Hydrometallurgical thermodynamics. I. Effect of sulfate ion-pairing and ion-solvation on the copper-iron redox equilibrium in aqueous acetonitrile

Abstract

The copper-iron redox equilibrium is shifted to the right

Cu^II+Fe^II↔Cu^I+Fe^III

by strong ion-pairing of sulfate ion with Fe^III and by specific solvation of Cu^I with acetonitrile. The equilibrium constant has been measured by direct e.m.f. and spectroscopic methods between pH 0-2 and found to be about 10⁷ higher for practical solutions of sulfates in acetonitrile/water than that calculated for perchlorates in pure water. Enthalpies and free energies of transfer of these ions from water to acetonitrile/water show that the shift in equilibrium to Cu^l+Fe^III in acetonitrile/water solution is strongly favoured by enthalpy changes associated with copper(I)-acetonitrile ion-solvation. Ion-pairing of sulfate ion with iron(III) results in an increase in entropy and a small shift of the equilibrium to the right.