Solvent extraction of some diprotic nitroaromatic acids and of their complexes with some actinides—I

J. inorg,nucl.Chem.,1971,Vol.33, pp. 839to843. PergamonPress. Printedin Great Britain

SOLVENT EXTRACTION OF SOME DIPROTIC NITROAROMATIC ACIDS AND OF THEIR COMPLEXES WITH SOME ACTINIDES---'I DISSOCIATION C O N S T A N T S A N D SOLVENT EXTRACTION OF 3-NITRO-p-CRESOL-5-SULPHONIC ACID WITH TRIBUTYLPHOSPHATE M. BERAN Department of Chemistry, Nuclear Research Institute of Czechoslovak Academy of Sciences, l~e~ near Prague, Czechoslovakia

(Received 13 April 1970) A b s t r a c t - T h e second dissociation constant, K2H = an[A2-]/[HA-], for 3-nitro-p-cresol-5-sulphonic acid (HzA) in 1.0M (H, Na)Cl at 25°C has been determined by potentiometric titration and u.v.-visible spectrophotometry as (2.71 +- 0-07). 10 -7 and (2.72-+ 0.16). 10 -7, respectively. The solvent extraction of H2A from l M (H, Na)Cl with TBP in n-dodecane at 25°C has also been studied. The extractable species appears to be H2A-3TBP, its apparent formation constant, /(n,A.aTBp= [H2A'3TBP]/aH [HA-I[TBP] a, is 2"27 +-0"06. The value of the first dissociation constant, K1H = aH[HA-]/[H2A], has been calculated from the extraction data as 2.06 -+0.23.

INTRODUCTION

AN EFFORTto improve the separation factors in solvent extraction TB P-processes for the recovery of irradiated nuclear fuel has resulted in new actinide complexes of diprotic nitroaromatic acids which can be effectively extracted with TBP[1]. We mean by "diprotic nitroaromatic acid" an aromatic hydrocarbon which is substituted with two acidic groups in the ortho-position and with one or more nitro-groups in the core. This group of compound can be represented, for example, by some nitrophenolsulphonic, nitrosulphobenzoic and nitrosalicylic acids. This paper presents the results of more detailed study of 3-nitro-p-cresol-5sulphonic acid and its solvent extraction with tributylphosphate. EXPERIMENTAL Reagents and apparatus. The monopotassium salt of 3-nitro-p-cresol-5-sulphonic acid (KHA) containing 8 molar per cent of free acid (Organic Synthesis Research Institute, Pardubice-Rybitvi) was used without further purification. Tributylphosphate was purified by vacuum distillation and diluted with pure n-dodecane. Distilled, carbon dioxide free water was used for potentiometric titrations. Carbonates were removed from sodium hydroxide by the method of S6rensen. All other reagents were of analytical grade. A Radiometer PHM-4c pH-meter with combined glass-calomel electrode G K 2024 C was used for the potentiometric measurements. U.V.-visible spectra were recorded using a Spectrophotometer VSU-I Zeiss Jena with 1 cm quartz glass cells. Spectrophotometry. U,V. and visible absorption spectra of 10-4M K H A solutions in I'0M sodium chloride were measured against a I-0M sodium chloride solution blank. The absorbance of K H A solutions at variable pan value was measured at 234,364 and 447 nm. 1. M. Beran, Paper at 5th Int. Conf. on Solvent Extraction Chem. Jerusalem (1968). 839

840

M. BERAN

Potentiometric titrations. 20 ml of 0'01M K H A solution in 1.0M sodium chloride was titrated with 0.464M sodium hydroxide in a nitrogen atmosphere. The N.B.S. standard buffer solutions [2] were used for standardization of the pH-meter. Solvent extraction. The aqueous phase containing 0.001-0.02M K H A in 1.0M (H, Na)CI was shaken with an equal volume of TBP in n-dodecane until equilibrium was reached. The distribution coefficient of H2A was determined spectrophotometrically by measurements at 364 nm in the initial and equilibrium aqueous phase. The hydrochloric acid concentration in the equilibrium aqueous phase was calculated from the hydrochloric acid added neglecting its extraction into TBP [3]. CALCULATIONS The hydrogen ion activity, an, in 1.0M (H, Na)CI at higher hydrogen ion concentration (h > 10-2 M), has been calculated, when the direct determination with glass electrode is impossible, by using the relation a H = h ' fHc~values from the equation: IOgfHCl = IOgfHcl¢o~-- OtHCICNaCI

