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Solvent extraction of cerium(III) from aqueous thiocyanate solutions by neutral organophosphorus compounds
135
Journal ~/ AIloys and Compounds, 192 (1993) 135-137 JALCOM 2088
Solvent extraction of cerium(III) from aqueous thiocyanate solutions by neutral organophosphorus compounds Taichi Sato* Faculty of Engineering, Shizuoka University, Hamamatsu (Japan) and Queen's University, Metallurgical Engineering Department, Kingston, Ont. (Canada)
Keiichi Sato FaculO, ~?/"Engineering, Shizuoka University, Harnamatsu (Japan)
Abstract The extraction of cerium(III) from aqueous solutions containing potassium thiocyanate by tributyl phosphate (TBP) or trioctyl phosphine oxide (TOPO) in kerosene or benzene has been investigated under different conditions. IR spectral studies have been carried out on the organic extracts. The distribution coefficient of cerium(III) for TBP increases monotonously with increasing aqueous thiocyanate concentration. As a result, it is found that the equilibrium equation for the extraction by TBP can be expressed as follows: Ce3+(aq)+ 3NCS ~(aq) Ce(NCS)3(aq), Ce(NCS)3(aq) + 4TBP(org) , " Ce(NCS)3.4TBP(org). A similar result was also obtained for the extraction by TOPO, although the extraction efficiency of TOPO is higher than that of TBP. The IR results indicate that the thiocyanate ion coordinates to the cerium ion through the N atom and that the cerium ion combines with the phosphoryl O atom of the extractants.
I. Introduction Thiocyanate has been utilized as a selective complexing agent in the spectrophotometric determination of metals and as a selective extraction agent for the separation of metals [1-4]. We have also investigated the extraction from aqueous solutions containing potassium thiocyanate of divalent metals [5, 6], aluminium (III) [7], thorium(IV) [8], titanium(IV) [9], vanadium (IV) [9, 10], zirconium(IV) [9] and uranium(VI) [11] by trioctylmethylammonium chloride (TOMAC) and of lanthanum(III) [12] and yttrium [13] by tributyl phosphate (TBP). The present paper extends the work to the extraction of cerium(III) from aqueous solutions containing potassium thiocyanate by TBP and trioctyl phosphine oxide (TOPO) in relation to the separation of rare earth elements.
2. Experimental details 2. 1. Chemicals TBP (Daihachi Chem. Ind. Co., Ltd.) was purified by the usual method [14] and TOPO ( H o k k o Chem. Ind. Co., Ltd.) was used without further purification. TBP
*Present address: 3-14-1, Daita, Setagaya-ku, Tokyo 155, Japan.
0925-8388/93/$6.00
and TOPO were diluted and dissolved in kerosene and benzene respectively. The stock solution of cerium was prepared by dissolving its chloride (CeC13.7H20) in an aqueous solution of potassium thiocyanate. Generally the aqueous metal concentration was 1 g dm 3 cerium chloride, except for the loading test. Other chemicals were of analytical reagent grade.
2.2. Extraction and analytical procedures Equal volumes (15 cm 3 each) of the aqueous and organic phases were shaken for a definite time in a 50 cm 3 stoppered conical flask in a thermostatted water bath at 20 °C, except for the experiments on the temperature effect. Preliminary experiments showed that the equilibrations were complete in 10 min. The mixture was centrifuged and separated and then aliquots of both phases were pipetted to determine the distribution coefficient (Ea°, the ratio of the equilibrated concentration of cerium in the organic phase to that in the aqueous phase). The cerium in the organic phase was stripped with 1 tool dm 3 hydrochloric acid. The concentration of cerium in the aqueous solution was determined by ethylenediamine tetraacetic acid titration using xylenol orange (XO) as indicator [15]. The concentration of thiocyanate in the organic phase was determined as described previously [6] and the water content of the organic phase by the K a r l - F i s h e r titration method.
