Technology , De par tment of Nuc lear Chemis try , GO teborg , Sweden .
VITA
David Edward Hobart was born in Midd letown , Ohio on January 5 , 1949 .
He at tended pub l ic schoo l in Miami , Florida , graduat ing from Miami Carol
City Senior High Schoo l in June 1967 .
In September o f tha t year he
e ntered Ro l l ins Co l lege , Winter Park , Florid a , and in May 19 7 1 he rec eived
a Bache lor of Ar ts degree with a maj or in Chemistry.
In Jul y 19 7 1 he was
married to the former Mi s s Barbara Jean Canaday in Winter Park .
In Oc tober 19 7 1 he entered ac tive mil itary service in the Uni ted
States Air Force during wh ich he served as an airc raft elec tronic sensor
sys tems spec ial i s t ( reconnais sance) for the Strategic Air Command .
He
graduated wi th honors at the Lowry AFB Technical Training Center , Denver ,
Co l orado , and wa s named Outs tanding Airman of the Month in hi s un it at
Be a le AFB , Cal i fornia .
He was also stat ioned in Okinawa and Thai land .
He
was honorab l y di scharged fr om ac tive duty serv ice in Augus t 19 75 .
In September 19 75 he ent ered the Graduate School of The Univer s i ty
o f Tenne s s ee , Knoxv il le , and was granted a graduate teaching as s i s tantship
in the Depar tment of Chemistry for two year s .
In September 1977 he was
g ranted a graduate research as sis tantship which enab led him to conduc t
spec ia l ized re search at the Transuranium Research Laboratory , Oak Ridge
Nat ional Laboratory , Oak Ridge , Tenne ssee .
Wh i l e conduc ting th i s
research , he had the opportuni ty t o collaborate on severa l research
projects wi th some international gue s t sc ient i s t s , wi th some proj ects
resu l t ing in pub l ications .
On Jul y 2 7 , 19 7 8 , the author ' s wi fe gave birth to a baby gir l ,
Miche l le Anne Hobar t .
153
1 54
The au thor received the Doc tor of Phi losophy degree from The
Univers i ty of Tenne s see , Knoxvi l le with a maj or in Chemis try in June
198 1 .
The author is a member o f the American Chemical Soc iety and its
Nuc l ear Chemis try and Technology Division .
He presented papers on hi s
research at the 3 0 th Southeas tern Regional Meet ing o f the Amer ican
Chemica l Soc ie ty , Savannah , Georgia in November 19 78 and at the
Combined Southea s t-Southwest Reg ional Mee t ing o f the American Chemic al
Soc ie t y , New Or leans , Loui s iana in December 1980 .
He has pub l i shed
ar t ic les in the Journal of Chemic a l Educat ion , Journal of Inorganic
and Nuc l ear Chemis try , and Inorganic and Nuc lear Chemis try Letters ,
and is co-author o f a chapter on Berke l ium in "The Chemis try o f the
Transuranium E l ements , " 2 nd Edit ion , J . J . Ka tz , G. T. Seaberg , and
L. R. Morss , Ed itors , to be pub l i shed by Chapman and Ha l l , London .