Kinetics of cathodic processes in Cu(II) gluconate solutions containing an excess of sulfate

Highlights: ► Kinetics of Cu(II) reduction in gluconate solutions, containing an excess of sulfate, is studied. ► Effective diffusion coefficients decrease with the molar part of Cu(II)-gluconate complexes. ► LPS voltammograms contain two current maxima conditioned by Cu(II) and H3O+ reduction. ► Reduction of surface Cu2O is observed at pH 5. - Abstract: Chronopotentiometry and linear potential sweep (LPS) voltammetry were applied to study cathodic processes occurring at the Cu electrode in Cu(II) solutions containing gluconate (L−) and excess of sulfate at 2 < pH < 5. The data obtained are indicative of sufficiently high lability of both CuL+ and CuSO4 complexes. The effective diffusion coefficients obtained from transition times (τ) are in the range from 4.4 × 10−6 to 3.7 × 10−6 cm2 s−1 and decrease with the molar fraction of CuL+. No adsorbed electrochemically active complexes (EAC) were detected. Additional τ′ observed at pH 5 results from the reduction of surface Cu2O that is formed at the open-circuit potential. LPS voltammograms contain two current maxima conditioned by Cu(II) and H3O+ reduction, respectively. The first of them contains irregularities that disappear when perchlorate is substituted for sulfate. Cu2+ aqua-ions take part in the consecutive transfer of two electrons whereas CuL+ complexes seem to be electrochemically inert. Cathodic charge transfer coefficients (α = 0.43 ± 0.03), determined by different methods, correlate well. A role of adsorption processes has also been discussed.