The effect of carbonate on the solubility of hydroxylapatite

The solubility of hydroxylapatite increased with increasing concentrations of bicarbonate in the solution. Solution CO 2 exchanged with solid P on a mole for mole basis. About one-fourth of the CO 2 in the carbonate exposed hydroxylapatite was lost upon heating to 300 °C, suggesting the presence of appreciable amounts of CO 2 in the form of bicarbonate. Increase of CO 2 in solid did not change the solubility of hydroxylapatite. Precipitates formed from metastable solutions of calcium and phosphate contained carbonate in proportion to the concentration of carbonate in the solution. The solubility of the precipitates increased with increase in the carbonate content. Heating experiments suggested that the CO 2 in the precipitate was present in the form of carbonate. X-ray diffraction studies showed that the precipitates were hydroxylapatite and that the presence of carbonate disturbed crystallization. The interrelationship between the carbonate phosphate system in saliva and tooth mineral is discussed.