The effect of glass composition on the thermodynamics of the Fe 2+/Fe 3+ equilibrium and the iron diffusivity in Na 2O/MgO/CaO/Al 2O 3/SiO 2 melts

Abstract

Melts in the system Na 2O/MgO/CaO/Al 2O 3/SiO 2 doped with 0.25 mol% Fe 2O 3 were studied using square-wave voltammetry at temperatures in the range from 1000 to 1600 °C. The voltammograms exhibited a distinct peak attributed to the Fe 2+/Fe 3+-redox reaction. The peak potential is equal to the standard potential of the redox pair and from the peak current, the iron diffusion coefficients can be calculated. With increasing temperature, the peak is shifted to less negative potentials, due to the shift of the redox equilibrium to the reduced state, Fe 2+. Increasing Na 2O concentrations in melts without alumina lead to a shift in the peak potential to more negative potentials and to smaller viscosity corrected iron diffusion coefficients. This can be explained by the stabilization of FeO 4 − tetrahedra by Na + ions. Introducing alumina leads to less negative peak potentials and increasing viscosity corrected diffusion coefficients. In analogy to Fe 3+, alumina is incorporated as AlO 4 − tetrahedra which need alkali for charge compensation. If the molar alumina concentration exceeds that of Na 2O, additional alumina is no longer charged balanced, the peak potentials get more negative and the viscosity corrected iron diffusion coefficients decrease again. If CaO is introduced into the melt, the maxima in the peak potentials (as a function of the Al 2O 3 concentration) and viscosity corrected iron diffusion coefficients do no longer occur at [Al 2O 3]=[Na 2O], but at [Al 2O 3]=[Na 2O]+[CaO]/2. This is evidence that the charge of AlO 4 − tetrahedra can also be compensated by Ca 2+; however, one Ca 2+ can stabilize only one AlO 4 − tetrahedron and not two. In magnesia containing melts, the maxima in the peak potentials and iron diffusion coefficients occur at [Na 2O]=[Al 2O 3]. Hence, Mg 2+ does not contribute to the stabilization of AlO 4 − and FeO 4 − tetrahedra; however, increasing MgO concentrations lead to more negative peak potentials. This can be explained by the similarities of the ionic radii and metal–oxygen bond lengths of Mg 2+ and Fe 2+. Increasing MgO concentrations hence lead to the incorporation of Fe 2+ in energetically less advantageous sites which favours the Fe 3+ redox state. Empirical equations are given which allow to calculate standard potentials, equilibrium constants and hence Fe 2+/Fe 3+ redox ratios from the chemical composition.