Thermodynamics of iron in alkali-magnesia-silica glasses

Alkali-magnesia silica glasses with lithium (74 SiO2, 16 Li2O, 10 MgO), sodium ((90 - x) SiO2, x Na2O, 10 MgO (x = 10, 15, 20, 25)), potassium ((90 - x) SiO2, x K2O, 10 MgO (x = 10, 15, 20, 25)) and caesium (74 SiO2, 16 Cs2O, 10 MgO) (all compositions given in mol%) doped with 0.2 mol% Fe2O3 were studied by means of square-wave voltammetry. Decreasing Na2O and K2O contents lead to an increase in the standard potentials of the Fe3+/Fe2+ redox pair. A comparison of the different alkali ions shows that the standard potentials increase from lithium to caesium. The standard potentials, standard enthalpies and standard entropies of iron in the alkali-magnesia-silica glasses are compared with literature data of alkali-lime-silica glasses. Empirical correlations enable a quantitative calculation of the standard potentials from the chemical composition. (C) 1998 Elsevier Science B.V. All rights reserved.