C R O A T I C A C H E M I C A A C T A 34 (1962)
CCA-242
41
541.18.041:542.938:346.791.2
Precipitation and Hydrolysis o£ Uranium(VI) in Aqueous Solutions:
Uranyl Nitrate-Potassium Hydroxide-Neutral Electrolyte*
B. Tomažič, M. Braniča, and B. Težak
Department oj Physical Chemistry, In stitute »Ruđer Bošković«
and Laboratory oj Physical Chemistry, Faculty oj Science,
U niversity oj Zagreb**, Zagreb, Croatia, Yugoslavia
R e c e iv e d F e b r u a r y 9, 1962
The precipitation and hydrolysis of uranium (VI) in aqueous
Solutions of uranyl n itrate — potassium hydroxide — neutral electrolyte, was investigated. Clear and stable systems of uranyl nitrate
and potassium hyđroxide, w ere studied, in equivalent concentration
of 2 X10-3 N. The pH value, of th e m entioned system was between
5 and 6. By addition of neutral electrolytes (potassium, calcium,
strontium , barium , yttrium , and lantanum nitrate) the precipitation
process was initiated and up to 70°/o of the uranium present was
precipitated.
The experim ental results showed, th a t the precipitation of
uranium (VI), in the observed system, was accomplished by several
mechanisms, t.e. hydrolysis, coagulation, and form ation of mixed
uranates.
IN T R O D U C T IO N
The aqueous solution of uranyl nitrate is acid due to the hydrolysis of
uranyl ions1-9. The addition of hydroxiđes augments the hydrolysis and above
a critical concentration the precipitation of uranium10-20 begins. The precipi­
tation of uranyl ions is sensitive to changes of the concentration14-16 of uranyl
ions. Some results on precipitation and coprecipitation of uranyl ions were
given elsewhere14'21.
Uranium (VI) is precipitated from hydroxide Solutions (pH values above 9)
even from very low concentrations of uranyl nitrate. In absence of neutral
electrolytes, and concentrations below 5X10'3N no precipitation of ura­
nium (VI) has been observed at pH 5—6. Therefore, in this work special
attention has been paid to precipitation, hydrolysis and coagulation phenomena in systems having a pH 5—6. The concentration of uranyl nitrate
was, in general, kept constant at 2X10~3N. At given conditions the Solutions
were optically clear, but by addition of neutral electrolytes, uranium (VI) was
precipitated.
EXEPERIMENTAL
Solutions have been prepared by dissolving an a ly tica lly pure Chemicals in
triple đistilled w ater. The concentration of uranyl n itra te has been determ ined
* Presented in p a rt at the Sym posium on Nuclear Fuels, Radovljica, Yugoslavia,
April 20—25, 1961.
** Contribution No. 99 of th e Laboratory of Physical Chemistry, Faculty of
Science.
B. T O M A Ž IČ ,
42
m
. b r a n ič a ,
and
b
.težak
by the precipitation of uranium w ith ammonium hydroxide or, altem atively, w ith
8-oxiquinoline, and weighing as U3Os22. The neutral electrolyte concentration was
determ ined by ion exchange (Dowex-50 X-8, 50/100 mesh) technique-f>. Standardizeđ
stock Solutions w ere diluted to the required concentrations w ith water.
The changes in turbidity w ere determ ined at 530 m« using a Zeiss tyndallom eter,
conneceted w ith a Pulfrich photometer. The glass cells w ith Solutions w ere kept
a t 20 + 0.1«C in a constant tem perature w ater bath. The same bath was also used
for keeping a constant tem perature in the cell com partm ent of the photometer.
The acidity was determ ined 24 hours after mixing of the coroponents w ith a
PYE - M aster pH m eter w ith a cup form ed m icro glass electrode. For each m easurem ent only 1—2 m l of the solution was needed; the sign of charge of the solid
phase particles was determ ined by the ultramicroscopic electrophoresis technique.
