Equilibrium Studies of Fe in Alkali Phosphate Glasses

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Abstract
The Fe2+‐Fe3+ equilibrium in binary Na2O‐P2O5, glasses was studied by equilibrating glass melts at different temperatures in air. The enthalpy change (δH) of the reaction 1/2Fe2O3⇌FeO+1/4O2 was calculated for 4 glasses. The results indicate that (1) the equilibrium shifts toward the oxidized state as the Na2O content of the glass increases (plots of log ([Fe2+]/[Fe3+]) vs mol% alkali were linear) and (2) ΔH values for glasses of different composition are nearly equal but differ from the standard (calculated) value for the reaction. The experimental ΔH values were nearly equal to that for the reaction FePO4→1/3 Fe3(PO4)2+ 1/6P2O5+1/4O2, indicating that Fe forms phosphate or polyphosphate configurations in the Na2O‐P2O5 glasses. In certain of the glasses studied a faint‐pink solid precipitated; its X‐ray diffraction pattern indicated that its principal component is crystalline Na2Fe111P3O10.

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