Cathodic processes in copper(II) solutions containing gluconic acid

The kinetics of cathodic processes proceeding in the acidic 0.01 M Cu(II) solutions containing gluconic acid and 0.5 M Na₂SO₄ as the supporting electrolyte is studied. According to the spectrophotometric data, in the moderately acidic solutions, a monoligand complex of CuL⁺ predominantly forms. Its concentration stability constant is 10^2.2 M⁻¹. In the cathodic voltammograms, a well-defined plateau of the limiting current is observed. The height of the plateau obeys the Levich equation. The effective diffusion coefficient decreases from 4.2 × 10⁻⁶ to 2.5 × 10⁻⁶ cm²/s with increasing complexation degree of the system. An analysis of normalized Tafel plots showed that the exchange current density of Cu²⁺ + e → Cu⁺ process decreases with increasing concentration of ligand or with increasing pH value. Thereby, the cathodic chargetransfer coefficient remains constant (0.33 ± 0.02). A comparison of the kinetic data with the results of deposit surface examination points to significant surface activity of the ligand. The gluconate chemisorption can be accompanied by the incorporation of the fragments, which were formed as a result of its destruction, into the electrodeposits.