The concentration of uranium in the m other liquor was determ ined polarographically using the Cambridge Instrum ent Co. Pen-Recording Polarograph. Aliquots
w ere taken after 24 hours from clear m other liquors. Systems in which the m other
liquors were turbid, even after 24 hours, w ere centrifuged in a Serwall SS-4 centri­
fuge, (rotor SM-24) in polypropylene tubes during 10 min. at 16.000 r. p. m. A fter
addition of a few drops of concentrated perchloric or sulphuric acid, the samples
w ere evaporated using an IR lam p and the residue dissolved in a supporting electrolyte consisting of 1.6 g. of salicylic acid, 4 ml. of concentrated sulphuric acid and
90 mg. of tymol per liter. To prevent the loss of uranium by coprecipitation
w ith barium sulphate, before dissolution a few drops of a saturated solution of
salicylic acid w ere added to samples containing barium ions.
Ali the precipitation systems w ere prepared by mixing the content of the
corresponding pairs of glass tubes, each containing 5 ml. of the Solutions of the
precipitating components. The concentration is always given for the who!e
volume i.e. for 10 ml. at 20°C. A fter mixing the turbidities w ere determ ined at
regular tim e intervals.
RESULTS
The titration curves of uranyl nitrate with potassium hydroxide show
two inflection points at characteristic UO„2+/OH“ ratio. The colour of the
light yellow solution is intensifded by increasing of the pH above 3.5.
Titration of uranyl nitrate of a concentration lower than 2X10“31V yields no
precipitate. At higher concentrations turbidity is observed and a precipitate is
obtained before the second inflection point. In presence of potassium nitrate
(5XKr3 N) at lower concentrations of uranyl nitrate (IX IO'3 N) a precipitate
was also obtained. Table I shows the characteristic hydroxiđe vs. uranyl
ion ratio at the first and second inflection points.
table
U 0 2(N 03)2
(N)
2 X 10-s
2 X 10-s
1 X 10-s
kno3
(N)
—
—
5 X 10 -s
i
1’* inflexion point 2!“‘ inflexion point
PH
o h -/ u o 22+
PH
4.7
4.5
4.85
1.52
1.62
1.58
8.0
7.1
7.0
OH-/UOas+
2.4
2.35
2.7
It was observed that some minimum pH value of the systems was
required to initiate the precipitation process. Visually, there are two different
precipitates. The first, a yellow green, is obtained at pH values (about 5)
and at higher uranyl nitrate concentrations (2X10'3W). At higher pH the
P R E C IP IT A T IO N A N D H Y D R O L Y S IS O F U R A N IU M (V I)
43
fo rm atio n of a y ello w -o ra n g e p re c ip ita te begins. A b ove p H 9.5 only th e la te r
k in d of p re c ip ita te is fo rm ed . F o r th e fo rm a tio n of th e o ra n g e -y e llo w p re c i­
p ita te a t low u ra n y l n itra te c o n c e n tra tio n s a m u ch h ig h e r h y d ro x id e to
u ra n y l ra tio is re q u ire d . T h e p re c ip ita te is o b ta in e d a t p o tassiu m h y d ro x id e
co n cen tratio n s of 2X 10"2 N in 2X IO-4 N u r a n y l n itr a te Solutions, w h ile
I X IO'4 N Solutions in u r a n y l re m a in clear.
TABLE
U 0 2(N 0s)2
(N)
1.0 X 10-3
2 X IO”3
3 X 10-3
5 X 10-3
7.5 X 10-3
II
OH-/UOo2+
pH values for the
ratio, for the
system 24 hours
appearence of the
after mixing
first precipitate
4
2.3
1.2
0.8
0.75
9.8
9.5
6.8
4.8
4.4
Colour of the precipitate
orange-yellow
orange-yellow
yellow
green-yellow
green-yellow
T ab le II show s p o tasisu m h y d ro x iđ e to u ra n y l n itr a te ra tio , a t w h ich
th e firs t p re c ip ita te is fo rm ed , in d ep en d en ce of u r a n y l n itr a te co n cen tratio n .