( 1)

describing the system of two strong electrolytes at constant total concentration are available[4]. For our systemfncl~o~ = 0.809 and anel = 0"039. (i)Spectrophotometric data treatment. The known graphical method[5] for the calculation of dissociation constants from the absorbance-pan plot at constant wave length has been used. (ii) Potentiometric titration data treatment. The average effective charge of the anions of N-protic acid HNA in the aqueous solution is defined as: n[HN_,A"-] o

(2) CA

where n is the degree of dissociation and CA the total concentration of acid in the solution. This quantity can be calculated from the potentiometric titration of a salt of the acid with sodium hydroxide. The following equation can be obtained from Equation (2) using the electroneutrality equation and neglecting the hydroxyl ion concentration: = CNaOH'~-CM-I-OH

(3)

CA

CNaor~and cM are the analytical concentrations of the sodium hydroxide added and cation from the titrated salt, respectively. l f t h e dissociation constant K, H .~ K~_~, we can easily calculate the K r~value: K.H = a . Z - n ( n z l ).

(4)

(iii) Solvent extraction data treatment. Assuming that only neutral species H2A can be extracted with TBP and considering the low acidity of the second acidic group, the distribution coefficient at higher hydrogen ion activity is given by: DA = ~ = KH,^.crw aHITBP] 8 cA 1 + an~K1n

(5)

where the symbols with stroke above refer to the organic phase and /~H,^.erav = [ ~ ] / a n [HA-][T---"ffP]'. As the total concentration of H2A in the organic phase is negligible compared to the total TBP concentration, the approximation [ T I ~ ] ~ PTaPwill be used in the following calculations. 2. 3, 4. 5.

R.G. Bates,Determination o f p H p. 76. Wiley, New York (1964). A. S. Kertes,J. inorg, nucl. Chem. 14, 104 (1960). R.A. Robinson and R. H. Stokes, Electrolyte Solutions p. 451. Butterworths, London (1968). B. Bud~insk~ and K. Haas,,4cta Chim.Acad. Sci. Hung. 39, 7 (1963).

Solvent extraction of some diprotic nitroaromatic acids

841

/~HzA'mTBPand Kl H are calculated by repeated back substitution to obtain the constant values from initially approximate values. Refering these values back to Equation (5), a theoretical distribution coefficient can be found. Comparison of an average square deviation between logarithms of the calculated and experimental distribution coefficients: (6)

O"cale-exv= -+-~ ~ (log DA,eale-- log OA,exp)2

m where m is number of experimental points, with an average square deviation of the experiment: °'exp= -- ~ / ~ (log/)A,exp- - log DA,exp)2

(7)

where/)A.exp is an arithmetical average of n parallel measurements, can then be used to evaluate the agreement between theory and experiment. This is satisfactory when Orealc.exo/O'exv < 2. RESULTS AND DISCUSSION (i) Spectrophotometry. T h e u.v. a n d p a r t l y v i s i b l e s p e c t r a o f K H A a q u e o u s s o l u t i o n s at t w o different p a a v a l u e s a r e s h o w n in Fig. 1. T h e c u r v e s 1 a n d 2 r e p r e s e n t t h e s p e c t r a o f H A - a n d A 2- s p e c i e s , r e s p e c t i v e l y . T h e s e c o n d d i s s o c i a t i o n c o n s t a n t , K2 " = a . [ A S - ] / [ A H - ] , h a s b e e n g r a p h i c a l l y d e t e r m i n e d f r o m t h e a b s o r b a n c e vs. Pall p l o t s at t h r e e different w a v e l e n g t h s (Fig. 2), as ( 2 . 7 2 ± 0 . 1 6 ) . 10 -~. (ii) Potentiometric titration. T h e r e s u l t s o f t h e c a l c u l a t i o n o f K2" a c c o r d i n g to E q u a t i o n s (3) a n d (4) a r e g i v e n in T a b l e 1. T h e r e s u l t i n g v a l u e , (2.71 ___0.07). 10 - r , I

I

I

I'0

o JCl

0.5

0.0

-

I

I

3OO

40O

I

500

Wavelength , n m

Fig. l. Absorption spectra of potassium 3-nitro-p-cresol-5-sulphonateaqueous solutions; CA= 10-4M, 1 -- pa. = 3, 2-- pa, = 9.

