Electrochemical determination of Gibbs energies of formation of calcium chromite and chromate

Abstract

The standard Gibbs free energies of formation of calcium chromite and calcium chromate have been determined employingsolid state galvanic cells Pt, Cr + CaO + CaCr₂O₄||ThO₂(Y₂O ₃)||Cr + Cr₂O₃, Pt in the temperature interval 1050 ≤T≤ 1475 K and Pt, O₂, CaO + CaF₂||CaF₂||CaF₂ + Cr₂O₃ + CaCrO₄, O₂, Pt for 870≤ T ≤1100 K. Calcium chromate(CaCrO₄) was found to be unstable in contact with Cr₂O₃ and O₂ (1.01 × 105 Pa) at T > 1125 K. The reversibility of the EMF was confirmed by microcoulometric titration. The temperature dependence of the standard Gibbs energy of formation ofthe ternary oxides can be represented by the equations CaO (r.s.) + Cr₂O₃ (cor.) #8594; CaCr₂O₄ (β), ΔG° = -60,970 - 11.29 T(±350) J mol^-1, and CaO (r.s.) + ½ Cr₂O₃ (cor.) + 3/4 O₂ (g) → CaCrO₄ (zir.) ΔG° = -219,250 + 121.76 T (±400) J mol^-1. Thermodynamic data for CaCr₂O₄ have been measured for the first time. The results obtained for CaCrO₄ are in good agreement with the limited data available in the literature. Periodic trends in the stability of alkaline earth chromates are analyzed. Based on the systematics, the enthalpy of formation of SrCrO₄ at 298 K is estimated as -250 (±5) kJ mol^-1.