Abnormal dissolution of octacalcium phosphate crystals at constant undersaturation

The dissolution of octacalcium phosphate (OCP), a possible important precur sor phase in the in vivo mineralization of hard tissue, has been studied at 37 degrees C over a range of undersaturations using a constant composition method (CC). The rate decreases markedly with time despite the sustained d riving force, and eventually it approaches zero even though crystals remain in the undersaturated solutions. These OCP crystals will not dissolve when exposed to calcium phosphate solutions of the same undersaturations. Howev er, the dissolution reaction resumes if the undersaturations are changed. F urthermore, at low undersaturation, the CC OCP dissolution curves show char acteristic stepwise profiles. These interesting unpredicted phenomena persu ade us to examine in more detail current crystal growth/dissolution models. (C) 2000 Elsevier Science B.V. All rights reserved.