Thermodynamic stability of Sb 2O 3 by a solid-oxide electrolyte e.m.f. technique

Abstract

The potentials of the solid-oxide electrolyte galvanic cells: Pt|C|Sb(cr)|Sb 2O 3(cr)|ysz|air { p( O 2 p ° )=0.21}|Pt , from 662 to 890 K, and Pt|C|Sb(l)|Sb 2O 3(l)|ysz|air { p( O 2 p ° )=0.21}|Pt , from 948 to 1051 K, where ysz denotes 15 mass per cent of Y 2O 3 stabilizing ZrO 2 have been measured over the temperature ranges shown. These have been used to derive the standard molar Gibbs energies of formation: Δ f G m °( Sb 2O 3,cr ,T)/(kJ·mol −1)=(−686.36+0.24594T/ K)±0.80 , Δ f G m °( Sb 2O 3,l ,T)/(kJ·mol −1)=(−663.42+0.21953T/ K)±0.65 . Even with the high precision of the e.m.f. technique, the orthorhombic-to-cubic transformation of Sb 2O 3 at 846 K could not be detected. Hence it has been assumed that Sb 2O 3 saturated with Sb remained in the orthorhombic form at least up to 895 K. A third-law treatment of the Δ f G m o(Sb 2O 3, cr, T) results has shown the absence of significant temperature-dependent errors in e.m.f. and has yielded a value of −(706.0 ± 2.0) kJ · mol −1 for Δ f H m o(Sb 2O 3, cr, orthorhombic, 298.15 K). The e.m.f.s in the range 895 to 950 K have been found to be irreproducible, the reasons for which have been discussed in the light of mutual solubility and proximity of the transition temperature of Sb and Sb 2O 3. The standard molar Gibbs energy expressions for solid and liquid Sb 2O 3 have yielded a value of 22.9 kJ · mol −1 for the difference in the two intercepts, in good agreement with 21.8 kJ · mol −1 calculated from the standard molar enthalpy of transformation of Sb 2O 3 from orthorhombic to cubic and of fusion of Sb and Sb 2O 3 reported in the literature (neglecting ∫ C p, m ° d T terms). A comparison of the Δ f G m o( T) results with those from earlier e.m.f. studies has also been made.