Oxalato complexes of neptunium(IV)

J. lnorg. Nucl. Chem., 1964, Vol. 26, pp. 799 to 805. PergamonPress Ltd. Printed in Northern Ireland

OXALATO

COMPLEXES

OF NEPTUNIUM(IV)

B. M. L. BANSAL and H. D. SHARMA Radiocbemistry and Isotope Division, Atomic Energy Establishment, Trombay, Bombay

(Received 6 July 1963; 23 October 1963) Abstract--The complex formation of neptunium(IV) ions with oxalate ions has been studied by measuring the equilibrium solubility at various oxalic acid concentrations of Np(C204)z.6HzO in perchloric acid medium maintaining an ionic strength of 1 at 26 2=_2°C. It was found that the data so obtained could be interpreted by assuming that the complexes in solution were Np(C~O,l) ~+, Np(C204)~ and Np(CzO4)a--. The values of the complexity constants (MA,) .

;~,

(M)~-)J

have been calculated from the data, the values being f~ ~:- (4.3 ~ 0-1) × 102; /33 = (7.65 _L 0-2) >~ 1@6; /53 = (4.9 ± 0'2) × 102a. These values were also checked by solvent extraction method using TTA as a chelating agent. The visible spectra of the complexes show a shift in their characteristic absorption bands when a ligand molecule enters the complex.

DURING the fuel processing and the recovery of 2aVNp from the raffinate from a TBP extraction processm from irradiated uranium, neptunium is ultimately precipitated as oxalate and finally calcined to NpO~. In the present work the complex formation of Np(IV) with oxalate ions has been studied using solubility as well as solvent extraction method. The only available data are those of KONDRATOVand GEL'MAN, (e) who determined the composition and equilibrium constants of the neptunium(IV) complexes in 0.5 M HC1 solution. EXPERIMENTAL All reagents used were analaR grade except TTA, which was obtained from Peninsular Chemical Research Inc. It was further purified by recrystallisation from benzene before use. 2aTNp obtained from AERE Harwell was free from ~41Am and ~a~pu as checked by alpha pulse analysis. Neptunium was reduced ~3~ to neptunium (IV) with 0.9 M hydrazine dihydrochloride and 0'18 M potassium iodide in 5 M hydrochloric acid, by heating to 80°C for 60 min. Np(IV) was precipitated as hydroxide, followed by several washings with distilled water and finally it was dissolved in 1 M hydrochloric acid. From this solution, the Np(C204)2.6H~O m was precipitated by the addition of 0.1 M oxalic acid. The crystalline green solid so obtained, was washed with water 2-3 times and then used for the solubility determination. The solubility measurements were made at 26 ~ 2°C, maintaining ionic strength of one and each sample in a sealed tube, was shaken for more than 72 hr. The tubes were centrifuged and the concentration of Np in the solution was determined radiometrically using Simpson type alpha proportional counter. The tubes were further shaken for 48 hr to ensure that the equilibrium was attained. The solubility value obtained after 120 hr equilibration was within 3-5 per cent of that obtained after 72 hr equilibration. The results are given in Tables 1 and 2 for solubility of Np(C204)2"6H20 in 1 M perchloric acid solution and 0"5 M perchloric acid and 0.5 M sodium perehlorate solution respectively. ~1) R. D. BAYBARZ,Report ORNL-3055. c-,~p. I. KONDRATOVand A. D. GEL'MAN, Radiokhimiya, 2, 315 (1960). ~a) j. M. TAUBE,ar. Inorg. Nucl. Chem. 15, 171 (1960). 799

x x x × x x X x ×

10 -8 10 -9 10 - a 10 - a 10 -~' 1 0 -~ I0 -s 10 -2 10 - t

2 x 10 - t 4 × 104

3 4 6 8'5 1"2 2 3 6 1

M

Oxalic a c i d concentration

1"392 1"856 2'784 3.944 5"568 9.280 1.392 2.784 4.640 9"280 1.856

x x x × x x x x × × x

10 -7 10 -7 10 - 7 10 -7 10 -7 10 -7 10 -e 10 -6 10 -6 10 -9 10 -5

Oxalate ion concentration [C~O42_ ]

3-89 3-06 2"34 1.77 1.43 1.32 1"36 1.60 2.10 3.41 6"25

Solubility o f neptunium S (moles]l) × 10 5 6'605 3"716 1"651 8.226 4"129 1"486 6.605 1"651 5-945 1"486 3"716

[Np ~÷] × × x × x × × x x x x

10 4 10 -6 10 -6 10 -7 10 -7 10 -7 10 - 8 10 - a 10 - 9 10 - 9 10 -1°

K, [C~O4j_p 5'889 8"235 14"17 21.52 34"63 88.83 205"9 969'1 3532"4 22947"5 168191"6

