482
J. Electrochem. Soc.: E L E C T R O C H E M I C A L
22. F. C. Anson, R. F. Martin, and C. Yarnitzky, J.
Phys. Chem., 73, 1835 (1969).
23. D. M. Mohilner in "Electroanalytical Chemistry,"
Vol. 1, p. 397, A. J. Bard, Editor, M. Dekker,
SCIENCE
A p r i l 1970
Inc., New York (1966).
24. F. Basolo and R. G. Pearson, "Mechanisms of
Inorganic Reactions," p. 37, John Wiley & Sons,
Inc., New York (1967).
Electrochemical Reduction of Chromate in the Presence
of Nickel Chloride in Molten Lithium
Chloride-Potassium Chloride Eutectic
Branko Popov~ and H. A. Laitinen*
Department of Chemistry and Chemical Engineering, University of Illinois, Urbana, Illinois
ABSTRACT
C h r o n o p o t e n t i o m e t r y of chromate in the presence of NiC12 in molten LiCIKCI eutectic reveals a diffusion controlled, three electron r e d u c t i o n step. I n
the presence of excess NiC12, chromate is reduced at --0.35V vs. P t ( I I ) / P t
reference electrode and the electroactive species responsible for the chronopotentiometric wave is estimated to have a diffusion coefficient of 1.06 9 10 -5
cm 2 sec - I at 450~
The stoichiometry of the reduction product depends
m a i n l y on the t e m p e r a t u r e at which the deposit is formed. At 500~ the
deposit approaches the composition LiNi2CrO4. X - r a y powder diffraction
shows the deposit to be a single compound with a face centered cubic lattice.
The length of the unit cell edge is estimated to be 4.14A. At 1400~ a weight
loss of the deposit is observed. X - r a y powder diffraction shows the presence of
two phases, which are identified as NiO and NiCr204. The weight loss is
a t t r i b u t e d to volatization of Li20.
L a i t i n e n and Propp (1) have shown that the electrcchemical reduction product of K2CrO4 in LiC1-KC1
eutectic containing dissolved MgC12 is a single unstoichiometric compound of formula LixMgyCrO4, where
x W 2y -~ 5. The values of x and y depended on the
conditions of the electrolysis, n a m e l y c u r r e n t density,
t e m p e r a t u r e , and the molar ratio of Mg (II) to C r ( V I )
dissolved in the melt. Typical values of x ranged bet w e e n 0.3 a n d 0.5.
H a n c k (2) observed that the reduction of chromate
in the presence of Z n ( I I ) was shifted from --1.0V vs.
P t ( I I ) / P t reference to --0.5V. Analysis of the deposit
indicated the composition to be LiZn~CrO4. The stoichiometry of the compound was not affected by the
electrolysis conditions.
The present investigation was u n d e r t a k e n to determ i n e w h e t h e r the reduction of K2CrO4 is affected by
the presence of NiC12 and to establish the composition of the reduction product. Spectrophotometric
m e a s u r e m e n t s failed to indicate a n y interaction between N i ( I I ) and chromate in the melt (3). N i ( I I ) is
k n o w n to exist as stable chlorocomplexes (4) in chloride melts, and it was of interest to determine w h e t h e r
a n y chloride is contained in the deposit. Also the reduction potential of N i ( I I ) lies about 0.2V below that
of chromate in the absence of divalent ions and it was
of interest to determine w h e t h e r the deposition of
metallic nickel could be avoided.
Experimental
Apparatus.--A H e v i - D u t y MK 3012-S vertical split
tube furnace ( H e v i - D u t y Electric Company, W a t e r town, Wisconsin) was utilized in this work. The t e m p e r a t u r e sensing element was a chromel alumel t h e r mocouple (Onega Engineering, Inc., Springdale, Connecticut). All experiments were made at 450~ except
as noted.
A Sargent Model IV coulometric c u r r e n t source was
used for the electrolytic p r e p a r a t i o n of the electrode
deposits. The constant c u r r e n t source for the chrono* E l e c t r o c h e m i c a l Society A c t i v e M e m b e r .
Z P r e s e n t a d d r e s s : F a c u l t y of E n g i n e e r i n g , U n i v e r s i t y of S k o p j e ,
Skopje, Y u g o s l a v i a .
potentiometric studies has been described previously
(1).