,,~" 1993 - Elsevier Sequoia. All rights reserved
136
T. Sato, K. Sato /Extraction of cerium(IIl) from aqueous thiocyanate solutions
2.3. IR and nuclear magnetic resonance ( N M R ) spectral measurements
IR spectra of the organic extracts were determined on JASCO Models IRA-I (4000-650cm -~) and IR-F (700-200 cm-l) using a capillary film between thallium halide plates or polyethylene films. N M R spectra were obtained for the organic extracts dissolved in carbon tetrachloride using a JEOL Model JNM-PMX6OSO N M R spectrometer with tetramethylsilane as internal reference.
3.2. IR spectra
3. Results and discussion 3.1. Extraction isotherm by TBP
The extraction of cerium(III) from aqueous solutions containing potassium thiocyanate .at different concentrations by TBP in kerosene gave the result that the distribution coefficient increases monotonously with increasing aqueous thiocyanate concentration. In this extraction system, log-log plots of E ° v s . [NCa-]ini t aq give straight lines with a slope of about 3, indicating that three thiocyanate groups combine with a cerium ion. In the extraction of cerium(III) from aqueous solutions containing potassium thiocyanate with 0.96%19% (v/v) TBP in kerosene, log-log plots of E ° vs. [TBP]init at a constant concentration of potassium thiocyanate of 1 - 5 m o l d m -3 give straight lines with a slope of about 4, e.g. slopes of 3.8, 3.7 and 3.7 at concentrations of potassium thiocyanate of 1, 2 and 5 mol dm -3 respectively. It is thus considered that the cerium(III)-thiocyanato complex is associated with four molecules of TBP. Accordingly, if we assume that the extraction of cerium(III) from aqueous solutions in the presence of thiocyanate by TBP proceeds through solvation, the equilibrium equation can be expressed as Ce3+(a) + 3NCS-(a) ~
the composition in the organic phase approaches a limiting value of the molar ratio [Ce]:[NCS]:[TBP]: [H20] of 1:3:4:0, which suggests the formation of the species Ce(NCS)3.4TBP with increasing aqueous cerium concentration. Similarly, species such as Ce(NCS)3.4TBP and Y(NCS)3.4TBP are also formed in the extraction of lanthanum(III) [12] and yttrium (III) [13] by TBP. According to E1-Yamani and Shabana [4], however, the solvation numbers of TBP with trivalent lanthanum, cerium and yttrium are five, four and three respectively.
" Ce(NCS)3(a )
Ce(NCS)3(a) +4TBP(o) .
" Ce(NCS)3.4TBP(o)
(1) (2)
where (a) and (0) represent aqueous and organic phases respectively. This is supported by the results for the continuous variation in [Ce]org as a function of the mole fractions [Ce]initaq/([Ce]inita q q - [ N C S - ] i n i t a q ) and [Ce]initaq/([Ce]initaq+[TBP]init) at fixed total concentrations of [Ce]i,it aq + [NCS-]init aq = 0.1 mo1 dm -3 and [Ce]i,it ,q + [TBP]init = 0.1 mol dm -3 respectively; ICe]ors shows maxima at the former and latter mole fractions of 0.25 and 0.2 respectively. In addition, the stoichiometry of the extracted species was examined by a loading test of cerium in the organic phase. As a result, it was found that in the extraction of cerium(III) from aqueous solutions containing 5.0 mol dm -3 potassium thiocyanate with 9.4% TBP in kerosene, the variation in