T he pH v alu e s a n d th e co lo u r of th e fo rm e d p re c ip ita te s a r e also given. T hese
d a ta h av e b een o b ta in e d 24 h o u rs a fte r m ix in g th e system s. T h e specific effect
of p o tassiu m ions ca n n o t be stu d ie d a t u ra n y l n itr a te co n c e n tra tio n s h ig h e r
th a n 2 X IO-3 IV. I n such a case th e re q u ire d p o ta ssiu m h y d ro x id e concen­
tra tio n fo r th e h y d ro ly sis p ro cess w o u ld be h ig h e r th a n 6 X 1 0~*N, th u s
ex ceeding th e c ritic a l p o tasisu m ion co n cen tratio n .
Fig. 1. show s tu rb id ity (ty n d a llo m e tric valu e) a n d pH v a lu e s in d ep en d en ce
of th e p o tassiu m h y đ ro x iđ e c o n cen tratio n s. T h e u r a n y l n itr a te co n c e n tra tio n
is co n stan t, 5X 10_3IV. Fig. 2. re p re se n ts th e sam e d ep en d en ce, b u t fo r a lo w er
F ig. l. C o n c e n tra tio n . ty n d a llo g ra m s o f th e s y s te m : u ra n y l n itr a te (5X10-3 N ) - p o ta s siu m
h y d r o x id e (v a ry ln g c o n c e n tra tio n s ) a t 1, 10, a n d 30 m in u te s . T h e u p p er cu rv e d e n o te s t h e
p H v a lu e s m e a s u re d 24 h o u r s a f t e r m ix in g .
44
B. T O M A Ž lC , M. B R A N IČ A , A N D B. T E Ž A K
CONCENTRATION 0F KOH. (M)
CONCENTRATION OF KOH (N)
F ig . 2. C o n c e n tra tio n ty n d a llo g ra m s o f th e sy s te m : u r a n y l n i t r a te (3X10-3 N ) - p o ta s siu m
h y đ r o x iđ e ( v a ry in g c o n c e n tr a tio n s ) a t 1, 10, a d 30 m in u te s. T h e u p p e r c u rv e d e n o te s th e pH
v a lu e s m e a s u re d 24 h o u r s a f t e r m ix in g .
F ig . 3. C o n c e n tra tio n ty n d a llo g r a m s o f th e s y s te m : u r a n y l n i t r a te (2X10-3 N )- p o ta s siu m
h y d r o x id e — p o ta s s iu m n i t r a t e (v a ry in g c o n c e n tra tio n s ) a t 1, 10, a n d 30 m in u te s. T h e c o n ­
c e n tr a tio n o f t h e p o ta s s iu m io n w a s k e p t c o n s ta n t (2X10-2 N) . T h e u p p e r c u rv e d e n o te s th e
PH v a lu e s m e a s u re d 24 h o u r s a f t e r m ix in g .
u ra n y l nitrate concentration (3X1(T3 N). S im ila r cu rves h av e b een o b tain ed for
o th e r u r a n y l nitrate c o n c e n tra tio n s. A m in im u m p o tassiu m c o n c e n tra tio n of
a b o u t 6 X 1 0 -3 N is required fo r th e fo rm a tio n of th e solid p h ase in d e p e n d e n t
of th e u r a n y l nitrate c o n c e n tra tio n , (Figs. 1. a n d 2.) w h ich in d icates th a t
th e p o tassiu m ions play an im p o rta n t ro le in th e p re c ip ita tio n process in th is
system . Therefore a constant u ra n y l n itr a te c o n cen tratio n of 2X10 ~3 N w as
chosen for further studies.
F ig . 4. T h e p e r c e n ta g e o f u r a n iu m (VI) in m o th e r liq u o r o f th e sy s te m : u r a n y l n i t r a te
(2X10-3 N ) - p o ta s s iu m h y d r o x id e -p o ta s s iu m n i t r a te (v a ry in g c o n c e n tra tio n s ) 24 h o u r s a f t e r
m ix in g . T h e c o n c e n tr a tio n o f th e p o ta s s iu m io n w a s k e p t c o n s ta n t, (4X10-* N). p H V alues
m ea su red 24 h o u r s a f t e r m ix in g a r e also p r e s e n te d .
