Original Paper

Solid Titration of Octacalcium Phosphate
H.-B. Pan^{a, b}, B.W. Darvell<sup>a

a</sup>Dental Materials Science, Faculty of Dentistry, and
^bDepartment of Orthopaedics and Traumatology, Faculty of Medicine, University of Hong Kong, Hong Kong, SAR, China

Address of Corresponding Author

Caries Res 2009;43:322-330 (DOI: 10.1159/000226231)

  Key Words

• Hydroxyapatite
• Octacalcium phosphate
• Solid titration
• Solubility

  Abstract

Octacalcium phosphate (OCP) is of considerable importance as a precursor in the formation of dental enamel and an intermediate phase in the precipitation of hydroxyapatite (HAp) in bone. However, agreement is poor on the solubility product (pK_sp), possibly due to the formation of the more stable phase HAp. The system was investigated using solid titration, which has shown reliability in work on HAp and related fluoride minerals, with OCP in 100 mM KCl at 37.0 ± 0.1°C. The constitution of the end point precipitate was determined by X-ray diffraction and selected-electron area diffraction; the particle morphology and elements present were examined by high-resolution field emission scanning, transmission electron microscopy and energy-dispersive X-ray analysis. The titration curve for OCP was found for pH ~3.4-7.4. The precipitate was HAp at pH 3.6 and 4.5; no residual OCP or other phase was detected. Dicalcium phosphate dihydrate (DCPD) was then found to form at pH 3.6 on further addition of OCP titrant after equilibrium had been achieved, possibly due to easier nucleation at lower pH. However, markedly crystalline HAp was formed in equilibrium for OCP titration with HAp seeding, verifying HAp as the more stable phase. A solubility isotherm for OCP was not obtained as HAp appears to be less soluble in the pH range studied. This adds weight to the view that HAp may be the most stable phase of all calcium phosphates, with further doubt being cast on DCPD being the most stable phase below pH 4.2. However, metastable DCPD may form in an Ostwald succession, depending on supersaturation and nucleation conditions.

Copyright © 2009 S. Karger AG, Basel

  Author Contacts

B.W. Darvell
Dental Materials Science
Prince Philip Dental Hospital
34 Hospital Road, Hong Kong, SAR (China)
Tel. +852 2859 0303, Fax +852 2548 9464, E-Mail b.w.darvell@hku.hk

  Article Information

Received: March 4, 2008
Accepted after revision: April 22, 2009
Published online: June 24, 2009
Number of Print Pages : 9
Number of Figures : 9, Number of Tables : 3, Number of References : 23