Reactions of octacalcium phosphate to form hydroxyapatite

Hydrolysis reactions of octacalcium phosphate (OCP) to form hydroxyapatite (HAp) were studied. OCP was mixed with the proportions of tetracalcium phosphate (TetCP) or calcium hydroxide needed to form stoichiometric HAp. The relationships between the rates of hydrolysis, microstructural evolution, and variations in solution chemistry were examined during the transformations of OCP to HAp. Isothermal calorimetry at 25, 38 and 60 degrees C and variations in solution chemistry showed that conversion of OCP to HAp occurred more rapidly at higher temperatures. Moreover, its transformation to HAp was more rapid when TetCP was present. Reactions involving TetCP and OCP showed heat evolution curves (dQ/dt versus t) characterized by the presence of two peaks. one due to mixing of the reactants and a second due to the conversion reaction. The rate of OCP hydrolysis to HAp was reduced when calcium hydroxide was present, if not entirely stopped. Calorimetric curves showed mixing peaks but the absence of reaction peaks when OCP hydrolyzed in the presence of Ca(OH)(2). The only reaction observed was the dissolution of Ca(OH)(2). There was an absence of any notable reaction with OCP once it had dissolved. The direct hydrolysis of OCP in the absence of other solids occurred the most slowly. This is consistent with the hydrolysis behavior of other calcium phosphates. Lastly, OCP reactivity depended on the manner in which it was produced. Although more crystalline, OCP made using calcium phosphate precursors was more reactive than OCP produced by precipitation from calcium acetate/sodium dihydrogen phosphate solution.