A Tektronix 503 oscilloscope served to record
chronopotentiograms and to m o n i t o r the potential of
the w o r k i n g electrode continuously d u r i n g the preparation of the electrode deposits. The P y r e x cell and
envelope have been described previously (5).
Electrodes.--The Pt indicator electrode used in this
study has been previously described (6). The electrode
had a geometric area of 0.5 cm 2 and was constructed so
the glass metal seal was always kept above the level
of the melt. The P t ( I I ) / P t reference electrode was
constructed as described by F e r g u s o n (8).
The p l a t i n u m gauze electrodes used to prepare samples of the film, as well as the carbon electrode which
served as the counterelectrode in all electrochemical
investigations in the melt, were constructed as described b y Propp (1).
Chemicals.--All chemicals used in this study were
reagent grade. Potassium chromate (J. T. B a k e r
Chemical Company, Phillipsburg, New Jersey) was
v a c u u m dried at 150~ before use. A n h y d r o u s NiC12
was prepared by heating the h e x a h y d r a t e (J. T. Baker
Chemical Company) in v a c u u m to 180~ over a three
day period. Analysis indicated it to be 99.3% pure.
The LiC1-KC1 eutectic was obtained from A n d e r s o n
Physics Laboratories, Inc., Champaign, Illinois. The
method of purification has been described (7).
Solid chemicals were added to the melt b y m e a n s of
a small glass spoon. A b l a n k e t of argon was kept over
the melt at all times to exclude oxygen and w a t e r
vapor. The purification t r a i n used in p u r i f y i n g the
argon has been described (8).
Experimental techniques.--Samples of the electrode
deposit resulting from the reduction of chromate in
the presence of NiC12 were obtained b y constant current electrolysis using p l a t i n u m gauze electrodes. The
gauze electrodes were cleaned in boiling HC104, rinsed
with distilled water, dried at 120~ for 16 hr, and
weighed before their insertion into the molten salt
solution. After electrolysis the gauze electrodes were
VoI. 117, No. 4
483
ELECTROCHEMICAL REDUCTION OF CHROMATE
12
o
o
A
1o
#
8
~~1.Oo.5
H
2
._
~.0
Fig. 1. Effect of nickel chloride concentration on reduction of
chromate. 1.56 x 10-2M K2CrO4. A. No NiCI2; B. 7.29 x 10-3M
NiCI2; C. 3.28 x 10-2M NiCI2.
washed w i t h deionized water, dried at 120~ and r e weighed. The electrode deposits w e r e dissolved in
boiling 72% perchloric acid. The nickel content of the
deposit was d e t e r m i n e d by an E D T A titration using
a m u r e x i d e indicator. The m e t h o d chosen for the
c h r o m i u m d e t e r m i n a t i o n is a modification of that of
Meier, Myers, and S w i f t (9). The p r o c e d u r e used has
been p r e v i o u s l y described (1). L i t h i u m was q u a n t i t a tively d e t e r m i n e d by flame photometry.
Results and Discussion
Chronopotentiometry
of K2CrO4-NiC12-LiC1-KC]
s y s t e m s . - - T h e effect of N i ( I I ) on the reduction of
c h r o m a t e was d e m o n s t r a t e d by successively increasing
the concentration of N i ( I I ) at constant c h r o m a t e concentration. F i g u r e 1 shows that at a N i ( I I ) concent r a t i o n of 7.29 9 10-~M, t w o transitions w e r e obtained:
one at 0.35V vs. P t ( I I ) / P t r e f e r e n c e and t he other
at --1.0V. A d d i t i o n a l increases in t h e nickel concent ra t i o n caused the w a v e at --0.35V to grow larger. At
a sufficiently high N i ( I I ) / C r (VI) c o n c e n tr a ti on ratio
(2.1), the w a v e at - - I . 0 V disappeared but a n e w w a v e
appeared at a p p r o x i m a t e l y --0.85V w h i c h corresponds
to t h e reduction of N i ( I I ) . F u r t h e r increase in the
N i ( I I ) concentration h a d no influence on the first r eduction step. This can be seen f r o m the fact that identical transition times w e r e obtained for 1.56 x 10-~M
K2CrO4 solution containing 3.28 x l O - 2 M NiC12 and
for one containing 5.6 x 10-2M NiC12.
