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)), potassi um ((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. Decreas ing Na2O and K2O contents lead to an increase in the standard potentia ls of the Fe3+/Fe2+ redox pair. A comparison of the different alkali i ons shows that the standard potentials increase from lithium to caesiu m. The standard potentials, standard enthalpies and standard entropies of iron in the alkali-magnesia-silica glasses are compared with liter ature data of alkali-lime-silica glasses. Empirical correlations enabl e a quantitative calculation of the standard potentials from the chemi cal composition. (C) 1998 Elsevier Science B.V. All rights reserved.