Fig. 3. shows the in flu e n c e of p o ta ssiu m h y đ ro x id e c o n cen tratio n on
tu rb iđ ity an d pH. By a d d in g p o tassiu m n itr a te th e co n c e n tra tio n of potassium
ions in so lu tio n was kept c o n sta n t a t 2X10_2iV. In Fig. 4. th e p ercen tag e of
u ra n iu m in the mother liq u o r, a n d th e pH is giv en fo r v a rio u s potassiu m
P R E C IP IT A T IO N A N D H Y D R O L Y S IS O F U R A N IU M (V I)
45
hyđroxide concentrations at constant potassium ion concentrations of 4X IO"2 N.
The curves in Figs. 3. and 4. indicate that for 2X1CT3 N uranyl nitrate, a critical potassium hyđroxide concentration of 2X10-3 N is required in order to
precipitate about 60% of the initial uranium content. Therefore, potassium
hyđroxide concentration was also kept constant at 2X10~3 N in ali experiments
where the influence of other neutral electrolytes on the formation of preci-
F ig. 5. C o n c e n tra tio n ty n đ a llo g ra m s o f th e s y s te m : u r a n y l n itr a te (2X10-3 N )- p o ta s s iu m
h y đ r o x id e (2X10-3 N) - y t t r i u m n i t r a te ( v a ry in g c o n c e n tr a tio n s ) at 1, 5, 15, 30 m in u te s a n d
1 a n d 3 h o u rs . T h e u p p e r c u r v e d e n o te s pH v a lu e s m e a s u r e d 24 h o u rs a f t e r m ix in g .
F ig . 6. T im e ty n d a llo g ra m s o f th e sy s te m : u r a n y l n i t r a t e (2X10-3 N )- p o ta ssiu m h y d r o x id e
(2X10-3 N ) - y t t r i u m n i t r a te ( v a ry in g c o n c e n tra tio n s ). T h e in te r s e c tio n s o f ta n g e n ts o n tim e
ty n d a llo g ra m s w ith th e a b sc issa g iv e s th e c r itic a l tim e o f th e co a g u la tio n
f o r Y (N O i)s c o n c e n tr a tio n s o f :
© 1.8X10-‘ N ,
( 2.5X10-3 N ,
SS 4.0X10-* N,
□ 6.0X10-* N
X 8.0X10-* N,
C 1.0X10-3 N,
• 1.2X10-3 N .
F ig. 7. D e p e n d e n c e o f th e c r itic a l tim e f o r t h e p r e c ip ita tio n o f u ran iu m (VI) o n t h e
c o n c e n tr a tio n o f y t t r i u m n i t r a te f o r t h e s y s te m : u r a n y l n i t r a t e (2 X 18-' K )- p o ta s s iu m
h y d r o x id e (2X10-» N).
46
B. T O M A 2 IC , M. B R A N IČ A , A N D B. T E Ž A K
pitates was studieđ. The pH of these systems was always between 5 and 6.
The hyđrolysis of uranyl nitrate in the pH range from 5—6 leads to formation
of colloids with a negative charge. This was shown by the electrophoretic
measurements. Neutral electrolytes: potassium, caicium, strontium, barium,
F ig. 8. C o n c e n tra tio n t.y n d a llo g ra m s o f th e sy s te m : u r a n y l n i t r a te (2X10~8 N ) - p o ta s siu m
h y đ r o x iđ e (2X10~3 N ) - p o ta s s iu m n i t r a t e (v a ry in g c o n c e n tra tio n s ) a t 1,10 a n d 30 m in u te s . T h e
u p p e r c u r v e d e n o te s t h e p H v a lu e s m e a s u re d 24 h o u r s a f t e r m ix in g .
