Measurement of Thermodynamic Property of Tri-calcium Phosphate Using Phase Relation in the CaO–SiO2–P2O5 Ternary System

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: $$3\text{CaO}\left(\text{s}\right)+{\text{P}}_{2}\left(\text{g}\right)+\left(5/2\right){\text{O}}_{2}\left(\text{g}\right)={\text{Ca}}_{3}{\text{P}}_{2}{\text{O}}_{8}\left(\text{s}\right)\Delta {G}^{\circ}/\text{kJ}\, {\text{mol}}^{-1}=-2306+0.562 T$$ 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 .