Talanta by Sushanta K Sahu

Talanta 51 (2000) 523 – 530 www.elsevier.com/locate/talanta

The synergistic extraction of thorium(IV) and uranium(VI) with mixtures of 3-phenyl-4-benzoyl-5-isoxazolone and crown ethers S.K. Sahu a, V. Chakravortty a, M.L.P. Reddy b,*, T.R. Ramamohan b a

Department of Chemistry, Utkal Uni6ersity, Bhubaneswar 751 004, India b Regional Research Laboratory (CSIR), Tri6andrum 695 019, India

Received 4 August 1999; received in revised form 30 September 1999; accepted 13 October 1999

Abstract The extraction of thorium(IV) and uranium(VI) from nitric acid solutions has been studied using mixtures of 3-phenyl-4-benzoyl-5-isoxazolone (HPBI) and dicyclohexano-18-crown-6, benzo-18-crown-6, dibenzo-18-crown-6 or benzo-15-crown-5. The results demonstrate that these metal ions are extracted into chloroform as Th(PBI)4 and UO2(PBI)2 with HPBI alone and as Th(PBI)4 · CE and UO2(PBI)2 · CE in the presence of crown ethers (CE). The equilibrium constants of the above species have been deduced by non-linear regression analysis. The addition of a CE to the metal chelate system enhances the extraction efficiency and also improves the selectivities between thorium and uranium. IR spectral data of the extracted complexes were used to further clarify the nature of the complexes. The binding to the CEs by Th(PBI)4 and UO2(PBI)2 follows the CE basicity sequence but with DC18C6 and DB18C6, steric effects become more important. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Synergistic extraction; Uranium(VI); Thorium(IV); 4-Acyl isoxazolone; Crown ether; Separation factors

1. Introduction The use of tri-n-butylphosphate on a commercial scale for the extraction of uranium(VI) from its ores is well known [1]. However, the selectivity of uranium over thorium is low [2]. Thus it has been a formidable challenge for the separation chemists to develop new extraction systems for the separation of actinides in fewer stages of * Corresponding author. Fax: +91-471-490186. E-mail address: reddy@csrrltrd.ren.nic.in (M.L.P. Reddy)

extraction. Montavon et al. [3] have reported a separation factor of 1000 between thorium(IV) and uranium(VI) with tetracarboxymethyl-p-tertbutyl Calix[4]arene. Synergistic extraction systems have also been applied to these elements and large increases in the extraction efficiency have been reported [4]. The extraction of thorium(IV) and uranium(VI) from nitric acid solution into ligroine solutions of 2-ethylhexyl phenylphosphonic acid, micellar dinonyl naphthalene sulphonic acid and a mixture of the two has been investigated by Otu [5] and reported a synergistic en-

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hancement factor of 66.9 and 10.8 for thorium and uranium respectively. b-Diketones such as 2-thenoyltrifluoroacetone (HTTA) and 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (HPBMP) are incapable of extracting uranium at pHB1.0. On the other hand, 3-phenyl-4-benzoyl-5-isoxazolone (HPBI), due to its lower pKa value (1.23) has been reported to be an efficient extractant for metal ions from acidic media [6,7]. The low pKa value of HPBI is due to the electron delocalisation by the isoxazolone group. This property makes isoxazolones an interesting class of b-diketones with potential application as reagents for the extraction and separation of metal ions from complexing strong acid media.This paper reports selective extraction of thorium(IV) over uranium(VI) using mixtures of HPBI and various crown ethers (CE) and the stoichiometry of the extracted metal complexes in the organic phase.

2. Experimental

2.1. Chemicals HPBI was synthesised in our laboratory by the benzoylation of 3-phenyl-5-isoxazolone (Aldrich) following the method of Krote and Storiko [8]. Elemental analysis calculated for C16H11NO3, C= 72.45; H= 4.15; N=5.28; found C= 72.27, H = 4.18, N = 5.17%; m.p. = 146°C. CEs were obtained from Aldrich Chemical Company and recrystallised from n-hexane. Stock solutions of thorium(IV) and uranium(VI) were prepared by dissolving appropriate amounts of Th(NO3)4 · 6H2O (LOBA-CHEMIE, India) and UO2(NO3)2 · 6H2O (LOBA-CHEMIE) in distilled water. One cubic centimetre of concentrated nitric acid was added to 100 cm3 of solution to suppress hydrolysis. The initial metal ion concentration was maintained at 1×10 − 4 mol/ dm3 for thorium(IV) and 1×10 − 3 mol/dm3 for uranium(VI) for all extraction studies. Extraction studies were carried out in 0.5 mol/dm3 nitric acid medium and 0.5 mol/dm3 NaNO3 to fix the ionic strength. It has been reported elsewhere that the extraction of sodium nitrate using CEs is negligi-

ble [9]. All organic phase solutions were prepared by dissolving weighed amounts of HPBI and desired synergist in chloroform and diluting to the required volume. Arsenazo(III) solution was prepared by dissolving 25 mg of the reagent in 25 cm3 of distilled water.

