Hydroxyapatite solubility in simple inorganic solutions

Objective: To use solid titration and laser-scattering end-point detection to determine the hydroxyapatite (HAP) solubility isotherm at 37 °C in a plain KCl solution, with CO2 present, and with additional phosphate. Design: Pulverised HAP solid was used to perform a titration to saturation. A very sensitive detector capable of detecting the light scattered at small forward angles from a laser beam was adopted to monitor the onset of precipitation (or failure to dissolve) near the equilibrium point. Each addition of HAP solid caused a step increase in scattered light and this signal decreased quasi-exponentially with time. When a steady but elevated scattering was obtained, small portions of 1 M HCl were added to lower the pH by about 0.5–2 units, according to need, to dissolve excess solid and allow a further titration. The scatter signal and pH data, plotted against the amount of HAP added, were used to estimate the actual end-point by interpolation and thus construct each point on the solubility isotherm. Results: The solubility isotherm for HAP in plain KCl solution in the absence of CO2 obtained now differs substantially from the results of previous solubility studies (i.e. much lower). The increase in solubility due to CO2 reported earlier was confirmed, while the effect of excess phosphate was found to be to increase the apparent solubility of HAP, contrary to elementary mass-action expectations. Conclusions: The solid titration method is a more reliable approach than that of conventional excess solid method with respect to the determination of the HAP-carrying capacity. The solubility of HAP appears to be substantially lower than previously reported, and a reconsideration of all aspects of the system is warranted.