The organic extracts of cerium(III) from aqueous solutions containing 5 . 0 m o l d m 3 potassium thiocyanate with 9.4% TBP in benzene were examined by IR spectroscopy. With increasing aqueous cerium concentration the spectra of the organic extracts show the following features in addition to the absorptions of the alkyl groups due to TBP: the OH stretching band at 3520-3400cm-~ and the OH bending band at 1640cm 1 decrease in intensity, indicating that the extracted species contains no coordinated water; the P - - ~ O stretching band of water-saturated TBP at 1270 cm l shifts to the lower frequency of 1220 cm J; the C e - O stretching band appears at 360 cm i; the CN stretching band appears at 2050cm -~ and the NCS bending band at 480 cm -j, implying that the thiocyanate ion coordinates to cerium through the N atom. The IR results confirm that cerium extracted into TBP by the solvating reaction (2) is bonded to the phosphoryl O atom. This suggests that the extracted species exists as the complex Ce(NCS)3.4TBP, displaying a coordination number of seven [16], in agreement with the results obtained in the extraction of lanthanum(III) [12] and yttrium(III) [13] by TBP. The organic phases for the repeated extraction of cerium(III) from aqueous solution containing 5.0 mol dm -3 potassium thiocyanate with 93% TBP were examined by N M R spectroscopy. The N M R spectrum for water-saturated TBP shows a sharp peak at 9.09 (z value) in a doublet due to the methyl protons, a multiplet at 8.50 which arises from most of the methylenic protons, and a quartet at 5.92 resulting from the methylenic protons attached to C atoms immediately adjacent to O atoms; in addition, the water proton resonance at 6.60 indicates the formation of the compound TBP.H20 [17]. In the extraction of aqueous solution containing potassium thiocyanate alone, the water proton signal is shifted to 6.48, confirming the formation of a hydrogen bond in the compound HNCS.TBP [ 18]. The organic solution from the extraction of cerium(III) reveals a shift of the water proton resonance to a lower field and its signal decreases in intensity, corresponding to the IR results.
T. Sato, K. Sato / Extraction o f ~erium(Ill),fi'om aqueous thiocyanate solutions
3.3. Temperature effect The extraction of cerium(Ill) from aqueous solution containing 1.0 mol dm 3 potassium thiocyanate with 38% TBP in benzene was carried out at temperatures between 10 and 40"C. The result indicates that the distribution coefficient for cerium(Ill) decreases with rising temperature. The heat of reaction ( - A H , change in enthalpy) is estimated to be 71.1 kJ mol -L.
3.4. Extraction by TOPO I n the extraction of cerium(lI I) from aqueous solutions containing potassium thiocyanate at different concentrations by TOPO in benzene, the distribution coefficient increases with increasing aqueous thiocyanate concentration at a constant TOPO concentration. L o g - l o g plots of E~ rs. [NCS ]i,~ ~,q give straight lines with a slope of about 3, suggesting the combination of three thiocyanate groups with a cerium ion. This is similar to the extraction by TBP, although the extraction efficiency for cerium(III) of TOPO is higher than that of TBP. In addition, log log plots of E ° vs. [TOPO]imt give straight lines with a slope of about 4, e.g. slopes of 3.9, 3.8 and 3.8 at concentrations of potassium thiocyanate of 0.1, 0.2 and 0.5 tool dm 3 respectively. Furthermore, the continuous variation in [Ce]org as a function of [Ce]mil~, q shows that the mole fractions [Ce]mitaq / [ N C S ]i,,i~,q and [Ce]ini, aq/[WOPO]init give [Ce]o,-g maxima of ~ and ~ at fixed total concentrations of [Ce]irm~, q -r- [NCS ]mit aq = 0. l mol dm -3 and [Ce]initaq + [TOPO]init = 0.05 mol d m 3 respectively. In the extraction of cerium(Ill) from aqueous solutions containing 