2.2. Sol6ent extraction procedure Distribution ratios were determined by shaking equal volumes of aqueous and organic phases for 60 min in a glass stoppered vial with the help of a mechanical shaker at 30391°K. The solutions were allowed to settle, centrifuged, separated and assayed spectophotometrically using a Hitachi 220 double beam microprocessor based spectrophotometer. Both thorium(IV) and uranium(VI) were determined spectrophotometrically as their Arsenazo III complexes in 1 mol/dm3 HCl solution at 660 and 656 nm respectively. The absorbances of the complexes were measured within 5 min of mixing. The metal concentrations in the aqueous phase were computed from the respective calibration graphs. The concentration of the metal ion in the organic phase was then obtained by a material balance. These concentrations were used to obtain the distribution ratio, D. All the computer programs were written in FORTRAN 77 and executed on a 32 bit mini-computer (HCl HORIZON III).

2.3. Preparation of metal complexes The metal complexes were prepared by following the general procedure: Stoichiometric amount of thorium nitrate or uranyl nitrate was added gradually to a well stirred solution of HPBI in chloroform and then refluxed for 1 h. To this, CE dissolved in chloroform was added and the entire mixture was refluxed for 4 h in order to ensure completion of the reactions. The precipitates formed were filtered, washed with chloroform and finally with diethyl ether and dried in a desiccator over fused calcium chloride. The Nicolet Impact 400 D IR spectrometer using potassium bromide pallet was used to obtain IR spectral data. The solutes studied were HPBI, DB18C6, B15C5, UO2(PBI)2 · DB18C6, UO2(PBI)2 · B15C5, Th(PBI)4, Th(PBI)4 · DB18C6 and Th(PBI)4 · B15C5.

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3. Results and discussion

3.1. Extraction of thorium(IV) and uranium(VI) with HPBI The extraction equilibria of thorium(IV) and uranium(VI) with the chelating extractant, HPBI, alone may be expressed as: Kex,0

Th4aq+ +4HPBIorg l Th(PBI)4org +4H+ aq 2+ 2 aq

Kex,0

(1)

+ 2HPBIorg l UO2(PBI)2org +2H

+ aq

(2)

where Kex,0 denotes the equilibrium constant. Since the partition coefficient (log PHPBI = 2.88) of HPBI was found to be very high, the complexa-

tion of isoxazolonate anion in the aqueous phase has been neglected. The distribution ratio, D0, of thorium(IV) is given by: [Th(PBI)4] D0 = (3) 4+ − 2 [Th ](1+b1[NO− 3 ]+ b2[NO3 ] ) where bi is the complex formation constant of Th4 + with nitrate ion in the aqueous phase. The values of the stability constants (log b1 = 0.1 and log b2 = 0.8) were obtained from the literature [10]. Then the distribution ratio of thorium(IV) can be written from Eq. (1) and Eq. (3) as: Kex,0[HPBI]4 (4) D0 = + 4 − 2 [H ] (1+b1[NO− 3 ]+ b2[NO3 ] ) Similarly the distribution ratio, D0, for uranium(VI) can be written as: Kex,0[HPBI]2 (5) [H+]2(1+b1[NO− 3 ]) The stability constant (log b1 = − 0.3) of uranium(VI) with nitrate ion was taken from the literature [11]. The extraction of thorium(IV) and uranium(VI) with HPBI alone in chloroform as a function of the hydrogen ion concentration and the extractant concentration, respectively, were studied. Plots (Fig. 1) of log D0 versus log [HPBI] had slopes of four for thorium(IV) and two for uranium(VI), indicating the extraction of the complexes Th(PBI)4 and UO2(PBI)2. These, in conjunction with the slopes of four and two observed for thorium(IV) and uranium(VI) respectively, with hydrogen ion variation (for the sake of conciseness, data not included) at constant HPBI concentration, confirm the above extraction equilibria. The formation of the above simple metal chelates was further confirmed by analysing the equilibrium data (presented in the Fig. 1) using Eq. (4) for thorium(IV) and Eq. (5) for uranium(VI). The equilibrium constants for the above complexes were determined by non-linear regression analysis (as described in our earlier publication [12]) and are shown in Table 1. The equilibrium constants thus calculated refer only to concentration quotients, calculated on the assumption that the activity coefficients of the species involved do not change significantly under the experimental conditions. D0 =

Fig. 1. Effect of HPBI concentration on the extraction of Th(IV) and U(VI). Aqueous phase, 0.5 mol/dm3 HNO3 +0.5 mol/dm3 NaNO3.