S X = [Np4+] 3-512 3-898 4-731 5-203 6.040 9"464 14"72 34"77 76.11 247"3 906'2

X1 × 10 -7

CONC"ENTRATtON ,U = 1, H + = 1, K,, = 1 . 2 8 × 10 - t 6

4-397 5-377 6"577 5"839 5-639 7.073 8"491 11.448 15.778 26"336 48.669

X~ -- fll [C~O42-] × 10 -18

X2 :

TABLE 1.--EQoILmRIUM SOLUBILITY OF N p ( C a O ~ ) ~ ' 6 H 2 0 IN 1 M PERCHI.,ORIC ACID AND VARIOUS OXALIC ACre

10-19

2.34 2"58 2-35 2.34 2-31 2-36

×

X2 -- f12

[C204 ~-]

Mean fla=2"35~

X9

o~ >

.v

O0 C, C'

3 5 8 1"5 3 6"0 1"0 2 3

× × × X × × × × ×

10 .3 10 -2 10 .3 10 -2 10 -~ 10 -2 10 -1 10 -1 10 -1

(M)

Oxalic acid concentration

5.07 8-45 13"52 25.35 50"7 101.4 169"0 338"0 507

x x x x x x × x ×

10 -7 10 -7 10 -7 10 -7 10 -7 10 7 I 0 -7 10 -7 10 -7

Oxalate ion concentration [C20,2--1

Np(CzO4)2.6H~O IN

1"44 1"27 1.31 1.596 2'43 4'24 6"83 12-8 19"5

Solubility of neptunium (moles/l) x 105 4.981 1,793 7.002 1"992 4-981 1"245 4'482 1'121 4'981

[Yp+q x x x × x x × × ×

10 - r 10 -7 10 - s 10 - s 10 -9 10 -9 10 -1° 10 lo 10 -11

K~ [C2042_p

OXALIC ACID CONCENTRATION ~tl =

TABLE 2.--EQUILIBRIUM SOLUBILITY OF

28.91 70.83 187.09 801.20 4878-54 34056-22 152387"32 1141837"6 3914876"0

X = [-N p + q

S

0"5,

10 -~a

5"505 8'264 13.764 31.566 96'204 335.850 90I'694 3378'214 7721'646

X t × 10 -7

1"28 ×

)(1 --/~1 [C~042_] x 10 -la

5.927 6.821 8.331 11.466 18.482 32.875 53"207 99"873 152'251

Xll -- ~g

2"22 2"39 2"61 2"63 2"70 2"77 2"86 2'81 2"91 Mean 2'66

[C20&] × 10 -1~

X a =

AND VARIOUS

(fl~ = 2.5 × 1 0 ' )

X~.

ACID, SODIUM PERCHLORATE

Ks ~

PERCHLORIC

1, n + ~

1 M

S

r-'

e-

g

o

N"

"O

G

©

802

B.M.L.

BANSAL and H. D. SHARMA

The dissociation constant of oxalic acid has been determined by potentiometric titration c4~ maintaining ionic strength of one using sodium perchlorate. The values of the first and second dissociation constants K ' and K" were calculated from the data as 1.08 × 10-1 and 4.76 × 10-~ respectively, The distribution coefficients of Np(IV) were determined at a constant ionic strength of one, hydrogen ion concentration 1 M, using perchloric acid and varying the oxalate ion concentration using 0.05 M theonyl trifluoro acetone (TTA) in benzene. The results are given in Table 2. The oxalic acid concentration could not be increased above 5 × 10-4 M as the value of the distribution coefficient could not be reproduced with accuracy.

Method of Calculations (a) Solubilitymethod. The general equilibrium may be represented in the following form: (4 -- 2x)H + + Np(C~O4)2"6H20 ~ N p ( C 2 0 , ) / ~ - ~ + (2 -- x)H2C20, + 6H20 Where x = 0, 1, 2, 3. . . . n

(1)

The solubility S of Np(C20~)~'6H20 is given by

S = ~ [Np(C204)=+~4-2=q X=0

~_,x[Np(C20,)=+~4-~q

or

d log S d log [H~C204]

(2)

:g

2 + .~ [Np(CzO4)~ +~-~q

(3)

Plotting log solubility against log oxalic acid concentration (Curve 1 of Fig. 1) it can be seen that 10 31

Experimenfol

~Colculofed

/

~2

if)

• iO ~5

I

2x10 3

I

I

I

I

r [ I

10-z 0xolic

B0 d acid concentrofion,

moMs/L

FIG. 1

the slope is --1 and changes to + 1 gradually as the oxalic acid concentration increases, indicating the presence of Np(C2OD ~+, Np(C~OD2 and Np(C~ODa~- species. From Equation (2) it follows

S=[Np+4](l+J~nflj[C20,2-] ~) j=l

~4~R. Gar,m and C. K. INC-,OLD,d.

