Hydrometallurgical Thermodynamics. II. Solvent Effects on the Activity and Free Energies of Transfer of CN-, Ag(CN)₂- and Au(CN)₂- in Ethanol-Water and Acetonitrile-Water Mixtures

Abstract

The free energies of transfer of CN^-, Ag(CN)₂^- and Au(CN)₂^- ions from water to a range of acetonitrile -water and ethanol-water mixtures are reported. The values are based upon measurements of solubility products and formation constants of AgCN and Ag(CN)₂^-, and upon measurements of the Au(CN)₂^-/Au couple, by using known values of ΔG_tr(Ag⁺) based on the tetraphenylarsonium tetraphenylborate ( tatb ) assumption. Addition of acetonitrile to water increases the activity of these ions in the order CN^- > Ag(CN)₂^- ≥ Au(CN)₂^- much more than the addition of ethanol. Solvent effects on CN^- and Ag(CN)₂^- are similar but slightly less than those on Cl ^- and AgCl₂^-, and show no special ion-solvent interactions between MeCN and CN^-. The relative activity of CN^- and Au(CN)₂^- in aqueous-organic mixtures rationalizes the ability of such mixtures to desorb gold from the activated carbon used in gold processing through an ion-exchange mechanism.