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

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.

► 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.