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 ΔGtr(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.

Gibbs Energies of Transfer of Anions from Water to Mixed Aqueous Organic Solvents

“…The redox strengths of oxidation−reduction couples in different solvents are of considerable interest for the development of new hydrometallurgical reactions, the rationalization of oxidation state stabilities, etc. , In principle, it is possible to establish universal scales for electrode potentials by measurements on cells such as provided that the liquid junction potential, E j , at the boundary between the solutions in the two solvents can be estimated reliably in some manner or be rendered negligible. Neither of these options is rigorously possible, with both approaches involving some assumption about the medium effect for single ions.…”

Ion-Exchange Equilibria for [Au(CN)2]-/Cl- and [Au(CN)2]-/SCN- on Purolite A500 in Mixed Solvents at 303 K

“…This type of interaction decreases as the charge density of the anion decreases. Thus, large polarizable anions, such as [Au(CN) 2 ] - , are less solvated than Cl - in water. , Dipolar aprotic solvents are much more polarizable than water and interact strongly with large polarizable anions. In most cases, however, anion solvation is more significant in water than in dipolar aprotic solvents.…”

“…The effect of modifying the solvent composition on selectivity is often more dramatic than the effect of changing variables such as ionic strength and temperature in an aqueous system. This phenomenon is attributed to the changes in the activities of the ions resulting from the solvation process in the mixed solvent. ,, …”

“…This phenomenon is attributed to the changes in the activities of the ions resulting from the solvation process in the mixed solvent. 10,13,14 Mixed solvents have been shown to be highly effective for the elution of [Au(CN) 2 ]from activated carbon. [15][16][17] In comparison to carbon, fewer studies have been devoted to the use of mixed solvents for the recovery of the complex from ion-exchange resin.…”

Cyanide Thermodynamics. 1. Stability Constants of Cadmium(ii) and Zinc(ii) Cyanide Complexes in Aqueous Acetonitrile Mixtures

“…Unfortunately, no quantitative data are available regarding the solvation of Cd2+ in MeCN/H,O mixtures although Zn2+ has been shown to be less well solvated in such mixtures than in water over the whole composition range. 29 Presumably, any enhanced solvation of Cd2+ over Zn2+ in MeCN/H20 mixtures is counterbalanced by a similar effect for the complex ions.…”