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Table 1 Two phase equilibrium constants of thorium(IV) and uranium(VI) — HPBI — crown ether — chloroform systems Metal ion

Th(IV) U(VI)

Log K1

Log Kex,0

8.71 90.03 1.6790.03

DC18C6

B18C6

DB18C6

B15C5

11.6390.03 4.9790.02

11.35 90.02 4.40 90.02

11.15 9 0.02 3.71 9 0.02

12.11 90.01 4.61 90.03

It is clear from Table 1 that the equilibrium constant (log Kex,0) value of thorium(IV) is about 7-fold higher than that of uranium(VI). A similar behaviour was reported by Mathur and Choppin [13] in the extraction of thorium(IV) and uranium(VI) with HTTA. Comparing the equilibrium constants of various chelating agents for the extraction of UO22 + , log KHDBzM = −6.20 [14], log KHTTA = −3.51 [14], log KHPMBP = − 1.15 [15] and log KHPBI =1.67 with their pKa values, dibenzoylmethane(HDBzM) = 9.35; HTTA= 6.25; 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (HPMBP)=4.10; HPBI=1.23, it can be concluded that the equilibrium constant value increases as pKa value decreases. It is clear from the results (Fig. 1) that the selectivity between thorium(IV) and uranium(VI) increases with increase in the concentration of HPBI at constant nitric acid concentration (DTh =9.55, DU =0.01 at 0.01 mol/dm3 HPBI, separation factor (S.F.)= 955; DTh =152.5, DU = 0.05 at 0.02 mol/dm3 HPBI, S.F.=2056; DTh = 2444, DU =0.20 at 0.04 mol/dm3 HPBI, S.F.= 1.22× 104).

The distribution ratio, D, of the synergistic extraction system is given by: D=

[Th(PBI)4]+ [Th(PBI)4 . CE] − 2 [Th4 + ]{1+b1[NO− 3 ]+ b2[NO3 ] }

(8)

From Eqs. (3), (7) and (8): K=

− 2 (D− D0)[H+]4{1+b1[NO− 3 ]+ [NO3 ] } 4 [HPBI] [CE]

(9)

Taking logarithms: log K= log (D− D0)− 4log [HPBI]− log [CE] +4log [H+] + log {1+ b1[NO3− ]+ b2[NO3− ]2}

(10)

The organic phase adduct formation reaction is represented as: KCE

Th(PBI)org + nCEorg l Th(PBI)4 . nCEorg

(11)

where KCE, the organic phase adduct formation constant, is given as: KCE =

[Th(PBI)4 . nCE] [Th(PBI)4][CE]n

(12)

From Eqs. (1), (7) and (12):

3.2. Extraction of thorium(IV) and uranium(VI) with mixtures of HPBI and crown ether The extraction equilibrium of thorium(IV) in presence of CE, may be represented as: Kn

Th4aq+ +4HPBIorg +nCEorg l Th(PBI)4 . nCEorg + 4H+ aq

(6)

The overall extraction constant, Kn, is given as: + 4

[Th(PBI)4 . nCE][H ] Kn = [Th4 + ][HPBI]4[CE]n where n= 0 or 1.

(7)

KCE =

Kn Kex,0

(13)

where n =1. For a synergistic extraction system employing a neutral donor, CE, the extraction equilibrium of U(VI) may be represented as: UO22 +aq + 2HPBIorg Kn

+ nCEorg l UO2(PBI)2 . nCEorg + 2H+ aq

(14)

where n =0 or 1. Then the distribution ratio, D, of a synergistic extraction system for uranium(VI) is given by:

S.K. Sahu et al. / Talanta 51 (2000) 523–530

[UO2(PBI)2]+ [UO2(PBI)2 . CE] [UO2 + ]{1 + b1[NO− 3 ]}

527

(15)

From Eqs. (5), (14) and (15): K=

(D − D0)[H+]2{1 + b1[NO− 3 ]} 2 [HPBI] [CE]

