Objectives: The solubility isotherm (S) of hydroxyapatite (HAp) is of fundamental importance to saliva chemistry, dental caries and related contexts. It has previously been shown that the locus of the S[HAp] is substantially lower than is commonly reported, and of different slope, probably due to HAps incongruent dissolution. The aim of the present study was to determine the S[HAp] over a wider pH range and to identify the precipitate formed at equilibrium in HAp solid titration.
Methods: The solid titration technique of Leung and Darvell (Leung VW-H, Darvell BW. Calcium-phosphate system in saliva-like media. J Chem Soc Faraday Trans 1991;87(11):1759-64.) was used to investigate the solubility behaviour of HAp at 37.0+/-0.1 degrees C in 100 mM aqueous KCl. The pH range studied overlapped that of earlier work from pH 3.6 to 5.2, for a reproducibility check and validation, and extended to pH approximately 2.9. XRD and EDX were used to identify the precipitates. SEM and TEM were used to observe the morphology.
Results: The previous S[HAp] reported by Chen et al. (Chen Z-F, Darvell BW, Leung VW-H. Hydroxyapatite solubility in simple inorganic solutions. Arch Oral Biol 2004;49(5):359-67.) was reconfirmed. An abrupt change of slope of S[HAp] was detected at pH approximately 3.9. No other phase than HAp was found at pH 3.2, 3.6 and 4.1. In particular, brushite (dicalcium phosphate dihydrate) was not detected, even below pH 3.9, where instead calcium-deficient HAp was formed.
Conclusion: The solid titration method was reconfirmed as reproducible and to yield HAp from pH 2.9 to 5.2. The expected brushite did not appear, but rather a stable calcium-deficient HAp was consistently formed. The chemistry of calcium phosphates needs to be reevaluated.