The effect of excess phosphate on the solubility of hydroxyapatite

The solubility of hydroxyapatite (HA) is critical in fields such as medicine, dentistry and geochemistry. Previously, it had been found that it was apparently slightly increased with 1 mmol/L excess phosphate present. This study was to determine the solubility of HA over the pH range 3.3–5.4 with a series of concentrations of such ‘excess’ phosphate with solid titration (ST), and to identify the precipitate formed at equilibrium with SEM, TEM, EDX, FTIR and XRD. For [PO4]XS=0–0.3 mmol/L, results followed closely the already-reported ST ‘low’ solubility isotherm. At [PO4]XS=0.5 mmol/L, the solubility surface switched abruptly to a ‘high’ position that could not be reconciled with either the ‘low’ isotherm or conventional calculations. Thus, HA solubility is critically dependent on the presence of excess phosphate. Such excess may account for the discrepancy between ST and excess-solid results, although the crystallographic explanation is as yet lacking.