Acco rd i n g to th e Sand equation th e p r o d u c t of I~ 1/2
is i n d e p e n d e n t of I for a semi-infinite linear diffusion
controlled process. T h e dependence of I T1/2 on I is
d e m o n s t r a t e d in Table I for t h e reduction of K2CrO4
Table I. Chronopotentiometry of K2CrO~ in the presence of NiCI2
A r e a of t h e P t f l a g = 0.5 cra2
CKzC r 0 4
C N i C 12
~
(M)
(M)
(mA)
7.8 • 10 -~
1.34 x 10 -2
1.56 x 1O-2
5.67 X l 0 ~
2.30 • 1O-2
6.67 • 10 -3
2.75 x 10~
5.67 • 10 -3
6.625
5.0
4.0
3.33
2.0
8.33
10.0
12.5
12.5
16.7
18.94
14.3
I6.7
18.94
12.0
CONC.
TIME
Er/4 = --0.35V
8.0
T1/2
(see1/"~)
~T1/2
( A see1/-")
0.497
0.665
0.850
1.00
1.605
9.80
0.656
0.53
0.775
0.565
0.490
0.79
0.67
0.566
3.29 x 10--3
3.25 • 10 -a
3 . 4 0 • 10 -8
3.33 • 10~
3 . 2 0 • 10 -3
6.664 • 10-~
6.56 • 10~
6.63 • 10-8
9.68 • 10-a
9.50 • l 0 s
9.77 • 10 ~
11.3 • l o s
11.25 • l 0 ~
11.30 • 10 -~
irw~
( A s e c ~/~
c mO
16.0
20.0
24.0
~8.0
I
32.0
( mmoles/llt er)
Fig. 2. Dependence of I"~ 89 on concentration of K2Cr04. 0.5
cm2 Pt flag electrode. CNiC12/CK2Cr04 > 2.1.
in the r an g e of 7.8 x 10-3M to 2.75 x 10-~M. The r e sults indicate that the reduction of c h r o m a t e in the
presence of N i ( I I ) is diffusion controlled over the
time i n t e r v a l investigated. Using n = 3 f r o m the controlled cu r r en t coulometry, a v al u e of 4.19 • 0.010
9 103 A sec ~/~ cm ~ mo1-1 for the transition t i m e constant and 0.5 cm 2 for the area of the electrode, w e
calculate D = 1.06 _ 0.06 9 10 -5 cm2 sec-1 for the
c h r o m a t e r e d u c t i o n in t h e presence of N i ( I I ) at 450~
F r o m the slope in Fig. 2 (plot of I 9 T1/2 vs. C) the
same v al u e for the diffusion coefficient was obtained.
In order to establish the chemical composition of the
electrode deposit, samples of the deposit, w h i c h had
been p r e p a r e d by constant c u r r e n t electrolysis o v e r a
wide range of p r e p a r a t i v e conditions w e r e analyzed.
To g u ar d against the deposition of metallic nickel,
t h e potential of the w o r k i n g electrode n e v e r was
allowed to b e c o m e m o r e cathodic t h a n --0.35 vs.
P t ( I I ) / P t . The influence of t e m p e r a t u r e , c u r r e n t d e n sity, and m o l a r ratio of N i ( I I ) to C r ( V I ) on the de posit composition was studied.
While holding the c u r r e n t density and the N i ( I I ) to
C r ( V I ) ratio constant, a series of deposits was p r e p ar ed at 400 ~ 450 ~ and 500~ The results of chemical
analyses of these deposits are shown in Table II. F r o m
this table one can see that L i ( I ) content increases
and the N i ( I I ) content correspondingly decreases as
the t e m p e r a t u r e of the m o l t e n salt b a t h is increased.
