Objective: Using solid titration with hydroxyapatite (HAp) and octacalcium phosphate, HAp has been found to be more stable than dicalcium phosphate dihydrate (DCPD) even at lower pH, inconsistent with the widely reported view that DCPD is less soluble than other calcium phosphates below pH 4.2. A check of the behaviour of other calcium phosphates (TTCP; Ca/P: 2.00 and beta-TCP; Ca/P: 1.33) is necessary.
Methods: Solid titration was used to determine the effective solubility of TTCP and beta-TCP in 100 mM KCl solution at 37.0+/-0.1 degrees C for pH approximately 2.9-9.2 and approximately 3-7.4 respectively. The constitution of the precipitate was determined by XRD, particle morphology was observed by SEM and TEM, and the precipitate Ca/P ratio was calculated by EDX.
Results: The only identified solid phase at equilibrium was HAp at both pH 3.60 and 4.50; no residual titrant or other phases were detected. A marked change of slope in the curve occurred at pH approximately 3.9 for TTCP.
Conclusion: HAp was verified to be more stable than other calcium phosphates, especially at lower pH. That DCPD is more stable below pH 4.2 is contradicted.