J. Inorg. Nucl. Chem., 1965, No 1127, pp. 2391 to 2395. Pergamon Press Ltd. Printed in Northern lreIand
SPECTROPHOTOMETRIC AND POTENTIOMETRIC S T U D I E S OF THE S Y S T E M C O N T A I N I N G UO~" , C,,O,'-' A N D H,,O+ N . S. KRISttNA PRASAD and K. M. ABUBACKER Atomic Energy Establishment Trombay, Chenristry Division, Bombay 28
(Receiced 13 September 1964; in rerised fi)rm 8 No~,ember 1964)
Abstract The interaction of uranyl ion with hydrogen peroxide in the presence of oxalate ions gives rise to complex species [(UO~)~OO(C~O~)~]~ and [UOeOO C._,O4]~ in the pH ranges 1 4.5 and 4.5 7.5 respectively as indicated by potentiometric and spectrophotometric data. This is supported by the isolation of the corresponding amnaonium salts in the solid state from solutions in these pFt ranges. The stability constants of the two complexes calculated from potentiometric data are 2 - I0: and 5-1 ~ 10:' respectively. l'i4t~ formation of a m m o n i u m uranyl peroxy oxalates (NH)4),,U.,O6(CeO4)~-2H_,O, (NH~)2U,)OsC.~O4.4HzO and (NH4)zUOIC204.3H.,O has been observed and reported by one of the authors. I1) in an attempt to elucidate the course of reaction leading to their formation, spectrophotometric and potentiometric studies of systems containing UOe"', C,)O,~2 and H20., have been carried out and the results are discussed in this paper. EXPERIMENTAL B.D.H. AnalaR grade chemicals were used in all experiments. A Beckman DU-2400 spectrophotometer and Beckman H2 pH meter were used for the investigation. The pH titrations were made using a micrometric device. RESULTS Spectrophotometric studies. The absorption spectra of mixtures of uranyl nitrate, ammoniunl oxalate and hydrogen peroxide of concentrations 10 3 M in the ratio 1 : 1 0 : 1 0 are given in Fig. 1. The absorbancy of uranyl nitrate or uranyl nitratehydrogen peroxide mixtures in the pH range 3 to 8 is not very appreciable. Mixtures o f uranyl nitrate and a m m o n i u m oxalate show absorption maxima at 310 and 410 ml~. In the presence of hydrogen peroxide these mixtures also show absorption maxima in the same wavelength region and differ little in absorbancy below pH 3. Bui with increasing pH, the absorbancy increases, the 310 mt~ peak showing a bathochromic shift to 340 m/~ and the 410 m/~ peak gradually disappearing. This indicates that hydrogen peroxide reacts with a m m o n i u m uranyl oxalate in solution to form one or more peroxy complex species. The pH absorbancy curves for mixtures of uranyl nitrate, a m m o n i u m oxalate and hydrogen peroxide at different wavelengths show a sharp rise in absorbancy in the p H range 4.5-6.5 indicating maximum complex formation in this range. The absorbancv remains constant after pH 6.5. ctl N . S. K R I S ; t N A PRASAD,
J. htort(. Nucl. Chern.
21,379
2391
(1962).
2392
N.S.K.
PRASAD a n d
K.
M . ABUBACKER
The composition of the peroxy species formed in solution at pH 6.5 was determined by carrying out spectrophotometric titrations of uranyl nitrate in the presence of constant excess of (a) hydrogen peroxide (twenty times) against ammonium oxalate and (b) ammonium oxalate (twenty times) against hydrogen peroxide. The first set URANYL
AMMONIUM HYDROGEN
CURVE
NITRATE
OXALATE
1 o
~3 M
15' M ,,
2
÷
,,
4
•
#
PEROXIDE
pH 3
~3 M u
-
/J
--
153 M 1
6 -o-
0'9
1~3M
5 u
153N
8 Js
.
.
0"8
0"7 0"6
"
~
. . . .
0"5 ..
