Adsorptive square-wave voltammetry applied to study the reduction mechanism of Cu–sulfoxine and Cu–ferron complexes

The reduction mechanisms of Cu(II) in solutions with the quinoline derivatives sulfoxine (Sox) and ferron (Fer) are studied by the simulation of their experimental square-wave voltammetric curves. Sox and Fer form very stable complexes with Cu(II) that can be accumulated on mercury electrodes by adsorption. Cathodic linear scan stripping voltammetry was performed for comparison purposes. The reaction schemes consider the reduction of an adsorbed oxidized complex with a ligand/metal stoichiometric ratio higher than that of the product. The remaining ligand may remain adsorbed or be released in solution. The fits between experimental and theoretical curves reveal the prevalence of the adsorbed species Cu(Sox)22− and Cu(Fer)22− at pH 6.4 and 5.3, respectively, which are the same as predicted by the distribution species diagram for the bulk of the solution. At these pH values, Cu(Sox)22− and Cu(Fer)22− complexes present essentially the same Ep and electrochemical rate constants ks=(1.3±0.1) s−1, and thus the same effective stability constants. The dependence of differential peak currents on the logarithm of the frequency describes the so-called quasi-reversible maximum (fmax). The frequency of this maximum is lower for solutions with high pH, indicating that the charge transfer rate is decreased when the stability of the complexes is enlarged. The fmax value is related to the ligand/complex concentration ratio and it is applied to the estimation of ks. Changes at the surface concentrations were evaluated considering the ligand to complex ratios at the electrode surface and at the solution bulk.