Chem. Soc. 2153 (1931).

(4)

Oxalato complexes o f n e p t u n i u m (IV)

803

Where

~p(C2OOp~'-~']. fl~ =

[ N p + q [ C 2 0 ,_]j ,

j = 1, 2

(5)

and Solubility p r o d u c t K = [Np'~][C2Ofl-] 2

(6)

F r o m E q u a t i o n s (4) a n d (6), one obtains S[C30&] ~

K~ = [Np~+l[C20&l ~ =

n

1 +

~

(7)

/Sj [C2Ofl-] j

j=l

T h e concentration o f [C2042-] was calculated by the f o r m u l a given by CROUTHMEL a n d MARTIN/5~ Cox,1 k'k"

[C~O&]

[H+P + k ' [ H +] + k'k"

where Coxal = Total c o n c e n t r a t i o n o f oxalate in solution. [H ÷] = H y d r o g e n ion concentration. Following the FRONAEUS m e t h o d ~ o f calculation o f stability c o n s t a n t s according to which the function Xj is defined as: Xy=I,2,..

• ,n -[C2042-]

Where X3= 1 +

~ flj[C~O,2-]jand/~0= 5=1

1.

It c a n be s h o w n t h a t L i m X~

=/3j

[C~O~ 2-] ~ 0

T h e e x t r a p o l a t i o n o f these functions, as given in Tables 1 a n d 2 for 1 M a n d 0-5 M in H + ion c o n c e n t r a t i o n , to zero ligand c o n c e n t r a t i o n gave t h e values of t h e various /3j, t h e values being fll = 2.92 ~ 0.1 x 107, fl2 = 4-88 :t: 0'2 × 1013, fls = 2-35 -t- 0.4 x 10~gandfl, = 2"5 :~ 0'3 × 10 ~ f12 = 4.2 ~ 0'4 × 10 x3 a n d f13 = 2'8 ± 0.6 × 10 TM respectively a n d K s = 1.28 ~ 0"1 × 10 13 for b o t h the cases. (b) T T A extraction method. T h e distribution coefficient of N p 4+ in T T A was determined at various oxalic acid c o n c e n t r a t i o n u p to a m a x i m u m o f 5 x 10 -4 M, keeping a n ionic strength one a n d h y d r o g e n i o n c o n c e n t r a t i o n 1 M, Following DAY a n d SOUGHTOS m e t h o d m o f calculation, the values of/3~ a n d 132 so obtained are 1.54 + 0.2 × 109 a n d 1.63 ± 0.5 x 1015 respectively. TABLE 3.--DISTRIBUTION COEFFICIENTS OF NEPTUNIUM (IV) /* ~ 1, [H +] = 1

C o n c e n t r a t i o n of oxalic acid 0.00 4.44 8"88 13.32 17-76 26"64 35'52 44.40

× X X X × X ×

10 -5 10 -5 10 -5 10 -5 10 -5 10 -a 10 -~

Oxalate ion concentration [C2042-]

Distribution coefficient K~

-x x X × × x ×

1-5400 1"0990 0"7590 0"6353 0"4728 0'2848 0"1990 0"1348

2-06 4"12 6'18 8"24 12"36 16"48 20"60

10 9 10 -9 10 -9 10 -9 10 -9 10 -9 10 -9

K~8 - Ka

kd ° ----

1

-0.401 1"029 1"424 2.257 4'407 6-739 10.424

~5~ C. E. CROUTHMEL a n d D. S. MARTIN,J. Amer. Chem. Soc. 72, 1382 (1950). le,i S. ~RONAEUS,Acta Chem. Scand., 5, 859 (1951). trl R. A. DAY a n d S. W. STOUGHTON, J. Amer. Chem. Soc. 72, 5662 (1950).

I

Kd

[C20~ ~-1 1.95 2.50 2.30 2.74 3.57 4.09 5.06

x x x x x x x

108 108 109 108 109 108 108

× × × × X

10 aa 10 x8 10 t6 10 xe 10 xs

3"2 × 101~

7"9 x 10 xe

4"9 4"2 1-63 7-9 3"6

.

ft.