F ig . 9. T h e p e r c e n ta g e o f u r a n iu m (VI) in m o th e r liq u o r o f t h e s y s te m : u r a n y l n i t r a te
(2X10~3 N ) - p o ta s s iu m h y d r o x id e (2X10~3 N ) - p o ta s s iu m n i t r a te (v a ry in g c o n c e n tra tio n s ), 24 h o u r s
a f te r m ix in g . T h e u p p e r c u r v e d e n o te s th e pH v a lu e s m e a s u re d 24 h o u r s a f t e r m ix in g .
F ig . 10. C o n c e n tra tio n ty n d a llo g ra m s o f th e sy s te m : u r a n y l n i t r a te (2X10 3 N ) - p o ta s s iu m
h y d r o x id e (2X10~3 N ) - c a ic iu m n i t r a t e ( v a ry in g c o n c e n tra tio n s ) a t 1, 10, a n d 30 m in u te s . T h e
u p p e r c u r v e d e n o te s t h e pH v a lu e s m e a s u re d 24 h o u r s a f te r m ix in g .
F ig . 11. C o n c e n tra tio n ty n d a llo g r a m s o f t h e sy s te m : u r a n y l n i t r a te (2X10-3 N ) - p o ta s s iu m h y d r o x id e (2X10~3 N ) - s tr o n tiu m n i t r a t e (v a ry in g c o n c e n tra tio n s) a t 1, 10, a n d 30 m in u te s . T h e u p p e r
c u r v e d e n o te s t h e p H v a lu e s m e a s u re d 24 h o u r s a f t e r m ix in g .
yttrium and lantanum nitrate, were chosen for further experiments. The
addition of neutral electrolyte to a clear solution of uranyl nitrate and
potassium hydroxide initiates the precipitation process.
The critical time for the precipitation process was determined using
tyndallometric techniques. The results obtained are illustrated on the example
of the influence of yttrium nitrate. The turbidity — concentration curves are
P R E C IP IT A T IO N A N D H Y D R O L Y S IS O F U R A N IU M (V I)
47
discontinuous, b u t a re w ell d efin eđ w ith re sp e c t to th e c o n cen tratio n of th e
n e u tra l electro ly te, a t w h ic h p re c ip ita tio n is in itia te d .
Fig. 8. show s th e in flu en ce of th e v a rio u s p o tassiu m n itr a te co n cen tratio n s
on th e tu rb id ity of th e m e n tio n e d system s. T h e c u rv es in Fig. 9. show th e
p ercen tag e of u ra n iu m in th e m o th e r liq u o r a n d th e pH o f th e system s
in d ep enden ce of v a rio u s p o tassiu m n itr a te co n cen tratio n s. In b o th cases
th e lo w est c o n c e n tra tio n o f p o tassiu m n itra te , a t w h ich th e p re c ip ita tio n of
u ra n iu m begins, is a b o u t 6X 1(T3 N.
F ig . 12. C o n c e n tra tio n ty n đ a llo g ra m s o f t h e sy s te m : u r a n y l n i t r a t e (2X10~3 N ) - b a r iu m n itr a te
( v a ry in g c o n c e n tra tio n s ) a t 1, 10, a n d 30 m in u te s. T h e u p p e r c u r v e d e n o te s th e p H v a lu e s
m e a s u re đ 24 h o u r s a f t e r m ix in g .
Fig. 13. T h e p e r c e n ta g e o f u r a n iu m (VI) in m o th e r liq u o r o f t h e s y s te m : u r a n y l n i t r a te
(2X10~3 N ) - p o ta s s iu m h y d r o x id e (2X10-3 N)~ c a lc iu m , s tr o n tiu m o r b a r iu m n i t r a te ( v a ry in g
c o n c e n tra tio n s ) 24 h o u r s a f te r m ix in g . T h e u p p e r c u rv e d e n o te s th e p H v a lu e m e a s u re đ
24 h o u r s a f t e r m ix in g .
