The effect of MgO on the thermodynamics of the Fe2+/Fe3+-redox equilibrium and the incorporation of iron in soda-magnesia-aluminosilicate melts

Melts with the basic compositions 10Na2O · 10MgO · xAl2O3 · (80−x)SiO2 (x=0, 5, 10, 15 and 20), 10Na2O · xMgO · 10Al2O3 · (80−x)SiO2 (x=5, 10, 15 and 20) and xNa2O · 10MgO · 10Al2O3 · (80−x)SiO2 (x=5, 10 and 15) all doped with 0.25 mol% Fe2O3 were studied using square-wave voltammetry. The temperatures applied were in the range of 1000–1600 °C. The square-wave voltammograms recorded show peaks caused by the reduction of Fe3+ to Fe2+. The attributed peak potentials measured decreased linearly with decreasing temperatures. Increasing the MgO-concentration led to more negative peak potentials. Introducing alumina in the melt first resulted in less negative peak potentials. If the molar Al2O3-concentration is equal to that of Na2O (=10 mol%) the peak potentials are least negative. Further increase of the Al2O3-concentration led to more negative peak potentials. The variation of the Na2O-concentration led to a maximum in the peak potentials at an Na2O-concentration of 10 mol%. An empirical formula which allows the calculation of standard potentials from the chemical composition is proposed. Furthermore, a structural explanation for the effect of the chemical composition is given. Especially, the incorporation of Al2O3 as AlO4−-tetrahedra at [Al2O3] < [Na2O] and as network modifier at larger concentrations was structurally explained by the similarities of Fe2+ and Mg2+, with respect to cation radii and metal–oxygen bond lengths.