Dissolution of Sparingly Soluble Inorganic Compound to Aqueous Suspension with Ion Exchange Resin (Part 11): Octacalcium Phosphate

The dissolution process of octacalcium phosphate (Ca8H2(PO4)6•5H2O, OCP) using a strongly acidic ion exchange resin with a hydrogen form (H-R) suspended in water was examined in detail to confirm the formation of CaHPO4•2H2O (DCPD) as an intermediate. The reactivity of OCP, α- or β- tricalcium phosphate (α- or β-TCP), or tetracalcium phosphate (Ca4(PO4)2O, TetCP), with Ca(H2PO4)2 solution to form DCPD was compared using calorimetry and chemical analysis. The reactivity increased in the order of TetCP=α-TCP»OCP≥β-TCP. The structure of OCP consists of “apatite layer” and “hydrated layer.” The “apatite layer” of OCP was examined by comparing the reactivity of DCPD+hydroxyapatite (HAp) and DCPD+β-TCP with H-R where HAp and β-TCP imitated the “apatite layer.” The results suggested that the “apatite layer” in OCP would be similar to β-TCP rather than HAp in terms of the reactivity.