2"35 x 10 ~ 2-9 × 1019 -2"5 × 108a 6"2 × 10 m

1"0 × 10 ~4

5-0 x I0 m

fls

--(Present work) - - ( P r e s e n t work) --(Present work) 3-2 X 10 ~Ttll) __ts~

2'5 x 10 ~7~2~

5.0 x 1027tx°l

3.2 x 1024t9~

f14

(~ M. Bose a n d D. CHOWDHURY,J. Indian Chem. Soc. 31, N o . 21, 111 (1954) a n d 32, N o . 10, 673 (1955), as cited in reference (10). i10) F. A. ZAKHAROVA a n d A. I. MOSKWN, Russ. J. Inorg. Chem. 5, N o . 6, 592 (1960). c11) A. I. MOSKVlN, P h . D . Thesis, Institute o f Physical C h e m i s t r y o f t h e A c a d e m y o f Sciences o f the U S S R , M o s c o w (1957), as cited in reference (10).

pu4+

10 ~ 102 l0 s 108 10 a

2"9 2"5 1"54 5"5 5"5

Solubility Solubility Sol. ext. Solubility Solubility

x × × × x

3"5 × 10 s

Solubility

N p *+

4-1 x 108

Solubility

U4+

.

.

Th4+

.

Method

fll

0"5 M HC1 K ' = 5 " 9 x 10 -2 , K # = 6 . 4 x 10 -~ #=I[H ÷ ] = 1 " 0 x 10 -1 # = I [ H + ] = 0 " 5 ) K ' = 1"08 x 10 -1 / ~ = 1 [H + ] = 0 " 5 ) K " = 4 " 7 6 x 10 -4 Not known 0"75 N H N O 3 K ' = 8-2 x 10 -~, K" ---- 1-78 x I0-4

0"5 M HC1 K ' = 5"9 x 10 -2, K" ~ 6'4 × 10 -~

N o t specified

Other conditions

4.--STABILITY CONSTANTS OF OXALATE COMPLEXES OF M(IV) IONS OF THE ACTINIDE ELEMENTS

i4+

Metal ion

TABLE

m

v'

>

.w

Oxalato complexes of neptunium (IV)

805

The value o f / ~ has much larger uncertainties because of uncertainties in the determination of Kd values which may be due to precipitation of neptunium oxalate and thereby true equilibrium may not be attained. The values of both fll and f12 are much higher as compared to the ones obtained by the solubility method which cannot be attributed to the formation for mixed complex species and extraction thereof into organic phase, which would in turn give lower values of t31 and f12 rather than the higher ones. The higher values for fll and fl~ can be due to lowering of concentration of oxalate ions in aqueous phase. The solubility of oxalic acid in benzene is very small as compared to its solubility in the aqueous phase but in presence of TTA, it is yet to be seen whether in the TTA-oxalic acid system the concentration of oxalate ions remains unaffected. The behaviour of TTA in presence of various anions like chloride nitrate etc. is under investigation and it may be possible to find out the cause of higher values of fll and f12 obtained by using this method. The value of fljs so obtained by these two methods are compared in Table 3. As the data under similar conditions, on oxalato complexes of other tetravalent actinide elements are not available it is not possible to draw any conclusion regarding the relative stabilities of these complexes. However, the available data as presented in Table 4 shows that the stability of these complexes Th(IV), U(IV), Np(IV), Pu(IV) is more or less of the same order, if the data for U(IV) and Pu(IV), is recalculated by taking the values of K' and K" for oxalic acid under the experimental conditions and also presence of chloride ions. The precise calculations for the values of /~j for these ions are not possible.

Characteristic absorption spectra of the various oxalate complexes The characteristic absorption band of Np(IV) in 1 M perchloric acid solutions c8~ which occurs in the visible (red region) at 725 m~, has been observed in three distinct positions in Np(IV) solutions of various oxalate ion concentrations. Thus in 1 M perchloric acid when the ratio H~C~OJNp 4+ = 1, the characteristic absorption band is found at 730 m/~. When another mole of oxalic acid per mole of neptunium is added to the above solution, the absorption band shifts to 732.5 m/~. Even though the Np(IV) oxalate is very insoluble under these conditions, it was still possible to observe this shift because the precipitation is slow if the solution is not vigorously stirred. Similar shift was observed in the case of Pu(IV) wherein the absorption maximum shifts by 2 m# when oxalate ion forms tho complex.~° When Np(C20~)2-6H20 is added to a solution containing 1 M HCIO4 and 0"4 M H2C~O4 and the solution after equilibration for 72 hours is viewed with a spectrophotometer after centrifugation, the characteristic absorption band is found at 735 m/~. These shifts in spectra also support the existence of complex species.

Acknowledgements--The authors are grateful to SHRI S. K. PA'I'IL for his helpful discussions. L~ R. SJOBLOMand J. C. HINDMAN,J. Amer. Chem. Soe. 73, 1744 (1951). ~a~W. H. R~AS, The Transuranium Elements, NNES, Division IV, Vol. 14B Paper 4.9. McGraw-Hill New York (1949).