5.0 tool dm ~ potassium thiocyanate with 0.01 mol dm ~ TOPO in benzene, the loading test of cerium in the organic phase as a function of [Ce]mit ~,q reveals that the molar ratio [CeJ:[NCSJ:[TOPO]: [ H 2 0 ] in the organic phase approaches a limiting value of 1:3:4:0 with increasing aqueous cerium concentration. These results suggest that the stoichiometric composition of the extracted species is Ce(NCS)~ .4TOPO. Hence the extraction of cerium(Ill) from aqueous solutions in the presence of thiocyanate by TOPO is expressed by the following equilibrium equation similar to eqn. (2): Ce(NCS)~(a) + 4 T O P O ( o ) ,
" Ce(NCS)3.4TOPO(o) (3)
The IR spectra of the organic extracts of cerium(Ill) from aqueous solutions containing potassium thiocyanate with TOPO in benzene show the following features: a shift of the P -,O stretching band of TOPO at 1146cm-~ to the lower frequency of 1120 cm ~, the Ce O stretching band at 375 cm ~, the CN stretching band at 2050 cm ~ and the NCS bending
137
band at 480cm ~. The result implies that the thiocyanate ion coordinates to cerium through the N atom and that cerium(lII) extracted into TOPO by the solvation reaction (3) is bonded to the phosphoryl O atom. From this it is deduced that the extracted species exists as the complex Ce(NCS)3-4TOPO, displaying a coordination number of seven, corresponding to the species Ce(NCS)3-4TBP formed in the extraction by TBP. When the temperature effect on the extraction of cerium(Ill) from aqueous solution containing 1.0 tool dm ~ potassium thiocyanate with 0.01 mol dm ~ TOPO in benzene was examined at temperatures between 10 and 40 C , it was found that the distribution coefficient decreases with rising temperature as seen in the extraction by TBP.
Acknowledgments The authors wish to thank H. Kamamori for assistance with the experimental work and also the Daihachi Chem. Ind. Co., Ltd. and the Hokko Chem. Ind. Co., Ltd. for supplying the TBP and TOPO respectively.
References 1 A. W. Wilson and O. K. McFarland, Anal. Chem., 35 (1963) 302. 2 C. E. O'Neile, V. A. Ettel, A. J. Oliver and 1. J. Itzkovitch, (kin. Min. Metall. Bull., 69(1976) 86. 3 W, Skey, Chem. Ind., (1967) 1780. 4 1. S. El-Yamani and E. 1. Shabana, J. Radioanal. Nucl. ('hem., 84 (1984) 307. 5 T. Sato, H. Watanabe and T. Kato, Proc. 1SEC 1977, Toronto, Vol. 1, Can. Inst. Min. Metall., Montreal, 1979, p. 143. 6 T. Sato, .I. Chem. Tech. Biotechnol., 29 (1979) 39. 7 T. Sato, K. Sato and T. Shiraishi, Proc. Syrup. on Solvent Extraction 1991, Osaka, Jpn. Assoc. Solvent Extr., Hamamatsu, 1991, p. 113. 8 T. Sato, S. Kotani and M. L. Good, .I. hu,'X,. Nucl. (Jwm., 35 (1973) 2547. 9 T. Sato, S. Kotani and M. L. Good, Amd. Chim. Acta, 84 (1976) 397. 10 T. Sato, S. Kotani and O. Terao, Proc. I S E C 1974, Lyon, Vol. 3, Soc. Chem. Ind., London, 1974, p. 2249. 11 T. Sato, S. Kotani and M. L. Good, J. hurry,,. Nut/. Chem., 36 (1974) 451. 12 T. Sato and K, Sato, Proc. Syrup. on Soh:ent Extraction 1989, Sendai, Jpn. Assoc. Solvent Extr.. Hamamatsu. 1989, p. 89. 13 T. Sato and K. Sato, Proc. Syrup. on Solvent Extraction 1988, Tokyo, Jpn. Assoc. Solvent Extr., Hamamatsu, 1988, p. 25. 14 K. Alcock, S. S. Grimley, T. V. Healey, J. Kennedy and H. A. C. McKay, Trans. Faraday Soc., 52 (1956) 39. 15 J. Kinnunen and B. Wennerstrand, Chemist-Analyst, 46 (1957) 92. 16 F. Matsumoto, T. Takeuchi and A. Ouchi. Bull. Chem. Soc. ,@n., 62 (1989) 2078, 17 T. Sato, Anal. Chim. Acta, 52 (1970) 183. 18 H. Yoshida, J. lnorg. Nucl. CTwm., 24 (1962) 1257.
