Voltammetric studies of Fe2+/Fe3+-redox equilibria in some Na2O · CaO · Al2O3 · SiO2 liquids

Thermodynamics of the Fe2+/Fe3+-equilibrium were studied in liquids with the basic mol% compositions of 10Na2O · 10CaO · xAl2O3 · (80 − x)SiO2 (x=0, 5, 10, 15, 20, 25 mol%) and 16Na2O · 10CaO · xAl2O3 · (74 − x)SiO2 (x=0, 5, 10, 15). The method used was the square-wave voltammetry. The temperatures applied were 1000–1600°C. With increasing temperatures, in all liquids the equilibrium was shifted to the reduced state. In the peralkaline range (([Na2O] + [CaO])/[Al2O3] > 1), with increasing alumina concentration, the equilibrium was also shifted to the reduced state. By contrast, in the peraluminous range, with further increasing the Al2O3 content, the equilibrium constant remains the same (at 1000°C) or was shifted to the oxidized state again. A structural explanation for this behavior is given.