Redox chemistry in glass melts out of equilibrium with their gaseous atmosphere

The redox behaviour of glass melts out of equilibrium with their gaseous atmosphere has been investigated. Melts having the composition Na2O·1.2SiO2·1.2B2O3 and containing antimony oxide at various concentrations were equilibrated with air at 1200°C and then rapidly cooled down to 950°C. The redox ratio ([Sb(V)]/[Sb(III)]) and the internal pressure of oxygen were measured, respectively, by voltammetry and potentiometry, before and after cooling as a function of the antimony concentration. The values of these two properties in the cooled melt are all lower as the total concentration of antimony is increased. Assuming an equilibrated reaction between the antimony and the oxygen redox systems, the equilibrium constant of this reaction may be calculated from the measured redox ratios and internal pressures of oxygen; the calculated equilibrium constant is independent of the total concentration of antimony. These results are discussed, thermodynamic functions are calculated and a general equation relating redox ratios at 1200°C and 950°C and the total concentration of antimony in the melt is proposed. This equation also allows one to calculate the solubility of oxygen in the melt at 1200°C from the measured redox ratios at high and low temperatures.