0"4 >(J Z .,(
'~
~ OVERLAPS 2 BELOW 4 0 0
mp
0"3'
0'Z-
m E
0 I/1 ID
,(
0'1
0'05 0"04 0'03,
0'02
4 0'01 L _ _ 300
320
340
360 ,~
380 WAVE LENGTH
400 IN
~'~----+ 420
440
mH
FIG. 1
of titrations (Fig. 2) showed two breaks corresponding to UO22+:C20~ 2- ratios of 1 : 1 and 1:2, while the second set (Fig. 2) showed only one break corresponding to UO22+: H202 = 1 : 1. This shows that peroxy oxalate complexes with UO22+ : H202 : C20~~ ratios of 1 : 1 : 1 and 1 : 1 : 2 are formed in solution under these conditions. The absorption spectrum of a solution of (NH4)2UO4C204.3H20 (which was isolated in the solid state from mixtures of uranyl nitrate, ammonium oxalate and hydrogen peroxide in the pH range 4.5-7"0) was studied in 0.1 M sodium perchlorate at various pH values in the ran ge 3.2-8.4 and at various concentrations from 2 x 10-4 M
Studies of the system containing UO2~, C20.~2 and H20.,
05
3,4~
1"0 ~
1"2
~
1-5 3
4 2 0
2393
20 330 mFtj m
~
1"0 ¢ 0"8
2 o
0.6
'~ 0-4
/,/
3,4 Hydrogen peroxide warying pH = 6"5
0"2
~_i ,z-o-s
lo
q__. 1.5 {o
d5
3'-0-3;
4:0
Note ratio FIG. 2
to 2 i< 10-a M. The single absorption maximum exhibited by the compound at various hydrogen ion concentrations undergoes a bathochromic shift from 310 to 340 m/~, the molar extinction coefficients decreasing with increasing concentration of the solution (Table 1). In this case there is also a sharp rise in absorbancy in the pH range 4"5 to 6"5. TABLE 1. URANYL
SHIFT 1N THE ABSORPTION MAXIM UM OF AMMONIUM PEROXY
OXALATE
[(NHDaUOaC204.3H20] W I T H
CONCENTRATION
Conc. of ammonium uranyl p e r o x y oxalate (10 ~M)
Absorption maximum hi,
Molar extinction coefficient
2.0 4-0 6-0 8.0 10.0 12-0 20.0
310 313 315 317 320 330 340
1435 1412 1334 1250 1192 1134 965
Potentiometric studies: The nature and composition of uranyl peroxy oxalate complexes were also investigated by potentiometric titrations of uranyl oxalate and uranyl oxalate-hydrogen peroxide mixtures with sodium hydroxide at constant ionic strength (0' 1 M sodium perchlorate). The results are given in Fig. 3. Curve 1 refers to 5 "~ 10-~ M uranyl oxalate. It shows two breaks corresponding to alkali equivalents of about 1-0 and 2"3 moles per mole of uranyl oxalate. Curve 2 refers to mixtures of uranyl oxalate and hydrogen peroxide, each 5 :~ 10 2 M, in the ratio 1:20. It shows a faint break in the pH range 3.5-4.5 and two sharp breaks in the range 4.5-7.0 and 7.0-10.0. Curve 3 refers to uranyl oxalate and oxalic acid, each 5 x 10 2 M, in the ratio 1:2. The two breaks in the curve correspond to the formation of sodium uranyl oxalate and sodium diuranate. Curves 4 and 5 refer to mixtures of uranyl oxalate,
2394
N . S . K . PRASADand K. M. ABUBACKER
oxalic acid and hydrogen peroxide with the ratios UO22+ : C2042- : HzO~ being 1:2 : 20 and 1:4:20. While curve 4 shows only two sharp breaks, curve 5 shows three distinct breaks in the p H ranges 4.0-5-0, 5.0--7-5 and 7.5-10-0. DISCUSSION The absorption spectra of mixtures of uranyl nitrate, ammonium oxalate and hydrogen peroxide resemble those of (NH4)2UO 4 C204-3HzO in aqueous solution with respect to shape and intensity. In both cases there is a sharp increase in the absorption in the pH range 4.5-6.5, the value remaining constant between pH 6.5 and 10.0. This fact, coupled with the isolation of solid (NH4)2UO4C204.3H20 in the pH range 4"5-7-0, suggests that the ion UO4C2042- is