Thermodynamic Measurement of Calcium Phosphates by Double Knudsen Cell Mass Spectrometry

Abstract

The pressures of gaseous phosphorus oxide in equilibrium with a mixture of (CaO)₄P₂O₅ and CaO or (CaO)₃P₂O₅ and (CaO)₄P₂O₅ under a phosphorus vapor pressure controlled by Cu-P alloy at 1523 to 1623 K were directly measured by double Knudsen cell mass spectrometry. The Gibbs energy changes of the following reactions calculated from the measured pressures were as follows:

$$ \begin{aligned} &4{\text{CaO}}\left( {\text{s}} \right) + {\text{ P}}_{ 2} ( {\text{g}}) + {\frac{5}{2}}{\text{O}}_{ 2} ( {\text{g}} ) = ( {\text{CaO}})_{ 4} {\text{P}}_{ 2} {\text{O}}_{ 5} ({\text{s}}) \\ & \quad \Updelta G^{^\circ } = - 2,211,000 + 520T( { \pm 15,000})\,{\text{J}}\\ & 3{\text{CaO}}({\text{s}}) + {\text{ P}}_{ 2} ({\text{g}}) + {\frac{5}{2}}{\text{O}}_{ 2} ({\text{g}}) = ({\text{CaO}})_{3} {\text{P}}_{ 2} {\text{O}}_{ 5} ({\text{s}}) \\ & \quad \Updelta G^{^\circ } = - 2,204,000 + 526T({ \pm 16,000})\,{\text{J}} \end{aligned}$$