Journal ~/ AIloys and Compounds, 192 (1993) 135-137 JALCOM 2088
Solvent extraction of cerium(III) from aqueous thiocyanate solutions by neutral organophosphorus compounds Taichi Sato* Faculty of Engineering, Shizuoka University, Hamamatsu (Japan) and Queen's University, Metallurgical Engineering Department, Kingston, Ont. (Canada)
Keiichi Sato FaculO, ~?/"Engineering, Shizuoka University, Harnamatsu (Japan)
Abstract The extraction of cerium(III) from aqueous solutions containing potassium thiocyanate by tributyl phosphate (TBP) or trioctyl phosphine oxide (TOPO) in kerosene or benzene has been investigated under different conditions. IR spectral studies have been carried out on the organic extracts. The distribution coefficient of cerium(III) for TBP increases monotonously with increasing aqueous thiocyanate concentration. As a result, it is found that the equilibrium equation for the extraction by TBP can be expressed as follows: Ce3+(aq)+ 3NCS ~(aq) Ce(NCS)3(aq), Ce(NCS)3(aq) + 4TBP(org) , " Ce(NCS)3.4TBP(org). A similar result was also obtained for the extraction by TOPO, although the extraction efficiency of TOPO is higher than that of TBP. The IR results indicate that the thiocyanate ion coordinates to the cerium ion through the N atom and that the cerium ion combines with the phosphoryl O atom of the extractants.
I. Introduction Thiocyanate has been utilized as a selective complexing agent in the spectrophotometric determination of metals and as a selective extraction agent for the separation of metals [1-4]. We have also investigated the extraction from aqueous solutions containing potassium thiocyanate of divalent metals [5, 6], aluminium (III) [7], thorium(IV) [8], titanium(IV) [9], vanadium (IV) [9, 10], zirconium(IV) [9] and uranium(VI) [11] by trioctylmethylammonium chloride (TOMAC) and of lanthanum(III) [12] and yttrium [13] by tributyl phosphate (TBP). The present paper extends the work to the extraction of cerium(III) from aqueous solutions containing potassium thiocyanate by TBP and trioctyl phosphine oxide (TOPO) in relation to the separation of rare earth elements.
2. Experimental details 2. 1. Chemicals TBP (Daihachi Chem. Ind. Co., Ltd.) was purified by the usual method [14] and TOPO ( H o k k o Chem. Ind. Co., Ltd.) was used without further purification. TBP
*Present address: 3-14-1, Daita, Setagaya-ku, Tokyo 155, Japan.
0925-8388/93/$6.00
and TOPO were diluted and dissolved in kerosene and benzene respectively. The stock solution of cerium was prepared by dissolving its chloride (CeC13.7H20) in an aqueous solution of potassium thiocyanate. Generally the aqueous metal concentration was 1 g dm 3 cerium chloride, except for the loading test. Other chemicals were of analytical reagent grade.
2.2. Extraction and analytical procedures Equal volumes (15 cm 3 each) of the aqueous and organic phases were shaken for a definite time in a 50 cm 3 stoppered conical flask in a thermostatted water bath at 20 °C, except for the experiments on the temperature effect. Preliminary experiments showed that the equilibrations were complete in 10 min. The mixture was centrifuged and separated and then aliquots of both phases were pipetted to determine the distribution coefficient (Ea°, the ratio of the equilibrated concentration of cerium in the organic phase to that in the aqueous phase). The cerium in the organic phase was stripped with 1 tool dm 3 hydrochloric acid. The concentration of cerium in the aqueous solution was determined by ethylenediamine tetraacetic acid titration using xylenol orange (XO) as indicator [15]. The concentration of thiocyanate in the organic phase was determined as described previously [6] and the water content of the organic phase by the K a r l - F i s h e r titration method.
