Orthophosphates. Part II. The transformations brushite → fluoroapatite and monetite → fluoroapatite in aqueous potassium fluoride solution

The transformations brushite → fluoroapatite and monetite → fluoroapatite have been investigated at 25 °C in aqueous acidic KF solutions, ca. 10^–1M in F^–. The transformations involve a first-order uptake of F^– by the secondary orthophosphates, followed by a solid-state rearrangement to fluoroapatite with the second-order release of H₂PO₄^–. The ΔG_f° for fluoroapatite has been calculated from the brushite → apatite transformation as –1524·1 kcal mol^–1, and confirmed by calculation of the ΔG_f° value for monetite as –401·1 kcal mol^–1. From these results, the ΔG_f° values of hydroxyapatite, Ca₅OH(PO₄)₂, Ca₃(PO₄)₂, and Ca(H₂PO₄)₂, and Ca(H₂PO₄)₂,H₂O were calculated as –1494·2, –925·1, –677·8, and –729·5 kcal mol^–1. The possibility of K substitution for Ca in fluoroapatite is discussed.