Thermodynamics of the redox equilibrium Fe2+/Fe3+ and the diffusivity of iron in 5Na2O·15MgO·xAl2O3·(80−x)SiO2 (x=0–20) melts

Melts with the basic compositions 5Na2O·15MgO·xAl2O3·(80−x)SiO2 (x=0,5,10,15,20) doped with 0.5 mol% Fe2O3 were studied by means of square-wave voltammetry at temperatures in the range from 1300 to 1600 °C. Increasing temperatures resulted in less negative standard potentials, i.e. in a shift of the redox equilibrium to the reduced state. The redox equilibria were notably affected by the alumina concentration. Introducing 5 mol% Al2O3 to a soda–magnesia–silicate melt resulted in less negative standard potentials, while further increasing the alumina concentration led to more negative standard potentials. Diffusion coefficients of iron could be fitted to Arrhenius equation. They increased with increasing alumina concentration. A structural model is proposed which explains the effect of the alumina concentration on the thermodynamics of the Fe2+/Fe3+-equilibrium and on the diffusivity of iron.