Thermodynamic stabilities of Cs 2CrO 4(β), Cs 3CrO 4 and M 2Cr 2O 7(l) (where M = Na, K or Cs) by solid electrolyte emf method

Abstract

The emf of the galvanic cells with Pt, Cs 2CrO 4 (α or β)|Cr 2O 3|Cs 3CrO 4 (cell I) and Pt, s 2CrO 4(β)|Cr 2O 3|Cs 2Cr 2O 7(l) (cell II) against air|Pt reference electrode using 15 mol% calcia stabilized zirconia as the electrolyte were measured to be E I(a)(mV) = (1345.88 ± 3.68) − (0.53174 ± 0.03376) T(K); (860–967 K) E I(b) (mV) = (1266.47 ± 1.50) − (0.45345 ± 0.04351) T(K); (1052–1097 K) E II (mV) = (344.42 ± 1.84) − (0.25715 ± 0.0079) T(K); (1025–1189 K). Using the literature data for the standard Gibbs energies of formation of α-Cs 2CrO 4 and Cr 2O 3 together with the above numerical expressions for the emf values of cell I, the transition temperature and the standard enthalpy of α-to-β phase transformation in Cs 2CrO 4 were found to be 1014 K and 12.8 kJ mol −1. In addition, the Δ G f 0 (Cs 3CrO 4) was derived to be as follows: (Δ G f 0 (Cs 3CrO 4) ± 2.0) (kJ mol −1 = −1531.72 + 0.38044 T(K). The emf results from cell II led to the expression (Δ G f 0 (Cs 2Cr 2O 7,1 ± 3.35) (kJ mol −1) = − 2066.47 + 0.55062 T(K). By making emf measurements on galvanic cells with Pt, M 2CrO 4 (β)|Cr 2O 3|M 2Cr 2O 7 (l) (where M = Na or K) against air|Pt as the reference electro configuration identical with cell II, the following expressions for Δ G f 0 of M 2Cr 2O 7(l) were derived. (Δ G f 0(Na 2Cr 2O 7,1 ± 2.0) (kJ mol −1) = −1942.68 + 0.51289 T(K), (Δ G f 0(K 2Cr 2O 7,1 ± 2.5) (kJ mol −1) = −2037.35 + 0.5406 T(K). The third law treatment of the emf data on cells I and II yielded the values of −1549.8 ± 1.4 kJ mol −1 and −2108 ± 4 kJ mol −1 r for the standard enthalpies of formation, Δ H f298 0 of Cs 3CrO 4(s) and Cs 2Cr 2O 7(s).