Octacalcium phosphate (Ca8H2(PO4)6·5H2O) is considered to be a 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.

A mechanism for incorporation of carbonate into apatite

Solubility of human dental enamel in H3PO4 was studied in the pH range of 4.5 to 7.6. Thermodynamic solubility of the enamel mineral was calculated in terms of the ion activity product, (Ca2+)5(PO4 3-)3 (OH-), for hydroxyapatite. The solubility product varied from 7.2 × 10-53 to 6.4 × 10-58 mol9 liter-9 depending on the cumulative amount of the dissolution of the solid in a series of repetitive sequences of solubility experiments.

Thermodynamic Solubility Product of Human Tooth Enamel: Powdered Sample

The hydrolysis of previously prepared amorphous calcium phosphate (ACP) was studied in a solution “saturated” with ACP; this eliminated the initial consumption of acid due to ACP dissolution. The procedure established that conversion of a high-concentration ACP slurry to an apatite involves two processes: the first process consumes acid and indicates the formation of a more acidic calcium phosphate intermediary with the solubility of octacalcium phosphate (OCP); the second process consumes base and indicates the conversion of the intermediary to apatite and, possibly, direct conversion of ACP to apatite. The thermodynamic analysis of the solution composition data suggests that ACP converts into a nonstoichiometric apatite when the OCP-like intermediary is formed, and a stoichiometric apatite is formed when no OCP-like intermediary is involved.

An intermediate state in hydrolysis of amorphous calcium phosphate.

Etude de la solubilite de l'OCP, important intermediaire dans la formation des dents et des os. Influence d'une atmosphere riche en CO 2

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Octacalcium Phosphate Solubility Product from 4 to 37-Degree-C

The ESR spectrum of X-irradiated carbonated apatites synthesized at low temperature was studied as a function of their carbonate content. Using13C-enriched samples, four different carbonate-derived radicals and a surface O− ion could be identified. Isotropic CO
 3
 −
 and CO
 2
 −
 ions are present at a B site in the apatite lattice, and anisotropic CO
 3
 −
 and CO
 2
 −
 radicals are located at the surface of the crystallites. Only the isotropic ESR signals increase with increasing carbonate content. The anisotropic signal ascribed to a surface CO
 2
 −
 radical is mainly responsible for the so-called asymmetric ESR signal near g=2. It is argued that this surface signal may still be composite and caused by several very similar CO
 2
 −
 ions. The consequences for phenomenological ESR studies of calcified tissues are discussed.

R L Dunn

Papers

Report on Phosphorus in Fertilizers II. Photometric Determination of Total Phosphorus