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

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 ?Gtr(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.