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 10Na(2)O . 10CaO . xAl(2)O(3) . (80 - x)SiO2 (x = 0, 5, 10, 15, 20, 25 mol%) and 16Na(2)O . 10CaO . xAl(2)O(3) . (74 - x)SiO2 (x = 0, 5, 10, 15). The method used was the square-wave voltammetry. The temperatures applied were 1000-1600 degrees 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 degrees C) or was shifted to the oxidized state again. A structural explanation for this behavior is given. (C) 2000 Elsevier Science B.V. All rights reserved.