Abstract The thermodynamic properties of Ca 3 P 2 O 8 at high temperatures were investigated using the CaO–SiO 2 –P 2 O 5 ternary phase relationship. The standard formation Gibbs energy of Ca 3 P 2 O 8 was measured at 1473 K to 1573 K by applying a chemical equilibrium method to the Ca 3 P 2 O 8 + CaSiO 3 + SiO 2 and Mg 3 P 2 O 8 + MgO oxide mixtures. The latter was used as a reference system for calibration to minimize intrinsic experimental errors. Additionally, a third-law analysis revealed excellent consistency between the measured Gibbs energy and literature data for the heat capacity and heat of formation of Ca 3 P 2 O 8. Consequently, the standard Gibbs energy of formation of Ca 3 P 2 O 8 from CaO, P 2, and O 2 was established as follows: 3CaO (s) +P₂ (g) + (5/2) O₂ (g) =Ca₃P₂O₈ (s) G^/kJ\, mol^-1=-2306+0. 562 T 3 CaO s + P 2 g + 5 / 2 O 2 g = Ca 3 P 2 O 8 s Δ G ∘ / kJ mol - 1 = - 2306 + 0. 562 T.
Saito et al. (Tue,) studied this question.