Effect of Carbonate on Hydroxyapatite Solubility

Carbonate is present in biological apatites (BAp) (enamel, dentine, bone, and pathological calcifications) by substitution at phosphate and hydroxide sites, tending to increase its solubility in comparison with pure hydroxyapatite (HAp). The role of solution carbonate, however, is poorly understood. Using the solid titration method, it was found that the apparent solubility of HAp increased greatly with an increase in pCO 2, and was sensitively-dependent at low values. No other phase was formed at the end point except HAp, and in particular no dicalcium phosphate dihydrate (DCPD) was found. However, no carbonate was detected in the end-point solid except for a small amount at pH ∼ 7.4, pCO 2 = 0.01−1.0 bar. The implications of these results need further investigation, bearing as they do on a central topic in oral chemistry.