413surface under oxidizing conditions. The foaming tendency diminishes under more reducing conditions. In particular they found that the foam becomes progressively destabilized in a process that begins with the appearance of the ferrous/ferric ratio. This corresponds to an oxygen activity of = 1 to 10 Wa from Fig. 1 . Thus, a practical upper bound for the control of foaming is suggested to occur at = 1 kPa, which corresponds to a ferrous/ferric ratio near 0.1. This upper bound corresponds to a ratio that can be readily measured and occurs in the linear range that is not affected by the mutual redox interactions described earlier.Schreiber et a l . * indicated that metallic phase formation (Ni, Fe, Co) is initiated in Savannah River glass at conditions more reducing than kPa. This oxidation state corresponds to a ferrous/ferric ratio from Fig. 1 at 1150°C of ;=.1.0. The ferrous/ ferric range between 0.1 and 1 is further narrowed upon considering other chemical constraints, such as the concentration of sulfur that is soluble in the melt over this range. Sulfur solubility is expected to be maximized at the oxidized extreme of this range for this West Valley glass.14 Under these conditions, less chemical processing of the waste would be required to diminish the sulfur concentration that eventually enters the melter. Therefore, a practical range for the vitrification process correlates with a ferrous/ ferric ratio between 0.1 and 0.5. V. ConclusionsThis study provides the first demonstration that the iron redox couple can be used as a quantitative redox indicator for the range of oxidation states anticipated for nuclear waste vitrification of complex borosilicate glasses that contain several other interacting redox species. Mutual redox interactions between iron and other multivalent ions do occur, but only under relatively oxidizing conditions which result in a departure from otherwise theoretical redox behavior for iron.

Ferroudferric equilibria were determined in an alkali-iron borosilicate glass of the type that is presently being considered for nuclear waste disposal. Theoretical redox behavior is found at conditions more reducing than 1 Wa. Deviation from this behavior at more oxidizing conditions results from mutual interactions between the iron redox couple and other multivalent ions in the glass. The results suggest that the ferrous/ferric ratio can be used as a quantitative indicator of the glass oxidation state in these complex glasses. Implications regarding redox control of the vitrification process are discussed.