Electrochemical processes in weakly acid solutions containing Cu(II)-glycine complexes

Rotating disc electrode (RDE) and linear potential sweep (LPS) voltammetry together with electrochemical quartz crystal microgravimetry (EQCM) were used for the investigation of cathodic processes in weakly acid solutions (pH 3-5) containing Cu(II)-glycine complexes and 0.3 M NaClO4 as a supporting electrolyte. The Cu2+ aqua-complex and monoligand complex CuL+ dominate in slightly acid Cu(II)-glycine solutions. Less than 1% of Cu(II) is converted to Cu(I) upon contact of solutions with a copper electrode. At pH > 4, the formation of surface layers containing Cu2O becomes possible. Due to the specific redistribution of species at the electrode surface, characteristic pre-waves appear on the RDE voltammograms, which are transformed into additional current peaks under LPS conditions. Using the model of mass transport of chemically interacting particles, changes in surface concentrations were estimated with further construction of linear Tafel plots normalized with respect to the surface concentration of the electrochemically active complex. The following values of kinetic parameters (the cathodic charge transfer coefficient, alpha(c1), and the exchange current density, i(01)) were determined for the rate-determining step CuL+ + e -> CuL: alpha(c1) - 0.23 and i(01) = 0.36 mA cm(-2) (pH 3); alpha(c1) = 0.44 and i(01) = 0.14 mA cm(-2) (pH 4). According to EQCM data, additional processes involving the formation of surface oxides arise at pH 5. (c) 2017 Elsevier Ltd. All rights reserved.