Oxygen-ion activity and its influence on redox equilibria in a ternary soda-lime-silica glass system

Abstract

The influence of oxygen-ion activity on redox equilibria in a ternary soda-lime-silica glass containing (in mol%) 20 Na2O , 10 CaO, 70 SiO2 was studied at 1400 °C in air atmosphere. On plotting lg K' (Κ' = [M^(x+n)+] / ([M^x+] [pO2]^n/4)) against the experimentally determined relative logarithm of oxygen-ion activity (Δlg O^2-) based on water-vapour solubility data in lithium and sodium silicate glasses at 1400 °C, linear relationships were found for Cu+ - Cu2+, Fe2+ - Fe3+, Ce3+ - Ce4+, Sn2+ - Sn4+, As3+ - As5+ and Cr3+ - Cr6+ redox reactions. The experimentally observed values of lg K' for Cu+ - Cu2+ and Cr3+ - Cr6+ redox pairs in ternary soda-lime-silica glass at 1400 °C in air were found to he on a vertical line corresponding to a particular value of Δlg O^2- for the glass. This value of Δlg O^2- for 20 Na2O · 10 CaO · 70 SiO2 glass indicates that under identical conditions the values of lg K' in this base glass should be around 1.80, 0.85, 0.50 and - 0.40 for Sn2+ - Sn4+, Fe2+ - Fe3+, As3+ - As5+ and Ce3+ - Ce4+ redox pairs, respectively. The predicted results of lg K' agree well with the results obtained by earlier workers. The observations open up a new possibility for predicting the values of lg K' for these redox pairs in ternary systems. The changes in coordinations and symmetries of the redox species during these redox reactions in silicate glasses greatly influence the slopes of the straight lines obtained for Ig K' versus Δlg O^2-.