Table II also shows t h a t the sum of t h e w e i g h t p e r centages of the t h r ee oxides, NiO, Li20, and Cr20~ is
v e r y close to 100%, indicating that the c h r o m i u m in
the deposit is in the ~ 3 oxidation state, and that no
chloride or potassium ion is present. Th e absence of
chloride in the deposit r e q u i r e s t h a t the nickel chloroc o m p l e x dissociate before the N i ( I I ) is incorporated
into t h e deposit. That such a dissociation occurs
r a p i d l y enough to p r e v e n t inclusion of chloride in the
deposit is r em ar k ab l e. F r o m Table II it is also obvious that in spite of the fact that the reduction potential of N i ( I I ) lies about 0.2V b el o w that of c h r o m a t e in t h e absence of d i v a l e n t m e t a l ions, no e v i -
mole -1)
422
426
439
427
410
426
427
424
421
412
425
411
410
411
Table II. Composition of electrode deposit as
function of temperature
CK2CrO 4 ~
CNim 2 = 0.198M
0.052M
Temperature
(~
Wt %
Li20
Wt %
NiO
Wt %
Cr 2 0 8
Total %
400
450
500
3.75
4.09
5.10
66.8
64.5
64.0
31.90
30.60
31.90
102.0
99,2
101.0
Formula
Lio. e~fl~i2.~CrO3.gt
Lio, e~Ni~. ]2CrO3.~
bio.~Ni2.~CrO~.~
484
J. Electrochem. Soc.: E L E C T R O C H E M I C A L
dence to the deposition of metallic n i c k e l was o b tained. The e m p i r i c a l f o r m u l a w h i c h best describes t h e
deposit composition is Li~NiyCrO4. As was o b s e r v e d
b y P r o p p (1) in his s t u d y of t h e MgC12-K2CrO4 system, x + 2y ----- 5 indicating t h a t no oxide is lost d u r ing the electrolysis. A t high t e m p e r a t u r e the deposit
approaches the composition LiNi2CrO4.
Table I I I indicates t h a t t h e r e is o n l y a small change
in t h e composition of t h e electrode deposit on changing
the m o l a r ratio of N i ( I I ) to C r ( V I ) . T h e r e is a slight
decrease in L i ( I ) content and an a c c o m p a n y i n g increase in N i ( I I ) content as the m o l a r ratio of N i ( I I )
to C r ( V I ) is increased. The effect of c u r r e n t d e n s i t y
on t h e composition of t h e electrode deposit on holding
the t e m p e r a t u r e and t h e m o l a r ratio of N i ( I I ) to
C r ( V I ) constant was o b s e r v e d and is indicated in
Table IV. C u r r e n t d e n s i t y in the r a n g e 1-10 m A / c m 2
does not a p p e a r to be a significant factor in d e t e r m i n ing t h e chemical composition of t h e electrode deposit.
X-ray powder di~raction studies.--Table V i n d i cates t h a t t h e deposit is a single compound w i t h a
face centered cubic lattice. The length of t h e unit cell
edge is e s t i m a t e d to be 4.14A.
Therma~ stabitity o~ electrode deposit.--A s a m p l e of
deposit, w h i l e still on t h e p l a t i n u m gauze electrode,
was h e a t e d in air and in an argon a t m o s p h e r e to
1000~ for a p e r i o d of 48 hr. W e i g h t losses w e r e only
0.5% in argon a n d 0.04% in air. Chemical analyses on
the h e a t e d samples w e r e in a g r e e m e n t w i t h the p r e vious analyses of u n h e a t e d samples T h e h e a t e d s a m ples contained 3.87% Li20, 66.7% NiO, and 37.50%
Cr_~Os corresponding to the e m p i r i c a l f o r m u l a
Li0.60Ni2.nCrO3.91. The x - r a y p o w d e r p a t t e r n of the
deposit a p p e a r e d to change slightly a f t e r h e a t i n g to
t e m p e r a t u r e s a b o v e 950~ The original face centered
cubic lines a p p e a r e d to shift to give l a r g e r "d" spacings and diffuse lines w e r e o b s e r v e d at 4~788 and
2.494A.
A p o w d e r e d s a m p l e of t h e deposit was heated to
1430~ in an air atmosphere. The x - r a y diffraction
p a t t e r n of the h e a t e d s a m p l e is shown in Table VI.
