ELECTROCHEMICAL DETERMINATION OF GIBBS ENERGIES OF FORMATION OF CALCIUM CHROMITE AND CHROMATE

The standard Gibbs free energies of formation of calcium chromite and calcium chromate have been determined employing solid state galvanic cells Pt, Cr + CaO + CaCr2O4 parallel-to ThO2(Y2O3) parallel-to Cr + Cr2O3, Pt in the temperature interval 1050 less-than-or-equal-to T less-than-or-equal-to 1475 K and Pt, O2, CaO + CaF2 parallel-to CaF2 parallel-to CaF2 + Cr2O3 + CaCrO4, O2, Pt for 870 less-than-or-equal-to T less-than-or-equal-to 1100 K. Calcium chromate (CaCrO4) was found to be unstable in contact with Cr2O3 and O2 (1.01 x 10(5) 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 of the ternary oxides can be represented by the equations CaO (r.s.) + Cr2O3 (cor.) --> CaCr2O4 (beta), DELTA-G-degrees = -60,970 - 11.29 T (+/- 350) J mol-1, and CaO (r.s.) + 1/2 Cr2O3 (cor.) + 3/4 O2 (g) --> CaCrO4 (zir.) DELTA-G-degrees = -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.