The effect of fluoride on the Fe2+/Fe3+-redox equilibrium in glass melts in the system Na2O/NaF/CaO/Al2O3/SiO2

Liquids with the base compositions (16 − x/2)Na2O · xNaF · 10CaO · 74SiO2 (x = 0, 1, 3, and 4) and (10 − x/2) · Na2O · xNaF · 10CaO · yAl2O3 · (80 − y)SiO2 (x = 0, 1, 3, 5 and y = 5 and 15) doped with 0.25 mol% Fe2O3 were studied by means of square-wave voltammetry in the temperature range from 1000 to 1500 °C. With increasing temperature, the redox equilibria were shifted to the reduced state. Also while increasing the alumina concentration, the Fe2+/Fe3+-redox equilibrium is shifted to the reduced state. In the soda-lime–silica melt the addition of fluoride shifts the equilibrium to the oxidized state, while in the aluminosilicate melts with 15 mol% Al2O3, the equilibrium is shifted to the reduced state. In the aluminosilicate melts with 5 mol% Al2O3, the equilibrium was not affected by the fluoride concentration. This is explained by the structure of the respective glass compositions.