Analyst, December, 1983 1525 SHORT PAPERS Solvent Extraction and Separation from Lanthanides Miss V. V. Mudshingikar and V. M. Shinde Department of Chemistry, Shivaji University, Kolhapur 416 004, India of Thorium(lV) Keywords : Thorium( I V ) separation ; solvent extraction ; mesityl oxide; lanthanides In this paper we propose a method for the solvent extraction, separation and purification of thorium. Thorium is extracted quantitatively from a 0.1 M sodium salicylate solution of pH 3.5-5 using mesityl oxide as an extractant. The extracted metal ion is stripped with nitric acid (0.5 M) and determined photometrically with Arsenazo I11 at 665 nm. Various solvents such as tributyl phosphate,l4-rnethylpentan-2-0ne,~ mesityl oxide,3 sulph- oxides4s5 and 2-butoxyethyl ether6 have been used for the extraction studies of thorium but these methods suffer from drawbacks such as multiple extraction, heating of the aqueous phase and the use of high concentrations of salting-out agents.Bis(2-ethylhexyl) hydrogen phos- hate,^^^ dibutyl hydrogen phosphate9 and trioctylphosphine OxidelOJl have also been used for the extraction of thorium from mineral acids but they have little practical application. High relative molecular mass amines such as t r i ~ c t y l a m i n e , ~ ~ ~ ~ ~ Aliquat,14J5 Amberlite LA-1 and LA-2,16s17 tridodecylamine,ls Primene JMT1S and Alamine-33620 have also been used for the extraction of thorium(1V) but in all these methods a systematic separation study of thorium is lacking. In this paper we describe a method for the separation of thorium(1V) from lanthan- ides such as scandium, yttrium, lanthanum , cerium, praseodymium , neodymium, samarium, gadolinium and dysprosium and uranium. Experimental Reagents TCtorium(IV) solution.Prepared by dissolving 6.43 g of thorium nitrate (analytical- reagent grade) in 5 ml of 0.5 M nitric acid and diluting to 500 ml with distilled water. This was standardised21 and diluted as required. Extractant. A 50% solution of doubly distilled mesityl oxide (Fluka, b.p. 125-128 "C) dis- solved in benzene was used for extraction studies. Arsenazo III. A 0.1 yo aqueous solution was used for the spectrophotometric determination of thorium.22 General Extraction Procedure An aliquot of solution (25 ml) containing 25-100 pg of thorium(1V) and 0.4 g of sodium salicylate (0.1 M) was adjusted to pH 3.5-5 with dilute nitric acid and sodium hydroxide solu- tion and shaken for 1 min in a 100-ml separating funnel with 10 ml of 50% mesityl oxide.After separation of the two layers thorium was removed from the mesityl oxide phase by shaking with 15 ml of 0.5 M nitric acid and determined as follows: add 2 ml of Arsenazo I11 solution, adjust the pH of the solution to 1.5-2 with dilute nitric acid and sodium hydroxide solution and read the absorbance of the dark violet complex at 665 nm.22 Results and Discussion Optimum Extraction Conditions The extraction of thorium(1V) was studied at various pH values (3-9), sodium salicylate concentrations (0.025-0.15 M) and mesityl oxide concentrations ( 10-lOO~o in benzene as diluent).The extraction of thorium(1V) is effectively quantitative at pH 3.5-5.0 from 0.1 to 0.125 M sodium salicylate medium (Fig. 1) with 10 ml of 50% mesityl oxide solution. Above pH 5, the extraction decreases. The percentage extraction (E) of the metal ion was calculated1526 SHORT PAPERS Analyst, Vol. 108 Sodium salicylate concentration/M 0.025 0.05 0.075 0.10 0.125 0.150 I------ 100. c -g 90. E c 8 0 . 0 c 70 2 60 1 I I I I I I 4 5 6 7 8 9 PH Fig. 1. Extraction of thorium(1V) as a function of pH (A) and salicylate con- centration (B). by stripping the metal ion from the organic phase with a subsequent photometric determination with Arsenazo 111. The distribution ratio (D) was calculated by using the relationship V W l V O x E D = where Vw and Vo are the volumes of aqueous phase and organic phase, respectively.Of the solvents tried xylene and toluene could also be used in place of benzene. Benzene was, how- ever, preferred as it gives clear phase separation although precautions are necessary as it is carcinogenic. 