Mixtures of the protic ionic liquids trioctylammonium bis(trifluoromethanesulfonyl)amide ([TOAH][NTf 2 ]) and trioctylammonium nitrate ([TOAH][NO 3 ]) were investigated as extractants for platinum-group elements. The mixture of 10 wt % [TOAH][NO 3 ] in [TOAH][NTf 2 ] has low viscosity (274.2 mPa s) and low aqueous solubility (2.6 Â 10 À5 mol dm À3 as the cation). With the ionic liquid mixture, Pd(II) and Pt(IV) were extracted almost quantitatively from 0.10 mol dm À3 hydrochloric acid. Under the same conditions, Na(I), Mg(II), K(I), Ca(II), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), Ru(III), Rh(III), and Cd(II) were only slightly extracted. It was found that the extraction of Pd and Pt increases with increasing content of [TOAH][NO 3] in the mixture. The metals could be back-extracted from the ionic liquid mixture with nitric acid solution, and selective stripping of Pt was possible by controlling the concentration of nitric acid. The metal extractability of the ionic liquid mixture after the back-extraction was equivalent to that of the fresh mixture, indicating that the species composition of the mixture returned to the original state through the back-extraction process. This extraction system provides an efficient separation method for Pd and Pt in acidic chloride media using ionic liquid mixtures, which are recyclable, easy to handle, safe, and environmentally friendly.

The 1:1 ion pair formation constants (K IP 0 ) of 1-alkyl-3-methylimidazolium ([RMeIm] + ; R ) butyl, hexyl, and octyl) and 1-butyl-2,3-dimethylimidazolium ([BuMe 2 Im] + ) ions with tetrafluoroborate ([BF 4 ] -), hexafluorophosphate ([PF 6 ] -), bis(trifluoromethanesulfonyl)amide ([NTf 2 ] -), and 2,4,6-trinitrophenolate (picrate, [Pic] -) ions have been determined conductometrically in dichloromethane at 25 °C. The K IP 0 determinations have also been made for symmetric tetraalkylammonium ions ([R 4 N] + ; R ) methyl, ethyl, propyl, and butyl) for comparison. For a given anion, the K IP 0 value of the [RMeIm] + salt is almost independent of the length of the alkyl chain (R), whereas that of the [R 4 N] + salt decreases with increasing alkyl chain length. Such a difference in the alkyl chain length dependence of the ion pair formation ability can be explained on the basis of the structures of the ion pairs calculated by density functional theory. The K IP 0 values of [BuMeIm] + , [BuMe 2 Im] + , and [Et 4 N] + , which are similar in the van der Waals volume, are in the order of [BuMeIm] + . [BuMe 2 Im] + ≈ [Et 4 N] + , showing that the C2-H atom on the imidazolium ring makes an important contribution to the strong ion pair formation ability of [RMeIm] + . For a given cation, the K IP 0 value is generally smaller for the larger anion, i.e., [BF 4 ]g [PF 6 ]g [NTf 2 ] -> [Pic]for [Et 4 N] + and [BuMe 2 Im] + , and [BF 4 ] -> [PF 6 ]g [Pic]g [NTf 2 ]for [RMeIm] + .

Ion-pair formation constants (mol(-1) dm3 unit), K(MX) for a univalent metal salt (MX) and K(MLX) for its ion-pair complex (ML+X-) with a crown ether (L) in water, were determined at various ionic strengths (I) and 25 degrees C by potentiometry with ion-selective electrodes for MX=NaPic, NaMnO4, NaBPh4, KPic, and KMnO4; and MLX = Na(18C6)Pic, K(18C6)Pic, and Na(18C6)BPh4, where Pic- and 18C6 denote a picrate ion and 18-crown-6 ether, respectively. Equations for analyzing I-dependence of logK(MLX) and logK(MX) were derived and fitted well to the I-dependence using a non-linear regression analysis. The equilibrium constants at I = 0 mol dm(-3), K(MLX) degrees and K(MX) degrees, were simultaneously obtained from the analysis. The experimental values of K(MLX) and K(MX) were only in agreement with the values calculated from K(MLX) degrees and K(MX) degrees , respectively, in the ranges of higher I.