(16)

The organic phase adduct formation reaction is represented as: KCE

UO2(PBI)2org +nCEorg l UO2(PBI)2 . nCEorg (17) where KCE, the organic phase adduct formation constant, is given by: Table 2 Synergistic enhancement factors of thorium(IV) and uranium(VI) with HPBI (0.01 mol/dm3) in the presence of crown ether (0.005 mol/dm3) Extraction system

HPBI+DC18C6 HPBI+B18C6 HPBI+DB18C6 HPBI+B15C5

Synergistic enhancement factor

Fig. 3. Effect of HPBI concentration on the extraction of U(VI) at constant CE concentration. Aqueous phase, 0.5 mol/dm3 HNO3 +0.5 mol/dm3 NaNO3; CE, 0.006 mol/dm3.

Th(IV)

U(VI)

KCE =

5.0 3.0 2.0 14.0

13.0 4.0 2.0 7.0

Fig. 2. Effect of HPBI concentration on the extraction of Th(IV) at constant CE concentration. Aqueous phase, 0.5 mol/dm3 HNO3 + 0.5 mol/dm3 NaNO3; CE, 0.005 mol/dm3.

Kn Kex,0

(18)

where n =1. The extraction of thorium(IV) and uranium(VI) from 0.5 mol/dm3 nitric acid + 0.5 mol/dm3 NaNO3 solutions with mixtures of HPBI [0.002– 0.009 mol/dm3 for thorium(IV)]; 0.04–0.1 mol/ dm3 for uranium(VI) and DC18C6 [0.001–0.008 mol/dm3 for thorium(IV) and uranium(VI)], B18C6 [0.002–0.01 mol/dm3 for thorium(IV) and uranium(VI)], DB18C6 [0.005–0.03 mol/dm3 for thorium(IV) and uranium(VI)] or B15C5 [0.002– 0.01 mol/dm3 for thorium(IV) and uranium(VI)] in chloroform were studied. It was found that extraction of these metal ions into chloroform with CE alone was negligible under these experimental conditions. However, with mixtures of HPBI (0.01 mol/dm3) and CEs (0.005 mol/dm3) considerable synergistic enhancement (synergistic enhancement factor=D/D0 where D= distribution ratio with HPBI+CE; D0 = distribution ratio with HPBI alone) in the extraction of these metal ions was observed (Table 2). It is clear from plots (Figs. 2 and 3) of log (D–D0) versus log [HPBI] that at constant CE [0.005 mol/dm3 for thorium(IV) and 0.006 mol/ dm3 for uranium(VI)] and constant nitric acid (0.5

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mol/dm3) concentrations, only four HPBI moieties in the case of thorium(IV) and two HPBI moieties in the case of uranium(VI) are attached to the synergistic species extracted into the organic phase. The plots (Figs. 4 and 5) of log (D−

Table 3 Stability constants for the organic phase synergistic reactions of thorium(IV) and uranium(VI)–HPBI chelates with crown ether in chloroform Metal ion Log KCE DC18C6 Th(IV) U(VI)

Fig. 4. Effect of CE concentration on the extraction of Th(IV) at constant HPBI concentration. Aqueous phase, 0.5 mol/dm3 HNO3 + 0.5 mol/dm3 NaNO3; HPBI, 0.006 mol/dm3.

Fig. 5. Effect of CE concentration on the extraction of U(VI) at constant HPBI concentration. Aqueous phase, 0.5 mol/dm3 HNO3 + 0.5 mol/dm3 NaNO3; HPBI, 0.06 mol/dm3.

B18C6

DB18C6

B15C5

2.92 9 0.03 2.6490.02 2.44 90.02 3.419 0.01 3.31 90.02 2.73 90.02 2.049 0.02 2.9590.02