,,~" 1993 - Elsevier Sequoia. All rights reserved
136
T. Sato, K. Sato /Extraction of cerium(IIl) from aqueous thiocyanate solutions
2.3. IR and nuclear magnetic resonance ( N M R ) spectral measurements
IR spectra of the organic extracts were determined on JASCO Models IRA-I (4000-650cm -~) and IR-F (700-200 cm-l) using a capillary film between thallium halide plates or polyethylene films. N M R spectra were obtained for the organic extracts dissolved in carbon tetrachloride using a JEOL Model JNM-PMX6OSO N M R spectrometer with tetramethylsilane as internal reference.
3.2. IR spectra
3. Results and discussion 3.1. Extraction isotherm by TBP
The extraction of cerium(III) from aqueous solutions containing potassium thiocyanate .at different concentrations by TBP in kerosene gave the result that the distribution coefficient increases monotonously with increasing aqueous thiocyanate concentration. In this extraction system, log-log plots of E ° v s . [NCa-]ini t aq give straight lines with a slope of about 3, indicating that three thiocyanate groups combine with a cerium ion. In the extraction of cerium(III) from aqueous solutions containing potassium thiocyanate with 0.96%19% (v/v) TBP in kerosene, log-log plots of E ° vs. [TBP]init at a constant concentration of potassium thiocyanate of 1 - 5 m o l d m -3 give straight lines with a slope of about 4, e.g. slopes of 3.8, 3.7 and 3.7 at concentrations of potassium thiocyanate of 1, 2 and 5 mol dm -3 respectively. It is thus considered that the cerium(III)-thiocyanato complex is associated with four molecules of TBP. Accordingly, if we assume that the extraction of cerium(III) from aqueous solutions in the presence of thiocyanate by TBP proceeds through solvation, the equilibrium equation can be expressed as Ce3+(a) + 3NCS-(a) ~
the composition in the organic phase approaches a limiting value of the molar ratio [Ce]:[NCS]:[TBP]: [H20] of 1:3:4:0, which suggests the formation of the species Ce(NCS)3.4TBP with increasing aqueous cerium concentration. Similarly, species such as Ce(NCS)3.4TBP and Y(NCS)3.4TBP are also formed in the extraction of lanthanum(III) [12] and yttrium (III) [13] by TBP. According to E1-Yamani and Shabana [4], however, the solvation numbers of TBP with trivalent lanthanum, cerium and yttrium are five, four and three respectively.
" Ce(NCS)3(a )
Ce(NCS)3(a) +4TBP(o) .
" Ce(NCS)3.4TBP(o)
(1) (2)
where (a) and (0) represent aqueous and organic phases respectively. This is supported by the results for the continuous variation in [Ce]org as a function of the mole fractions [Ce]initaq/([Ce]inita q q - [ N C S - ] i n i t a q ) and [Ce]initaq/([Ce]initaq+[TBP]init) at fixed total concentrations of [Ce]i,it aq + [NCS-]init aq = 0.1 mo1 dm -3 and [Ce]i,it ,q + [TBP]init = 0.1 mol dm -3 respectively; ICe]ors shows maxima at the former and latter mole fractions of 0.25 and 0.2 respectively. In addition, the stoichiometry of the extracted species was examined by a loading test of cerium in the organic phase. As a result, it was found that in the extraction of cerium(III) from aqueous solutions containing 5.0 mol dm -3 potassium thiocyanate with 9.4% TBP in kerosene, the variation in