842

M. B E R A N I

i

i

f

i

bO

--

-

i-

N

°

0.5

•

s° 0.0

°l'~°

4

,,,,~'~"1 Q,

ql~"'-,~

I

6 pa.

5

uJ nil

I

7'

I

8

~

l

9

Fig. 2. Determination of the second dissociation constant of 3-nitro-p-cresol-5-sulphonic acid from spectrophotometfic measurements; CA = 10-", ©--234 rim, t D - 3 6 4 rim, 0 447 nm.

is in good agreement with that obtained from the spectrophotometric measurements. (iii) Solvent extraction. It can be seen from Table 2, that the distribution coefficient of 3-nitro-p-cresol-5-sulphonic acid is independent of its equilibrium Table 1. Potentiometric titration of potassium 3-nitro-p-cresoi-5-sulphonate ~NaOH

K2 H

(/.t 1)

pall

;~

(X 107)

160 180 200 220 240 260 280 300 320 340 360 380 400 420 440

6' 16 6"27 6"36 6"45 6.54 6'62 6"71 6"79 6"89 6-98 7.09 7"21 7'36 7"55 7"84

1.29 1"34 1"39 1.44 1.48 1"53 1"58 1"62 1"67 1"72 1"76 1"81 1'86 1"91 1-95

2'82 2"79 2.79 2"78 2"68 2"69 2"69 2-61 2.64 2"67 2"57 2"62 2'69 2"86 2"72

K~H = (2"71 --+0'07). l0 -7.

Solvent extraction of some diprotic nitroaromatic acids

843

concentration in the aqueous phase. This indicates that aggregates of H2A are probably not present in the organic phase and that Equation (5) fits the treatment of the extraction results. Although there are only three points in the plot log DA vS. log (TaP at constant hydrogen ion activity, the straight line fitted to these points indicates a slope of c a . 3. Thus, it can be assumed in the next calculations, that the extracted complex is probably solvated with three molecules o f T B P (s = 3). Calculation of/~H2A3TBP a n d K1 H according to Equation (5) gives the values 2.27 -- 0.06 a n d 2.06 _ 0.23, respectively. The values of the distribution coefficients calculated by refering these constants to Equation (5) are given in the last but one column in Table 2. The average square deviation between the calculated and experimental distribution coefficients (O'catc-exo), obtained from Equation (6), is _ 0.011. As the accuracy of the distribution measurements depends on the absolute quantity of the distribution coefficient in our case, three series divided into the whole experimental region with five parallel points in each have been used to calculate the average square deviation of the measurement (O'exo) according to Equation (7). It has a value o f _+ 0.011 and O'calc.exp/Orexp is then equal to unity. Thus, the formation of H2A.3TBP satisfactorily fits the experimental results. Table 2. Solvent extraction of 3-nitro-p-cresol-5-sulphonic acid with T B P

DA.exl,

CA (X 103)

aH

0-43 0'43 0-43 0-43 0"43 0'73 0'69 0"68 0"75 0"71 1"38 1'45 1"42 1"40 1"39 1"70 1"82 1"78 1'78 1"67 0"053 0"18 0"98

0-70 1"4 3"5 7-0 14 0"58 1-1 2"7 5"5 11 0"41 0"81 1"9 4"0 8'0 0"37 0'71 1"8 3"6 7"5 9-5 8'5 5"5

0' 153

CTBp

1" 1

0'270

0"595

0"808

0'389

0'44 0"66 1"1

/~H2A.3TBP

gl a

2"27 2'27 2'27 2'27 2"27 2"30 2'18 2" 15 2"37 2"24 2"26 2"37 2"32 2"29 2"27 2"22 2"37 2"32 2"32 2" 18

2"04 2'04 2"04 2"04 2'04 2"30 1"48 1"35 3'08 1"81 1"96 2"48 2'24 2"09 2"03 1"85 2"37 2"17 2"17 1-75

2'26

1"95

DA.cal,.

0"43

0"72

1"39

1"74

0"063 0"21 0'98

log

OA talc

0 0 0 0 0 -- 0-006 +0-018 + 0"025 --0"017 +0"006 +0"003 --0'018 --0'009 -- 0"002 0 + 0"010 --0'019 --0"010 --0-010 + 0"017 +0"075 +0.067 0

/~H2A'3TBP= 2'27 -----0"06:K1H = 2.06 _--_0"23.

Acknowledgement-The author gratefully acknowledges valuable discussions with Dr. S. Havelka and the experimental assistance of Mr. P. Franta.