Orthophosphates. Part IV. Stability relationships of orthophosphates within the systems CaO–P2O5–H2O and CaF2–CaO–P2O5–H2O under aqueous conditions

Abstract

Data relating to the ionic activities of supernatant solutions arising from the fluoride-induced transformations of monetite, CaHPO₄, and brushite, CaHPO₄,2H₂O, were used to calculate ΔG_f° of Ca₂F(PO₄), Ca₃(PO₄)₂, Ca(H₂PO₄)₂, and Ca(H₂PO₄)₂,H₂O as –600·3, –926·1, –675·2, and –731·8 kcal mol^–1 respectively. The stabilities of the various calcium phosphates in the system CaO–P₂O₅–H₂O have been investigated under aqueous conditions relative to the pH. The relationship at 25°C is of the form Ca(H₂PO₄)₂,H₂O ⇌ CaHPO₄,2H₂O (pH 2·3); CaHPO₄,2H₂O ⇌ Ca₈H₂(PO₄)₆,5H₂O (pH 4·85); Ca₈H₂(PO₄)₆,5H₂O ⇌ Ca₅OH(PO₄)₃(pH 5·95); Ca₅OH-(PO₄)₃⇌ Ca₄O(PO₄)₂(pH 6·55). The Ca₈H₂(PO₄)₆,5H₂O ⇌ Ca₅OH(PO₄)₃ and Ca₅OH(PO₄)₃⇌ Ca₄O-(PO₄)₂ transformations are dependent on the activity of Ca²⁺, and show increase in pH with decrease in the activity. Preliminary results have also been obtained of the relationship existing under aqueous conditions at 25°C in the system CaO–P₂O₅–CaF₂–H₂O, which may be summarised as Ca₄O(PO₄)₂⇌ Ca₂FPO₄(pH 8·05); Ca₂FPO₄⇌ Ca₅F(PO₄)₃, (pH 6·63); Ca₅F(PO₄)₃⇌ CaF₂(pH 2·10).