Thermodynamics of the Cu+/Cu2+-redox equilibrium in alumosilicate melts

Glass melts with the basic mol% composition 26 Na2O · x Al2O3 · (74 − x) SiO2 (x = 5, 10, 15 and 20); x Na2O · 10 CaO · 10 Al2O3 · (80 − x) SiO2 (x = 10, 15, 20 and 25) and x Na2O · 20 Al2O3 · (80 − x) SiO2 (x = 10, 15 and 20) doped with 1 mol% CuO were studied by means of square-wave voltammetry in the temperature range from 800 to 1150 °C. Each voltammogram shows two distinct maxima. The first maximum occurs at potentials in the range from −100 to + 150 mV and is caused by the Cu+/Cu2+-redox pair. The second maximum is observed at more negative potentials and is due to the Cu+/Cu0-redox transition. With decreasing temperature both peaks are shifted to more negative (or less positive) values. From the linear dependency of the peak potentials upon the temperature ΔH°, ΔS° and the Cu+/Cu2+-redox ratio at 1000 °C were calculated. Furthermore the effect of the glass composition on the Cu+/Cu2+-redox equilibrium was discussed with respect to the melt structure and the incorporation of the copper ions into the alumosilicate network.