Extraction of Gold(III) from Hydrochloric Acid into Various Ionic Liquids: Relationship between Extraction Efficiency and Aqueous Solubility of Ionic Liquids

The extraction of gold(III) from 0.10 mol dm –3 hydrochloric acid into various aprotic ionic liquids (ILs) was investigated at 25 °C. Eleven kinds of ILs were examined as the extraction solvents; the cations were 1-butyl-3-methylimidazolium ([BMIm] + ), 1-hexyl-3-methylimidazolium, 1-methyl-3-octylimidazolium, 1-butyl-2,3-dimethylimidazolium, 1-butyl-1-methylpyrrolidinium, and methyltrioctylammonium; the anions were tetrafluoroborate, hexafluorophosphate, bis(trifluoromethanesulfonyl)amide ([NTf 2 ] − ), and bis(pentafluoroethanesulfonyl)amide. The metal was extracted as anionic tetrachloroaurate ([AuCl 4 ] − ) into all the nonprotic ILs with high distribution ratios (>100). The distribution ratio of gold(III) differs largely (480-fold at most) depending on the IL species and also depends on the amount of extracted gold(III). These dependences of the distribution ratio were quantitatively explained as functions of the solubility product of the IL used, where we considered the equilibrium model comprising the ion pair extraction of [AuCl 4 ] − with the IL cation in the aqueous phase and the ion exchange of [AuCl 4 ] − with the IL anion in the IL phase. From a practical purpose, the back extraction of gold(III) from an IL ([BMIm][NTf 2 ]) phase was also examined; nearly quantitative stripping was possible using acidic thiourea solutions.