Potentiometric determination of the Gibbs free energy of formation of cadmium and magnesium chromites

Abstract

The standard Gibbs free energy of formation of magnesium and cadmiumchromites have been determined by potentiometric measurements on reversible solid-state electrochemical cells in the temperature range 500°-730°C, and Pt, Cr + Cr₂O₃Y₂O₃-ThO ₂/Cr + MgCr₂O₄ + MgO, Pt in the temperature range 800°-1200°C. The temperature dependence of the freeenergies of formation of the ternary compounds can be represented by the equations CdO(r.s) + Cr₂O₃(cor)→ CdCr₂O₄(sp) ΔG⁰= -42,260+ 7.53T (±400)J and MgO(r.s) + Cr₂O₃(cor)→ MgCr2O4(sp) ΔG⁰= -45,200+ 5.36T (±400)J The entropies of formation of these spinels are discussed in terms of cationdisorder and extent of reduction of Cr³⁺ ions to Cr²⁺ ions. Thermodynamic data on the chromates of cadmium and magnesium are derived by combiningthe results obtained in this study with information available in the literatureon high temperature, high pressure phase equilibria in the systems CdO-Cr₂O₃-O₂ and MgO-Cr₂O₃-O₂.