An intermediate state in hydrolysis of amorphous calcium phosphate.

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 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.