Thermodynamics of the Cu+/Cu2+-redox equilibrium in soda-silicate and soda-lime-silicate melts

The thermodynamics of the redox equilibrium of Cu+/Cu2+ were determined by square-wave voltammetry in glass melts with the base mol% compositions x Na2O · (100 − x) SiO2 (x = 15, 20, 26 and 33) and (26 − x) Na2O · x CaO · 74 SiO2 (x = 0, 5, 10 and 15) doped with 1 mol% CuO in the temperature range from 850 to 1150 °C. All recorded voltammograms showed two maxima attributed to the reductions of Cu2+ to Cu+ and Cu+ to metallic copper. Both peaks are shifted to smaller potentials with decreasing temperature. With increasing melt basicity, the [Cu+]/[Cu2+]-ratio first increases, and remains constant for optical basicities >0.56. The effect of composition on the redox equilibrium is explained by the incorporation of both Cu+ and Cu2+ in octahedral coordination into the melt structure.