Determination of standard Gibbs free energy of formation for Ca2P2O7 and Ca(PO3)2 from solid-state EMF measurements using yttria stabilised zirconia as solid electrolyte

The equilibrium reactions: 3Ca2P2O7(s)+6Ni(s)⇄2Ca3(PO4)2(s)+2Ni3P(s)+52O2(g) and 2Ca(PO3)2(s)+6Ni(s)⇄Ca2P2O7(s)+2Ni3P(s)+52O2(g) were studied in the temperature range 890 K to 1140 K. The oxygen equilibrium pressures were determined using galvanic cells incorporating yttria stabilized zirconia as solid electrolyte. From the measured data and using the literature values of standard Gibbs free energy of formation for Ca3(PO4)2 and Ni3P, the following relationship of the standard Gibbs free energy of formation for Ca2P2O7 and Ca(PO3)2 were calculated:ΔfG∘(Ca2P2O7)±11/(kJ·mol-1)=-3475.9+1.5441(T/K)-0.1051(T/K)·ln(T/K)andΔfG∘(Ca(PO3)2)±12/(kJ·mol-1)=-3334.8+6.1561(T/K)-0.6950(T/K)·ln(T/K).