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, $Cr + CaO + CaCr_20_4\|ThO_2(Y_20_3)\|Cr + Cr_20_3$, Pt in the temperature interval $1050 \leq T \leq 1475 K$ and Pt, $0_2, CaO + CaF_2\|CaF_2\|CaF_2 + Cr_20_3 + CaCrO_4, 0_2$, Pt for $870 \leq T \leq 1100 K$. Calcium chromate $(CaCrO_4)$ was found to be unstable in contact with $Cr_20_3$ and $O_2 (1.01 \times l0^5 Pa)$ 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 $CaO (r.s.) + Cr_20_3 (cor.)\rightarrow CaCr_20_4 (\beta), \Delta G^o = -60,970 - 11.29 T (\pm 350) J mo1^{-1}, and \hspace{2mm} CaO (r.s.) + 1/2 Cr_20_3 (cor.) + 3/4 O_2 (g)\rightarrow CaCrO_4 (zir.) \Delta G^o = -219,250 + 121.76 T (\pm 400) J mol^{-1}$. Thermodynamic data for $CaCr_20_4$ have been measured for the first time. The results obtained for $CaCrO_4$ 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 $SrCrO_4$ at 298 K is estimated as $-250 (\pm 5) kJ mol^{-1}$