Solubility of thermal synthesized fluorhydroxyapatites

Fluorhydroxyapatites were synthesized by solid state reaction at 1000°C. The solubility was determined in diluted phosphoric acid. At pH 4 and 5 a degree of substitution x of OH− by F− ions of only a few percent was sufficient to reduce the solubility by 50%, At pH 6 and 7 very high degrees of substitution were necessary to reach this effect. Addition of F ions to the diluted phosphoric acid had no effect on the solubility in the ranee x > 0.3, but it caused a reduction in solubility with up to 88% for hydroxyapatite at pH 7. Addition of Ca2+ ions to the diluted phosphoric acid had no effect on the solubility of hydroxyapatite (OHA). However, it increased the solubility of fluorapatite (FA) and of OHA - FA solid solutions considerably.