The p a t t e r n shows the presence of two phases; a
d i a m o n d cubic phase w i t h a unit cell edge of 8.305A
and a face centered cubic p h a s e w i t h a unit cell edge
Table III. Composition of electrode deposit as function of NiCI2
concentration
Temperature
= 450~
I = 1.26 m A / c m - "
CK2Cr0 4, M
CNicl2, M
Total Wt %
Formula
0.078
0.068
0.152
0.225
0.312
0:481
100.1
99,8
Lio.~lNi2.oeCrO3.o~
Lio.a~Ni2.1oCrO~.9~
Lio.ezNi~. 1~CrO3.~
Lio.e2Ni2.18CrOe.s
0.058
0,056
99.1
99.0
Table IV. Effect of current density on composition of electrode
deposit
CK~CrO4 -----0.0482 M
C~lcl~ = 0.124M
I, m A / e m ~-
Total Wt %
1.21
2.82
4.31
9.82
99.2
99.2
98.5
98.9
Temperature
= 450~
Formula
Lio.er/~i--,. l s C r O3..~n
Lio.~Ni2.19CrO4.oo
Lio.asNi~. 2oCrO~.~
Lio. ~Ni2. ~ C r O s .
April 1970
SCIENCE
Table V. X-ray data for electrode deposit
d (A)
I/Io
hkl
a (A)
2.3840
2.0657
1.4622
1.2476
1.1957
1.0352
60
100
60
111
200
220
311
222
400
4,129
4,137
4,138
30
20
10
4,178
4.1403
4,141
Table VI. X-ray powder pattern of electrode deposit heated to
1430~
d(A)
I/Io
4,7533
2.9180
2.493
*2.4024
*2.0798
2.0850
1.6960
1.5938
"1.4730
1.4696
1.2635
"1.2599
1.207
10
15
100
60
49
20
30
90
20
100
10
25
10
10
1.0798
* F a c e c e n t e r e d c u b i c lines.
of 4.169A. The face c e n t e r e d cubic phase corresponds
r e a s o n a b l y w e l l to NiO (a = 4.176A) and the diamond cubic phase to NiCr204 (a = 8.32A). No lines
w e r e o b s e r v e d w h i c h could be assigned to the u n heated nickel compound or to p u r e Li20.
Chemical analyses of the electrode deposit heated
to 1400~ a r e in a g r e e m e n t w i t h x - r a y data. The d e posit contained 67.01% NiO, 32.3% Cr203, and no
Li20, w h i c h corresponds to t h e e m p i r i c a l f o r m u l a
Ni2nlCrO3.61.
The e m p i r i c a l f o r m u l a of an u n h e a t e d s a m p l e was
found to be Li0.vNi~.t0CrO4. If t h e Li20 is lost during
heating, t h e r e m a i n d e r of t h e c o m p o u n d m u s t have
the e m p i r i c a l f o r m u l a Ni2.10CrO~.6 which is in a g r e e m e a t w i t h chemical analyses of t h e h e a t e d sample.
M a n u s c r i p t s u b m i t t e d Sept. 23, 1969; r e v i s e d m a n u script r e c e i v e d ca. Dec. 1, 1969.
A n y discussion of this p a p e r w i l l a p p e a r in a Discussion Section to be p u b l i s h e d in the D e c e m b e r 1970
JOURNAL,
REFERENCES
1. H. A. L a i t i n e n and J. H. Propp, Ana~. Chem., 41,
645 (1969).
2. K. W. Hanck, Ph.D. Thesis, U n i v e r s i t y of Illinois,
(1969).
3. K. W. Hanck, P e r s o n a l communication, u n p u b l i s h e d
results.
4. J. Brynestad, C. R. Boston, and G. P. Smith, J.
Chem. Phys., 47, 3179 (1967).
5. H. A. L a i t i n e n a n d W. S. Ferguson, Anal. Chem.,
29, 4 (1957).
6. H. A. L a i t i n e n and R. D. B a n k e r t , ibid., 39, 1790
(1967).
7. H. A. Laitinen, R. S. Tisher, and D. K. Roe, This
Journal, 107, 546 (1960).
8. W. S. Ferguson, Ph.D. Thesis, U n i v e r s i t y of Illinois
(1956).
9. D. J. Meier, R. T. Myers, a n d E. H. Swift, J. Am.
Chem. Soc., 71, 2340 (1949).