MEASUREMENTS OF STANDARD GIBBS ENERGIES OF FORMATION OF SB2O3, MGSB2O4 AND CA4SB2O7 BY AN EMF METHOD

Magnesium and calcium antimonites, which can coexist with MgO and CaO, respectively, were synthesized by a phase-equilibration technique and identified by X-ray diffraction to be MgSb2O4 and Ca4Sb2O7. To determ ine the standard Gibbs: energies of formation of those antimonites, em f values of the following galvanic cells have been measured: Pt, Re/Sb (s,l), Sb2O3(s,l)/ZrO2 + CaO(or Y2O3)/air, Pt, Pt, Re/Sb (l), MgSb2O4 (s), MgO(s)/ZrO2 + CaO/air, Pt, Pt, Re/Sb (l), Ca4Sb2O7(s), CaO(s)/ZrO 2 + CaO/air, Pt. The following free energies of formation were determi ned: Delta G(0)/kJ mol(-1) +/- 0.4 = - 687.7 + 0.2425 T (773-904 K), f or 2 Sb(s) + 3/2 O-2(g) = Sb2O3(s), Delta G(0)/kJ mol(-1) +/- 0.3 = - 730.2 + 0.2895 T (904-929 K), for 2 Sb(l) + 3/2 O-2(g) = Sb2O3(s), Del ta G(0)/kJ mol(-1) +/- 0.2 = -683.1 + 0.2385 T (929-1095 K), for 2 Sb( l) + 3/2 O-2(g) = Sb2O3(l), Delta G(0)/kJ mol(-1) +/- 0.7 = -78.9 + 0. 0495 T (929-1101 K), for Sb2O3(l) + MgO(s) = MgSb2O4(s), Delta G(0)/kJ mol(-1) +/- 0.7 = - 134.7 + 0.0017 T (972-1101 K), for Sb2O3(l) + 4Ca O(s) = Ca4Sb2O7(s).