Compositional dependence of redox equilibria in sodium silicate glasses

Abstract

The effects of melt composition on the redox equilibria of cerium, chromium, copper, iron manganese and uranium have been individually determined in sodium silicate glass melts at constant oxygen fugacity and temperature. Increased melt basicity shifts the Ce 4+ Ce 3+ , Cr 6+ Cr 3+ , Mn 3+ Mn 2+ and U 6+ U 5+ redox couples in the direction of the oxidized state, and shifts the Cu 2+ Cu + couple to the formation of the reduced state. The compositional shift is dependent on the solvation of the individual redox ions, in particular whether the oxidized or reduced state has the higher coordination number in the resulting glass. The Fe 3+ Fe 2+ redox couple becomes more reduced as the melt composition becomes more basic up to a molar Na/Si ratio of about unity, but then becomes more oxidized as the melt is made even more basic. This compositional dependence is consistent with a change in coordination of Fe 3+ with composition; in acid melts, the Fe 3+ ion is in octahedral coordination, whereas in basic melts it changes to the FeO 2 − ion in tetrahedral coordination.