Potentiometric Determination of the Gibbs Free Energy of Formation of Cadmium and Magnesium Chromites

The standard Gibbs free energy of formation of magnesium and cadmium chromites have been determined by potentiometric measurements on reversible solid-state electrochemical cells [GRAPHICS] in the temperature range 500 degrees-730 degrees C, and Pt, Cr + Cr(2)O(3)/Y(2)O(3)-ThO(2)/Cr + MgCr(2)O(4) + MgO, Pt in the temperature range 800 degrees-1200 degrees C. The temperature dependence of the free energies of formation of the ternary compounds can be represented by the equations CdO (r.s.) + Cr(2)O(3) (cor) -> CdCr(2)O(4)(sp) Delta G degrees = -42,260 + 7.53T (+/- 400) J and MgO (r.s.) + Cr(2)O(3) (cor) -> MgCr(2)O(4) (sp) Delta G degrees = -45,200 + 5.36T (+/- 400) J The entropies of formation of these spinels are discussed in terms of cation disorder and extent of reduction of Cr(3+) ions to Cr(2+) ions. Thermodynamic data on the chromates of cadmium and magnesium are derived by combining the results obtained in this study with information available in the literature on high temperature, high pressure phase equilibria in the systems CdO-Cr(2)O(3)-O(2) and MgO-Cr(2)O(3)-O(2).