The effect of alumina on the Sn2+/Sn4+ redox equilibrium and the incorporation of tin in Na2O/Al2O3/SiO2melts

Glasses with the basic compositions 10Na(2)O . 10CaO . xAl(2)O(3) . (80 - x)SiO2 (x = 0, 5, 15, 25) and 16Na(2)O 10CaO . xAl(2)O(3) . (74 - x)SiO2 (x = 0, 5, 10, 15, 20) doped with 0.25 - 0.5 mol% SnO2 were studied using square-wave-voltammetry at temperatures in the range from 1000 to 1600 degreesC. The voltammograms exhibit a maximum which increases linearly with increasing temperature. With increasing alumina concentration and decreasing Na2O concentration the peak potentials get more negative. Mossbauer spectra showed two signals attributed to Sn2+ and Sn4+. Increasing alumina concentrations did not affect the isomer shift of Sn2+; however, they led to increasing quadrupole splitting, while in the case of Sn4+ both isomer shift and quadrupole splitting increased. A structural model is proposed which explains the effect of the composition on both the peak potentials and the Mossbauer parameters. (C) 2004 Elsevier B.V. All rights reserved.