F ig . 14. T h e p e r c e n ta g e o f u r a n iu m (VI) in m o th e r liq u o rs o f th e s y s te m : u r a n y l n i t r a te
(2X10-3 N ) - p o ta s s iu m h y đ r o x id e (2X10~3 N )- la n th a n u m o r y t t r i u m n i t r a t e ( v a ry in g c o n c e n tra tio n s )
24 h o u r s a f t e r m ix in g . T h e u p p e r c u rv e d e n o te s t h e pH v a lu e s m e a s u r e đ 24 h o u r s a f t e r m ix in g .
T he cu rv es in Figs. 10., 11., and. 12 show th e tu rb id ity an d th e pH v alu es
fo r th e sam e sy stem s to w h ic h th e va-rious co n c e n tra tio n s of calcium , s tro n tiu m
and b a riu m n itr a te a re d iffe re n t.
T he cu rv es in Fig. 13. in d ic a te th e p e fc e n ta g e of u ra n iu m in th e m o th e r
liqu or, an d th e pH v a lu e s fo r v a rio u s co n c e n tra tio n s of calcium , s tro n tiu m and
b a riu m n itra te . T h e re is no d ire c t c o rre la tio n betvveen th e ty n d a llo m e tric
48
B. T O M A Ž IČ , M. B R A N IČ A , A N D B. T E Ž A K
maximum shown in Figs. 10., 11., and 12 and the percentage of uranium in the
mother liquor. In spite of the smaller turbidity at high neutral electrolyte
eoncentrations, these systems show equal, or even higher percentage of
uranium precipitated.
The influence of the trivalent yttrium, and lantanum cations on the
percentage of uranium left in the mother liquor is shown in Fig. 14.
D IS C U S S IO N
The titration curves of uranyl nitrate with hydroxides show two
inflection points. The curves could not be reproduced by titrating an alkaline
system with perchlordc or nitric acid. This is due to »hysteresys«, observed
already by Kraus and Nelson5, which indicates the irreversibility of the uranyl
hydrolysis. The formation of the solid phase is the cause of the irreversibility.
In the course of hydroxide addition the uranyl ion hydrolyzes over a series
of consecutive polynuclear hydroxy - complexes. The first and second
complex have pK values 6.1 and 5.8 respectively. They exist in acid solution
at pH lower than 48>9. No colloid particles could be found in the optical field
of the ultramicroscope in such acid systems. Neither did the addition of
neutral electrolyte influence the turbidity, nor did it initiate the precipitation
process. This indicates that the process is taking place in a real ionic solution.
At more basic uranyl nitrate and potassium hydroxide Solutions (pH above
4), the solid phase could be formed, either at high eoncentrations of both precipi­
tation components, or by addition of neutral electrolytes at lower eoncentrations.
There is disagreement in literature with respect to the composition of the formed
precipitates and the mechanism of their formation1011’12-20. In most cases it was
suggested that either Na2U20 7 or Na,U70 , 2 are formed. According to the results
shown in Fig. 4., it is probable that in the presence of a high concentration of
potassium ions (4X1CT2 N) the precipitate might be potassium diuranate, K,U2Ot.
This conclusion follows from the fact that a complete uranyl (2X1CT3 N)
precipitation is achieved at 3 X IO"3 N potassium hydroxiđe, while the supernatant solution is neutral (pH is about 7). We obtain the ratio U 0 22+/OH" = 1 : 3 ,
i.e. and the reaction corresponds to the equilibrium:
2 U O /2 + 6 OH" + 2 r $
K2U20 7 + 3 H20
With reference to the given data, it is obvious that tyndallometric measurements
indicate only the critical concentration at which the precipitation process
begins. But the measurements are simple, easy to perform and copious
experimental data can be obtained in a short time. The composition of the
formed precipitates and hydrolyzates, can be determined only by using
quantitative analytical techniques, which are more elaborate and, above ali,
more time consuming. Combining both methods a rapid and good review
of the precipitation phenomena is achieved.
