Kinetics of zinc and copper reduction in gluconate–sulfate solutions

RDE and LPS voltammetry together with energy-dispersive spectrometry was applied to study partial and net processes occurring at 2 < pH < 7 in 0.01 M Cu(II) and Zn(II) solutions containing 0.01–0.05 M gluconate and 0.5 M Na2SO4. Simulations using material balance equations, which include stability constants of complexes and these of protonated ligands, show that Cu2+ and Zn2+ aqua-ions together with monoligand sulfate and gluconate complexes are formed. Furthermore, the formation of OH-containing ZnLOH complexes (L− is a gluconate anion) is evident at pH > 5. Equilibrium potentials and partial cathodic processes in the system are arranged in the following order: Cu(II) reduction, hydrogen evolution and Zn(II) reduction.Rate of Cu(II) reduction drastically decreases with increasing pH. This process is accompanied by a strong chemisorption of gluconate with subsequent destruction and incorporation of oxygen and carbon in the deposits. Kinetics of hydrogen evolution on zinc and copper is similar. Its rate also decreases with solution pH and becomes insignificant in neutral media. Gluconic acid functions in this process as a proton donor. No significant suppression of Zn(II) reduction was observed in acidic and neutral media. The exchange current density for this process is comparatively high and amounts to 0.1 mA cm−2 at pH 6.All previous regularities act upon the codeposition of copper and zinc. Zn content in brass increases with cathodic polarization over a wide region of potentials (−1.05 < E < −0.5 V). Significant depolarization of Zn(II) reduction (Zn underpotential deposition) is consistent with the thermodynamics of alloy formation.