A computer simulation of the reactions between hydroxyapatite and fluoride in aqueous acidic solutions

A precipitation reaction between hydroxyapatite (HAp) and fluoride solution was analyzed by computer simulation. The system comprised excess HAp and 0.05-0.2 M acetic buffer solutions containing 0.5-2.0 mM potassium fluoride. The amounts of HAp dissolved and calcium salts precipitated and activities of all the species present in the solution were calculated during the process. In the systems, calcium fluoride was initially precipitated when applying the fluoride solution to HAp, followed by precipitation of fluorapatite (FAp) and re-dissolution of the previously precipitated calcium fluoride at a critical pH at which the ion activity product exceeded the solubility product of FAp. The critical pH was increased with pH of the buffer, whereas it was scarcely affected by fluoride ion concentration, and it was necessary to introduce phosphate ions in advance into the buffer for predominant precipitation of FAp in this system.