The conditional stability constant and the distribution constant of [M(dtp)3](M3+=As3+ or Sb3+, dtp-=0,0'-diethyl dithiophosphate)were determined by analyzing the relationship between the distribution ratio of arsenic or antimony and the concentration of dtp-. The values for arsenic( III) complex were 10112 (mol dm-3)-3 (in 2 mol dm-3 sulfuric acid solution) and 1030, respectively. Those for antimony(Ill) complex were 1010 4(mol dm-3)-3 (in a solution of pH 1.6) and 1024, respectively.Both arsenic(Ill) and antimony(Ill) were quantitatively extracted into hexane with Kdtp of 2 mmol dm-3 from 1-2 mol dm-3 sulfuric acid solution, only antimony(l I l) was extracted from a solution of pH 1.3 -2.5, and neither of them was extracted at pH 4.2 or above. The separation method of arsenic(Ill) from antimony(l I I) and the reverse are proposed.
KeywordsSolvent extraction, hexane, arsenic (III), antimony( I I I ), 0,0'-diethyl dithiophosphate 0, O'-Dialkyl dithiophosphates, especially diethyl derivative, have been used as chelating reagents for the solvent extraction of metal ions.2 The reagents are classified into the category of soft bases3 because they coordinate to metal ion through two sulfur atoms. They are one of a few soft bases used in analytical chemistry, and thus it is of interest to clarify their characteristic. Bode et al.2 reported the extensive data on the analytical application of the reagents. Many other reports have also appeared, but the characteristics of the reagents are not clarified sufficiently because of lack of thermodynamic properties such as stability and extractability of their metal complexes.The authors have been studying the analytical application of the reagents on the basis of thermodynamic information, and have reported the solvent extraction methods for some metal ions with 0, 0'-diethyl dithiophosphate(dtp-).4-6 Arsenic(III) and antimony(III) can be extracted into carbon tetrachloride with dtp-as [As(dtp)3] and [Sb(dtp)3], respectively.2 Puttemans et al.' demonstrated that arsenic(III) can be separated from arsenic(V) because arsenic(V) can only be extracted into organic solvent after preliminary reduction to a trivalent state.On the other hand, Bode et al.2 described that antimony(V) is partially extracted as [Sb(dtp)3] by reduction to a trivalent state with dtpitself.However, there is no information about the thermodynamic stability of these complexes. In this paper, the conditional stability and the distribution constants of the complexes are determined by a solvent extraction method. Hexane is used as an solvent in order to carry out the experiment spectrophotometry.organic by UV Experimental Reagents Potassium 0, 0'-diethyl dithiophosphate(Kdtp) was synthesized and purified according to the method described in a preceding paper4, and its purity (above 99%) was confirmed by iodometry.8The stock solution of arsenic(III) (0.1000 mol dm-3) was prepared by dissolving 9.891 g of a primary standard diarsenic trioxide in 50 cm3 of 20% potassium hydroxide solution, neutralizing the solution ...