Solvation of ions. Some applications. II. Electrolysis of copper(I) sulphate in water-nitrile mixtures

Abstract

Organic nitriles stabilize acidic aqueous copper(I) sulphate solutions. Electrolysis of such solutions provides an alternative for the electrowinning and electrorefining of copper. The Cu⁺/Cu^O and Cu²⁺/Cu⁺ standard reduction potentials in aqueous solutions containing acetonitrile or 3-hydroxy-propanenitrile (hydracrylonitrile) differ from those in water, due to the specific solvation and stabilization of the copper(I) ion by either nitrile. In the mixed solvent, the solvation sphere around the copper(I) ion contains organic nitrile, rather than water, and this reduces the mobility of Cu⁺ and slows its diffusion to the electrode surface in a working cell. The copper(II) ion contains water in its solvation shell in the mixed solvent. Thus for dilute solutions, the diffusion coefficient of the copper(I) ion in 10 mole % aqueous acetonitrile is about two-thirds and in 10% 3- hydroxypropane-nitrile-water is about one-fifth that of the copper(II) ion in water. In concentrated solutions containing sulphuric acid, the copper(II) ion diffuses as ion pairs and this reduces its mobility. This effect is not observed for copper(I) ions. Cell voltages measured during the electrolysis of copper(I) sulphate solutions by using soluble copper, insoluble platinum or insoluble carbon anodes are analysed in terms of the ohmic resistance, the equilibrium potential, the charge-transfer overpotential and the diffusion polarization of the working cell. There is a significant charge-transfer overpotential for the oxidation of Cu⁺ to Cu²⁺ at an insoluble graphite anode in 6 mol dm^-3 acetonitrile-water solutions; the rate constant is 4 x 10^-4 cm s^-1. At platinum the rate constant is 1.6 x 10^-2 cm s^-1.