Redox processes in a tin doped melt with the basic composition 20Na2O · 80SiO2 studied by square-wave voltammetry and impedance spectroscopy

Redox processes in a tin doped melt with the basic composition 20Na2O · 80SiO2 were studied by square-wave voltammetry and impedance spectroscopy. At temperatures of 1350 and 1450 °C, the recorded square-wave voltammograms exhibited a distinct peak attributed to the reduction of Sn4+ to Sn2+. Impedance spectra recorded at the same temperatures could be simulated with a simple equivalent circuit assuming a diffusion controlled electrode reaction. At temperatures T ⩽ 1250 °C, and small step times, the voltammograms exhibited a second peak which was much stronger affected by the frequency (step time) applied than expected from a diffusion controlled electron transfer reaction. In order to fit the impedance spectra at these temperatures, a more complex equivalent circuit was necessary, taking into account adsorption phenomena. In all impedance spectra, the Warburg parameter obtained from simulation, which stands for the diffusion from and to the electrode, exhibited a minimum which was equal to the voltammetric peak at more negative potentials. The voltammetric peak at less negative potentials is caused by the reduction of Sn4+ to an adsorbed state. Within this reduction step, a monolayer of Sn2+ species is formed.