The precipitation of uranium from uranyl nitrate Solutions at concentrations higher than 5X10"3 N was also observed in acid media, in absence
of neutral electrolyte. At lower uranvl nitrate eoncentrations the precipitates
appear only at the pH above 9 (see e.g. Figs. 1. and 2., Table II).
Further investigations have shown that it is possible to precipitate uranium
(VI) from clear Solutions consisting of uranyl nitrate (2xl0"3 N) and potassium
hydroxide, at pH values below 7, when the potassium ions are in excess. This
P R E C IP IT A T IO N AN D H Y D R O L Y S IS O F U R A N IU M (V I)
49
is illustrated in figures 4. and 3. The pH values of Solutions containing
equivalent quantities of uranyl nitrate and potassium hydroxide (2X1(TSIV))
is about 6.
The turbidity of the systems is time dependent and was successfully
measured (Figs. 5., 6., and 7.). Figures 9 to 14 represent the precipitation
of uranium (VI) vs. neutral electrolyte concentrations. They show that pre­
cipitation of uranium (VI) is accompanied by a simultaneous decrease of pH.
It was suposed that the hydroxyl ions from the solution are bound to uranyl
ion in the precipitation process. Because of the dissolution of the precipitate
at pH below 5 the system becomes self-buffering and there is little change in
pH. For this reason in the system of the given composition, a maximum of 70°/o
of uranium (VI) is precipitated.
OONCENTUMION O f ,HEUTWAi"EI£CTROLYTCS^M* (N O ,l [Ni
Fig.
15. C o n c e n fr a tio n ty n d a llo g ra m s o f th e s y s te m :
h y d r o x id e (2X10-* N ) - n e u tr a l e le c tr o ly te s ( v a ry in g
u r a n y l n i t r a te
c o n c e n tra tio n s )
(2 X 10-* N ) -p o ta s s iu m
a t 30 m in u te s.
A survey of »coagulation« values i.e. of the lowest concentrations of
neutral electrolytes, necessary for the initiation of the precipitation in the
same system is given in Fig. 15. The values for uni-, bi-, und trivalent cations
are not in accordance with the Schulze-Hardy rule. The specific action is best
shown for the bivalent cations (calcium, strontium and barium). Such
behaviour cannot be attributed to the differences of their ionic radia, or
solvation behaviour. This is probably a specific influence due to the
building-in and formation of the corresponding uranates.
The obtained results indicate that neutral electrolytes influence the pre­
cipitation of uranium through several mechanisms including coagulation,
salting out and formation of mixed uranates.
Further investigations of the quantitative determination of the composition
of the precipitates, as well as the influence of the neutral electrolyte, and
pH on the precipitation of uranium (VI), are in progress.
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IZVOD
Taloženje i hidroliza urana(VI) u vodenim otopinam a: uranil n itrat-k alijev hidroksidneutralni elektrolit
B. Tamažič, M. Braniča i B. Težak
Istraživani su uvjeti taloženja i hidrolize urana (VI) u vodenoj otopini: uranil
nitrata - kalijeva hidroksida - neutralnog elektrolita. Osobito su bili proučavani (sta­
bilni i bistri) sistem i uranil n itra ta i kalijeva hidroksida u ekvivalentnoj kon­
centraciji od 2XlCr3 N. Takvi sistem i im aju pH vrijednost od 5 do 6. Taložni proces
počinje dodatkom neutralnog elektrolita (kalij, kalcij, stroncij, barij, itrij i lantan
nitrat), te se m aksim alno istaloži do 70%> prisutnog urana. Eksperim entalni rezul­
tati pokazuju, da je taloženje u ran a (VI), u prom atranom e sistemu, posljedica neko­
liko mehanizama, kao što su: hidroliza, koagulacija i stvaranje m iješanih uranata.
Taloženja su praćena tindalom etrijski i pH_m etrijski, dok je postotak urana
u matičnici određivan polarografski.
IN S T IT U T »PU D E R BOŠKO VIC«
ZA G R E B
P rim lje n o 9. v e lja č e 1962.