Numerous biological and chemical studies involve the use of calcium hydroxyapatite (HA), Ca10(PO4)6(OH)2. In this study detailed physicochemical characterization of HA, prepared from an aqueous solution, was carried out employing different methods and techniques: chemical and thermal analyses, x-ray diffraction, infrared and Raman spectroscopies, scanning and transmission microscopies, and Brunauer, Emmett, and Teller (BET) surface-area method. The contents of calcium (Ca2+), phosphate (PO43−), hydroxide (OH−), hydrogenphosphate (HPO42−), water (H2O), carbonate (CO32−), and trace constituents, the Ca/P molar ratio, crystal size and morphology, surface area, unit-cell parameters, crystallinity, and solubility of this HA were determined. This highly pure, homogeneous, and highly crystalline HA is certified as a National Institute of Standards and Technology (NIST) standard reference material, SRM 2910.

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Apatites containing CO₃ and/or F were synthesized and exposed to acid buffer. The extent of dissolution was determined (as mMCa/ml buffer solution) and the apatites characterized by X-ray diffraction, IR absorption, and chemical analyses before and after acid exposure. Results showed that: (i) the extent of dissolution was directly proportional to the CO₃ contents but that the simultaneous presence of F in the apatite minimized the adverse CO₃ effect; (ii) the extent of dissolution during the second exposure was much less than during the first exposure; (iii) the lattice parameters, crystal-Unity and CO₃ and F contents of the apatites differed before and after exposure to the acid buffer, i.e., larger a-axis, initial decrease then increase in crystallinity, lower carbonate and higher fluoride contents of apatites after acid exposure. Results from this study suggest that the vulnerability of synthetic and biological apatites to acid dissolution is largely due to their carbonate constituent and that the caries process may involve a combination of dissolution of carbonate-rich/fluoride-poor apatites and reprecipitation of carbonate-poor/fluoride-rich enamel apatites and that the reprecipitated apatite is rendered more resistant to acid dissolution.

Octacalcium phosphate (Ca8H2(PO4)6 . 5H2O) is considered to be precursor in the formation of apatite in bones and teeth; a crucial step for incorporation of impurities appears to occur during its hydrolysis. The present study examines the role that octacalcium phosphate plays in the process of incorporation of carbonate into apatite. Chemical, X-ray diffraction, and infrared techniques were used. When octacalcium phosphate is hydrolyzed in the presence of sodium and carbonate ions in aqueous media, approximately one sodium and one carbonate ion seem to substitute for a calcium and phosphate ion, respectively, in forming apatite, and the a axis is shortened. The infrared spectrum of the product indicates that the carbonate is in the type B site, which is presumed to be a phosphate site. This mechanism is of particular importance since the presence of carbonate in human enamel appears to be related to caries susceptibility. A structural mechanism for the incorporation of impurities during hydrolysis of octacalcium phosphate is presented.