100- E Nature of the Extracted Species The log - log plot of distribution ratio 'ucysus mesityl oxide concentration at fixed pH and salicylate concentration gave a slope of 1.8, indicating the presence of two solvent molecules in the extracted species. The extraction mechanism thus involves solvation of thorium salicylate salt. Period of Equilibration mesityl oxide is needed for complete extraction of thorium(1V). adverse effect on the extrattion of thorium. Variation of shaking time showed that a single extraction for 1 min with 10 ml of 50% Prolonged shaking has no Tolerance of Diverse Ions The extractions were carried out by the recommended procedure in the presence of a number of foreign ions in order to observe their interference in the extraction and subsequent spectro- TABLE I TOLERANCE OF DIVERSE IONS IN THE SEPARATION OF 100 pg OF THORIUM Ion added Tolerance limitlpg Ag(I), Fe(II), Hg(II), Pd(II), Bi(III), Au(III), Cr(III), Pt(IV), Mo(VI), Cu(II), Ni(II), Mn(II), Zn(II), Cd(II), Zr(IV), ascorbate, citrate, phosphate, sulphate .... .. .. .. .. .. 600 EDTA, tartrate .. .. .. .. .. .. .. .. None W(VI), Cr(V1) .. .. * . .. .. 2 000 Ru(III), Nb(V), V(V),'Ta(V), oxalate, thiourea, chloride,' hitrate' . . 1000December, 1983 SHORT PAPERS 1527 photometric determination of thorium.The results in Table I show that a large number of cations and anions did not interfere. Notable interferences are given by ions such as EDTA and tartrate. Separation of Thorium( IV) from Scandium( 111), Yttrium( 111), Lanthanum( 111), Cerium( IV), Praseodymium( 111), Neodymium( 111), Samarium(III), Gado- linium( 111), Dysprosium(II1) and Uranium(V1) Elements such as scandium, yttrium, lanthanum, cerium, praseodymium, samarium, gadolinium, dysprosium and uranium are only partially extracted (ca. 30%) when thorium is extracted from 0.1 M sodium salicylate solution (pH 4) with 10 ml of 50% mesityl oxide dis- solved in benzene. Large amounts of added ions remain in the aqueous phase (A). The partially extracted metal ions are selectively removed from the organic phase by back-washing with distilled water or with 0.01 M sulphuric acid (for cerium only).The back-stripped aqueous phase (B) is combined with the original aqueous phase (A) and in the combined aqueous phase metal ions are determined by standard procedures. Thorium does not strip with either water or 0.01 M sulphuric acid and remains in the organic phase. It is subsequently removed from the organic phase by back-washing with 0 . 5 ~ nitric acid and determined spectrophotometrically with Arsenazo I11 as described under General Extraction Procedure. TABLE I1 SEPARATION OF THORIUM(IV) FROM BINARY MIXTURES AND MULTI-COMPONENT SYSTEM Composition of mixturelpg Th, 100; Sc, 500 . . .. Th, 100; Y, 500 . . .. Th, 100; La, 500 . . .. Th, 100; Ce, 200 .. .. Th, 100; Pr, 500 . . .. Th, 100; Nd, 500 . . .. Th, 100; Sm, 500 . . .. Th, 100; Gd, 500 . . .. Th, 100; Dy, 500 . . .. Th, 100; U, 500 .. Th, 100; Sc, 100; Y,' 100; La, 100; Pr, 100; Nd, 100; Sm, 100; Dy, 100; U, 100; Gd, 100 . . .. .. Recovery of thorium from triplicate analysis/ pg 99.6 100.0 99.5 99.1 99.6 99.3 99.6 99.3 99.3 99.1 99.0 Relative error, yo 0.4 0.0 0.5 0.9 0.4 0.7 0.4 0.7 0.7 0.9 1 .o Recovery of added ion from triplicate analysis/ pg 100.0 99.0 100.0 99.5 99.5 99.8 99.2 100.0 100.0 100.0 Relative error, % 0.0 1.0 0.0 0.5 0.5 0.2 0.8 0.0 0.0 0.0 Spectrophotometric reagent used for added ions Arsenazo Pa Arsenazo P4 Arsenazo I*6 Arsenazo 111'6 PARz7 PARB7 Arsenazo Arsenazo P4 PAR'7 PAR'@ * Thorium is determined with Arsenazo I11 as described under General Extraction Procedure.The recovery of thorium and that of the other added ions was greater than 99.0%. Thorium(1V) was also separated from the multi-component system. The results of the separa- tion for binary mixtures as well as the multi-component system are reported in Table 11. The reproducibility of the method was satisfactory and the determination required only 20 min. Six determinations with 100 pg of thorium(1V) gave a mean absorbance value of 0.6 & 0.02 with a standard deviation of 1.4 x 10-2 and a coefficient of variation of 2.3%. The proposed method is precise and accurate. The authors thank the Council of Scientific and Industrial Research (New Delhi) for award- ing a fellowship to one of them (V.V.M.). References 1. 2. 3. 4. 5. 6.Cheng-Chyuan Chen, and Gann Ting, J . Chin. Chem. Soc. (Taipei), 1977, 24, 25. Norio, I., Talanta, 1971, 18, 21. Jen Chun-Hwa, and Cheng Chen Mei, Chemistry, Taipei, 1968, 4, 148. Markl, P., Mikrochim. Acta, 1973, 907. Reddy, A. S., and Reddy, L. K., Sep. Sci., 1977, 12, 641. Jenkins, I. L., Lidington, D. H., and Wain, A. G., J . AppZ. Cheun., 1969, 19, 213.7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 1528 SHORT PAPERS Analyst, Vol. 108 Mitsugashira, T., Yamana, H., and Suzuki, S., Nippon Kagaku Kogyo, 1977, 50, 2918. Percival, D. R., and Martin, D. B., Anal. 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