Kinetics of Sn(II) reduction in acid sulphate solutions containing gluconic acid

Voltammetry, electrochemical quartz crystal microgravimetry (EQCM) and electrochemical impedance spectroscopy (EIS) were applied to study kinetics of cathodic processes occurring in acidic Sn(II) gluconate solutions containing an excess of sulphate. Simulations based on the material balance equations showed a rather complicated composition of the solutions involving gluconate, sulphate and hydroxide complexes of Sn(II). Equilibrium potentials following from the Nernst equation satisfactorily coincide with experimental open-circuit potentials. According to EQCM data, Sn(II) reduction is the main process occurring at sufficiently low cathodic polarizations (ΔE). An onset of hydrogen evolution was observed at ΔE ≈ −0.3 V (pH 4). The rotating disc electrode (RDE) and linear potential sweep (LPS) voltammetric data were analyzed using equations based on the charge transfer and diffusive mass transport regularities. The observed considerable decrease in the rate of Sn(II) reduction with solution pH is embodied in the respective diminution of the effective Sn(II) diffusion coefficient. Its values obtained from the limiting (RDE) and peak (LPS) current densities vary from ∼6 × 10−6 to 6 × 10−7 cm2 s−1 at 2 < pH < 5. Nyquist plots (EIS data) contain two semicircles that become clearly pronounced on subtraction of the double layer impedance. It is assumed that the electroreduction of Sn(II) in gluconate solutions is accompanied by adsorption phenomena whose inhibitive character enhances with solution pH.