The organic extracts of cerium(III) from aqueous solutions containing 5 . 0 m o l d m 3 potassium thiocyanate with 9.4% TBP in benzene were examined by IR spectroscopy. With increasing aqueous cerium concentration the spectra of the organic extracts show the following features in addition to the absorptions of the alkyl groups due to TBP: the OH stretching band at 3520-3400cm-~ and the OH bending band at 1640cm 1 decrease in intensity, indicating that the extracted species contains no coordinated water; the P - - ~ O stretching band of water-saturated TBP at 1270 cm l shifts to the lower frequency of 1220 cm J; the C e - O stretching band appears at 360 cm i; the CN stretching band appears at 2050cm -~ and the NCS bending band at 480 cm -j, implying that the thiocyanate ion coordinates to cerium through the N atom. The IR results confirm that cerium extracted into TBP by the solvating reaction (2) is bonded to the phosphoryl O atom. This suggests that the extracted species exists as the complex Ce(NCS)3.4TBP, displaying a coordination number of seven [16], in agreement with the results obtained in the extraction of lanthanum(III) [12] and yttrium(III) [13] by TBP. The organic phases for the repeated extraction of cerium(III) from aqueous solution containing 5.0 mol dm -3 potassium thiocyanate with 93% TBP were examined by N M R spectroscopy. The N M R spectrum for water-saturated TBP shows a sharp peak at 9.09 (z value) in a doublet due to the methyl protons, a multiplet at 8.50 which arises from most of the methylenic protons, and a quartet at 5.92 resulting from the methylenic protons attached to C atoms immediately adjacent to O atoms; in addition, the water proton resonance at 6.60 indicates the formation of the compound TBP.H20 [17]. In the extraction of aqueous solution containing potassium thiocyanate alone, the water proton signal is shifted to 6.48, confirming the formation of a hydrogen bond in the compound HNCS.TBP [ 18]. The organic solution from the extraction of cerium(III) reveals a shift of the water proton resonance to a lower field and its signal decreases in intensity, corresponding to the IR results.
T. Sato, K. Sato / Extraction o f ~erium(Ill),fi'om aqueous thiocyanate solutions
3.3. Temperature effect The extraction of cerium(Ill) from aqueous solution containing 1.0 mol dm 3 potassium thiocyanate with 38% TBP in benzene was carried out at temperatures between 10 and 40"C. The result indicates that the distribution coefficient for cerium(Ill) decreases with rising temperature. The heat of reaction ( - A H , change in enthalpy) is estimated to be 71.1 kJ mol -L.
3.4. Extraction by TOPO I n the extraction of cerium(lI I) from aqueous solutions containing potassium thiocyanate at different concentrations by TOPO in benzene, the distribution coefficient increases with increasing aqueous thiocyanate concentration at a constant TOPO concentration. L o g - l o g plots of E~ rs. [NCS ]i,~ ~,q give straight lines with a slope of about 3, suggesting the combination of three thiocyanate groups with a cerium ion. This is similar to the extraction by TBP, although the extraction efficiency for cerium(III) of TOPO is higher than that of TBP. In addition, log log plots of E ° vs. [TOPO]imt give straight lines with a slope of about 4, e.g. slopes of 3.9, 3.8 and 3.8 at concentrations of potassium thiocyanate of 0.1, 0.2 and 0.5 tool dm 3 respectively. Furthermore, the continuous variation in [Ce]org as a function of [Ce]mil~, q shows that the mole fractions [Ce]mitaq / [ N C S ]i,,i~,q and [Ce]ini, aq/[WOPO]init give [Ce]o,-g maxima of ~ and ~ at fixed total concentrations of [Ce]irm~, q -r- [NCS ]mit aq = 0. l mol dm -3 and [Ce]initaq + [TOPO]init = 0.05 mol d m 3 respectively. In the extraction of cerium(Ill) from aqueous solutions containing 