the absorbing species in solution. It is also probable that, in solutions containing excess ammonium oxalate, the species [UO4(C204)2] 4- corresponding to the 1:1:2 complex indicated by spectrophotometric titrations also contributes to this absorption. KOMAROV et al (2) assumed that the strong absorption of a mixture of uranyl oxalate, ammonium oxalate and hydrogen peroxide containing excess of oxalate ion was due to the formation of the species [(UO,,)zOO(C204)4] 6-, but they failed to isolate this complex in the solid state; this was explained as being due to its high solubility. The present study gave us indication of the existence of such a complex. The bathochromic shift noticed with increasing concentration of the complex (NH4)2UO4C204 is in agreement with the general observation that the absorption centre will move from the blue to red region with increase in weight or complexity of the chromophore. (3) The decrease in molar extinction coefficient with increase in concentration is probably due to progressive polymerisation of the species in solution as in the case of peroxy-titanic acid.(4) Conductivity and pH measurements (5) have established that in aqueous solutions uranyl oxalate dissociates according to the schemes (a) and (b) UO~C20~ ~ UO22+ + C2042UO2C204"H20 ~ [UO2C204(OH)]- + H +
(a) (b)
The two breaks observed in curve 1, Fig. 3 correspond to the neutralisation of the proton liberated according to Equation (b) and the final formation of UO2(OH)z, in agreement with earlier work. (5) In uranyl oxalate--hydrogen peroxide mixtures, the potentiometric curves show three breaks both in the presence and absence of excess oxalate, the first of which is not very prominent. The three breaks correspond to alkali equivalents of neutralisation of the two protons in each case according to the equations 2UO2C204 q- H202 ---, [(UO2)2OO(C204)2] 2- q- 2H +
[(U02)200(C204)2] 2- -- H202 --~ 2[UO4C204] 2- -]- 2H + 2[UO4C204] 2- q- 2 O H - ---~ [UzO~]2- + 2C2042- -}- H20
(1) (2) (3)
12)E. V. KOMAROVand A. M. GUREVICH,13V. Akad. Nauk. USSR., Otdel. Khim. Nauk. 3, 547, 1959. (a) W. G. BERL,Physical methods in chemical analysis Vol. I, p. 231. Academic Press, New York (1950). (~ R. E. REEVESand H. B. JONASSEN.,J. Amer. Chem. Soc. 76, 5354 (1954). (~) A. A. GREENBERG,B. V. PTITSINand N. E. TEKSTER.,Report of radium institute of academy of sciences, USSR, Vol. 7, p. 74 (1956).
Studies of the system containing UO~2~, C.,O~2 and H,,O.,
2395
The stability constants K 1 and K s for [(UO2)2OO(C20~L]" and [UO4C204]'-' were determined by applying BJERRUMS method as modified by CALVIN and WILSON"~ from ~ vs. log (l/A) plots and were found to be 2 .' 107 and 5.1 >< 105 respectively. The validity of Equations (1) to (3) is supported by the isolation, from solutions
11 10 1
9
3
8 7
~5
5 n
5
Oxalic
4 3 2 1 0
3
--
4 5
5m[
0"5mr
,,
1"0 mt
i
i
i
i
i
i
i
•1
'Z
"3
'4
'5
"6
•7
~Vo!,ume of a t k a [ i
"8
(rn[)
FIG. 3 o f a m m o n i u m uranyl o x a l a t e - H 2 0 ~ mixtures, of the corresponding solid complexes in the pH ranges covering the breaks in the potentiometric curves.(1, v) Acknowledffement--The authors wish to thank Dr J. SHANKAR,Head, Chemistry Division, Atomic
Energy Establishment, Trombay for his keen interest in the work. (") M. CALVINand K. W. WILSON,J. Amer. Chem. Soc. 67, 2003. ~7) N . S. KRISHNA PRASAD,
J. Inofig. Nucl. Chem. 27, 2311 (1965).