D0) versus log [CE] at constant HPBI concentrations gave slopes of unity for both metal ions indicating the participation of only one CE molecule in the extracted species in the synergistic extraction reaction. This, in conjunction with the slope of four and two observed for the extraction of thorium(IV) and uranium(VI) respectively with H+ variation experiments at constant HPBI+ CE, indicates the extraction of the complexes Th(PBI)4 · CE and UO2(PBI)2 · CE. Since the partition coefficients of the CEs used (log KD, DC18C6 =3.52 [16], log KD, DB18C6 = 3.9 [17] and log KD, B15C5 = 2.5 [17]) are known to be quite large, no correction is necessary for the partitioning of these CEs in the aqueous phase. It was assumed that this is also true for B18C6. The above extracted species were further confirmed by analysing the equilibrium data using Eq. (9) for thorium(IV) and Eq. (16) for uranium(VI). The equilibrium constants of the above species for these metal ions were deduced by non-linear regression analysis and are given in Table 1. The stability constant, KCE, for the organic synergistic reaction of Th-HPBI chelate or UHPBI chelate with various CEs were calculated and are given in Table 3. The sharp decrease in the complexation for both uranium(VI) and thorium(IV) from DC18C6 to B18C6 and to DB18C6 mostly reflects the increase in steric effects and decrease in basicity of CEs. A similar trend was observed by Mathur and Choppin [13] on the extraction of thorium(IV) and uranium(VI) with HTTA in the presence of these CEs. The maximum in log KCE for Th(PBI)4 · B15C5 is likely to

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be associated with a good fit of Th4 + (ionic diameter, 0.20 nm [18]) in the cavity of the B15C5 (cavity size, 0.17 – 0.22 nm [19]). On the other hand, the UO22 + (ionic diameter, 0.15 nm for U in UO22 + [18]) values increase regularly to DC18C6 (cavity size, 0.26 – 0.32 nm [19]). It can be concluded that the relationship between the cavity size and the ionic diameter is not the determining factor in the complexation of UO22 + . The unusual behaviour observed in the present investigations of CEs with f-elements in the presence of HPBI, may be a multifaceted problem, as pointed out by Mathur and Choppin [13] with the evidence of thermodynamic, NMR and hydration data in the extraction of f-element cations with crown ehers in the presence of HTTA. The CEs do exhibit synergistic extraction behaviour but the fit of the cation and the crown cavity size does not seem to be a significant factor in HPBI complexes. Further, these metal ions may interact with only a few of the potential donor oxygens and steric effects are probably significant in establishing this number. The adduct formation constant (log KCE = 3.31) of UO2(PBI)2 · DC18C6 is higher than that of UO2(DBzM)2 · DC18C6 (log KCE =2.52) and UO2(TTA)2 · DC18C6 (log KCE =3.20) [14]. It is well known that stable adduct formation reaction is usually brought out by strong acidic extractants (pKa of HPBI =1.23; HTTA =6.25; HDBzM = 9.35) as observed in the present systems. Table 4 gives the S.F. between thorium and uranium defined as the ratio of respective distribution ratios in the HPBI (0.01 mol/dm3) and Table 4 Separation factors between thorium(IV) and uranium(VI) with HPBI (0.01mol/dm3) and HPBI (0.01 mol/dm3)+CE (0.005 mol/dm3) systems Extraction systems

HPBI HPBI+DC18C6 HPBI+B18C6 HPBI+DB18C6 HPBI+B15C5

529

HPBI (0.01 mol/dm3)+ CE (0.005 mol/dm3) systems. Although addition of DC18C6 or B18C6 to the chelate system can bring a decrease in S.F. values, it is interesting to note that the addition of B15C5 or DB18C6 improves the S.F. values. The reasons for such higher separation factors between these metal ions with HPBI alone and with mixtures of HPBI and CE are worth investigating. Thus HPBI and HPBI+CE systems may be useful for the separation of actinides from nuclear wastes.

3.3. IR spectral data The IR spectra of the extracted complexes of the macrocyclic polyethers are quite complex and are similar. The stretching frequency of the C O group of HPBI has shifted from 1694 to 1627– 1620 per cm in the complexes, which indicates that the carbonyl group is involved in bonding. The other strong absorption occurring around 939–933 per cm may be assigned to n(O U O) of UO22 + [14]. The bands at 1190–1180 per cm are assigned to n(C–O) of polyether, suggesting the involvement of oxygen of CE in the adduct formation. 4. Conclusion The extraction equilibria of thorium(IV) and uranium(VI) with HPBI and with mixtures of HPBI and CEs have been investigated. The CEs do exhibit synergistic behaviour but the fit of cation in the crown cavity size does not seem to be a significant factor in these HPBI complexes. Very high separation factors have been observed between uranium(VI) and thorium (IV) when extracted from complexing strong acid solution using HPBI or HPBI+ CE systems.

Separation factor (S.F.) Th/U

Acknowledgements

955 380 758 1131 2185

This work was supported by Council of Scientific and Industrial Research, New Delhi. The authors wish to thank Dr G. Vijay Nair, Director, Regional Research Laboratory, Trivandrum for giving permission to publish this work.

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