5.0 tool dm ~ potassium thiocyanate with 0.01 mol dm ~ TOPO in benzene, the loading test of cerium in the organic phase as a function of [Ce]mit ~,q reveals that the molar ratio [CeJ:[NCSJ:[TOPO]: [ H 2 0 ] in the organic phase approaches a limiting value of 1:3:4:0 with increasing aqueous cerium concentration. These results suggest that the stoichiometric composition of the extracted species is Ce(NCS)~ .4TOPO. Hence the extraction of cerium(Ill) from aqueous solutions in the presence of thiocyanate by TOPO is expressed by the following equilibrium equation similar to eqn. (2): Ce(NCS)~(a) + 4 T O P O ( o ) ,
" Ce(NCS)3.4TOPO(o) (3)
The IR spectra of the organic extracts of cerium(Ill) from aqueous solutions containing potassium thiocyanate with TOPO in benzene show the following features: a shift of the P -,O stretching band of TOPO at 1146cm-~ to the lower frequency of 1120 cm ~, the Ce O stretching band at 375 cm ~, the CN stretching band at 2050 cm ~ and the NCS bending
137
band at 480cm ~. The result implies that the thiocyanate ion coordinates to cerium through the N atom and that cerium(lII) extracted into TOPO by the solvation reaction (3) is bonded to the phosphoryl O atom. From this it is deduced that the extracted species exists as the complex Ce(NCS)3-4TOPO, displaying a coordination number of seven, corresponding to the species Ce(NCS)3-4TBP formed in the extraction by TBP. When the temperature effect on the extraction of cerium(Ill) from aqueous solution containing 1.0 tool dm ~ potassium thiocyanate with 0.01 mol dm ~ TOPO in benzene was examined at temperatures between 10 and 40 C , it was found that the distribution coefficient decreases with rising temperature as seen in the extraction by TBP.
Acknowledgments The authors wish to thank H. Kamamori for assistance with the experimental work and also the Daihachi Chem. Ind. Co., Ltd. and the Hokko Chem. Ind. Co., Ltd. for supplying the TBP and TOPO respectively.
References 1 A. W. Wilson and O. K. McFarland, Anal. Chem., 35 (1963) 302. 2 C. E. O'Neile, V. A. Ettel, A. J. Oliver and 1. J. Itzkovitch, (kin. Min. Metall. Bull., 69(1976) 86. 3 W, Skey, Chem. Ind., (1967) 1780. 4 1. S. El-Yamani and E. 1. Shabana, J. Radioanal. Nucl. ('hem., 84 (1984) 307. 5 T. Sato, H. Watanabe and T. Kato, Proc. 1SEC 1977, Toronto, Vol. 1, Can. Inst. Min. Metall., Montreal, 1979, p. 143. 6 T. Sato, .I. Chem. Tech. Biotechnol., 29 (1979) 39. 7 T. Sato, K. Sato and T. Shiraishi, Proc. Syrup. on Solvent Extraction 1991, Osaka, Jpn. Assoc. Solvent Extr., Hamamatsu, 1991, p. 113. 8 T. Sato, S. Kotani and M. L. Good, .I. hu,'X,. Nucl. (Jwm., 35 (1973) 2547. 9 T. Sato, S. Kotani and M. L. Good, Amd. Chim. Acta, 84 (1976) 397. 10 T. Sato, S. Kotani and O. Terao, Proc. I S E C 1974, Lyon, Vol. 3, Soc. Chem. Ind., London, 1974, p. 2249. 11 T. Sato, S. Kotani and M. L. Good, J. hurry,,. Nut/. Chem., 36 (1974) 451. 12 T. Sato and K, Sato, Proc. Syrup. on Soh:ent Extraction 1989, Sendai, Jpn. Assoc. Solvent Extr.. Hamamatsu. 1989, p. 89. 13 T. Sato and K. Sato, Proc. Syrup. on Solvent Extraction 1988, Tokyo, Jpn. Assoc. Solvent Extr., Hamamatsu, 1988, p. 25. 14 K. Alcock, S. S. Grimley, T. V. Healey, J. Kennedy and H. A. C. McKay, Trans. Faraday Soc., 52 (1956) 39. 15 J. Kinnunen and B. Wennerstrand, Chemist-Analyst, 46 (1957) 92. 16 F. Matsumoto, T. Takeuchi and A. Ouchi. Bull. Chem. Soc. ,@n., 62 (1989) 2078, 17 T. Sato, Anal. Chim. Acta, 52 (1970) 183. 18 H. Yoshida, J. lnorg. Nucl. CTwm., 24 (1962) 1257.