Solutions of copper(II) and copper(I) sulphate in water containing acetonitrile have applications in the hydrometallurgy of copper. The potentials (n.h.e.) of the Cu+/Cu; the Pt/Cu2+, Cu+, and the Cu2+/Cu electrodes in water containing a large molar excess of various copper(I) bases and various counter-anions have been measured. Free energies of transfer of copper(I) ions from water to water containing the bases are calculated. ΔGtr(Cu+) at 25�C becomes less exoenergetic for water containing the bases in the following order: CN- >> S2O32- > Me2NCHS > KI > NH3 > C5H5N > KBr > KCl > C3H5OH ≥ MeCN, CO, C2H4 > C3H6. The reduction potentials of CuSO4/Cu2SO4 in acidic saturated copper(II) sulphate solutions and a range of Cu2SO4 concentrations in acetonitrile- water mixtures show that CuSO4/MeCN/H2O is a powerful oxidant, comparable in strength to acidic iron(III) sulphate in water. Acetonitrile is the preferred base for the processing of copper by way of solutions of copper(I) sulphate.

The kinetics and electrochemistry of the dissolution of a rotating copper disc in various acidified acetonitrile-water mixtures containing iron(III) as the oxidant are described. The reaction kinetics were investigated by a chronopotentiometric method in which the time taken for a copper film of known thickness to dissolve was determined. The reaction was shown to be diffusion-controlled over the temperature range (270-304 K) and composition range (1.0-14.5 mol dm-3 acetonitrile) investigated, good agreement being obtained between rate constants calculated from kinetic measurements and those calculated from electrochemical (cathodic polarization) measurements. From these and kinematic viscosity data, diffusion coefficients for iron(III) in acetonitrile-water mixtures were calculated. It was found that, associated with a discontinuous change in the kinematic viscosity at 290 K, there was a corresponding change in the diffusion coefficient. The activation energy for the high-temperature diffusion process was lower than that for the low-temperature process. It was also found that the diffusion coefficient decreased with increasing acetonitrile concentration. Corrosion potential measurements as a function of disc rotation speed and iron(III) concentration are reported. From these measurements, it was inferred that the anodic reaction is under mixed control.