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: $$3\text{CaO}\left(\text{s}\right)+{\text{P}}_{2}\left(\text{g}\right)+\left(5/2\right){\text{O}}_{2}\left(\text{g}\right)={\text{Ca}}_{3}{\text{P}}_{2}{\text{O}}_{8}\left(\text{s}\right)\Delta {G}^{\circ}/\text{kJ}\, {\text{mol}}^{-1}=-2306+0.562 T$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>3</mml:mn> <mml:mtext>CaO</mml:mtext> <mml:mfenced> <mml:mtext>s</mml:mtext> </mml:mfenced> <mml:mo>+</mml:mo> <mml:msub> <mml:mtext>P</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> <mml:mfenced> <mml:mtext>g</mml:mtext> </mml:mfenced> <mml:mo>+</mml:mo> <mml:mfenced> <mml:mn>5</mml:mn> <mml:mo>/</mml:mo> <mml:mn>2</mml:mn> </mml:mfenced> <mml:msub> <mml:mtext>O</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> <mml:mfenced> <mml:mtext>g</mml:mtext> </mml:mfenced> <mml:mo>=</mml:mo> <mml:msub> <mml:mtext>Ca</mml:mtext> <mml:mn>3</mml:mn> </mml:msub> <mml:msub> <mml:mtext>P</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mtext>O</mml:mtext> <mml:mn>8</mml:mn> </mml:msub> <mml:mfenced> <mml:mtext>s</mml:mtext> </mml:mfenced> <mml:mi>Δ</mml:mi> <mml:msup> <mml:mrow> <mml:mi>G</mml:mi> </mml:mrow> <mml:mo>∘</mml:mo> </mml:msup> <mml:mo>/</mml:mo> <mml:mtext>kJ</mml:mtext> <mml:mspace/> <mml:msup> <mml:mrow> <mml:mtext>mol</mml:mtext> </mml:mrow> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mo>=</mml:mo> <mml:mo>-</mml:mo> <mml:mn>2306</mml:mn> <mml:mo>+</mml:mo> <mml:mn>0.562</mml:mn> <mml:mi>T</mml:mi> </mml:mrow> </mml:math> .