The stepwise reduction of Cu(II) at a platinum electrode in water + N, N-dimethylformamide mixtures

Abstract

The cathodic reduction of Cu(II) at platinum electrodes has been studied in N, N-dimethylformamide (DMF) and in water +DMF mixtures with the use of the rotating disc (RDE) and ring-disc (RRDE) electrode techniques. The voltammetric curves obtained at the disc electrode split into two waves which correspond to the successive charge transfer steps: Cu(II)+e − → Cu(I) and Cu(I)+e − → Cu, with soluble monovalent copper as an intermediate. In water-rich mixtures, the first wave is a small one, but its height increases with increasing DMF content in the solution. On the basis of the ring-disc measurements, it was established that the current efficiency for Cu(I) generation is equal to 100% within the region of the first wave. Using coulometric and potentiometric techniques, the formal potentials of the Cu(II)/Cu(I), Cu(II)/Cu and Cu(I)/Cu redox couples as well as the equilibrium constants of the reproportionation reaction Cu(II) + Cu ⇌ 2 Cu(I) have been determined. Formal potentials expressed in the ferrocene electrode scale were utilized to calculate the Gibbs energies of transfer for the Cu(II) and Cu(I) ions from water to water + DMF mixtures. Both Cu(II) and Cu(I) are solvated more strongly by DMF than by water. In addition, spectrophotometric measurements were performed in order to determine the equilibrium constant of solvent exchange at Cu(II) in water+DMF mixtures. The diffusion coefficients of Cu(II) and the kinetic parameters of the Cu(II)/Cu(I) electrode reaction were determined and discussed.