Solid Titration of Octacalcium Phosphate

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.

A Novel Kinetic Method to Measure Apparent Solubility Product of Bulk Human Enamel

“…The inconsistency in results is in spite of both experiments using powdered enamel as the substrate which again highlights the advantage of using bulk enamel as the substrate rather than powdered. Such measurements will also confirm or otherwise the marked change in slope of the solubility isotherm for HAp at around pH 3.9 as reported by Pan and Darvell ( 2009a , b ).…”

Calcium orthophosphates (CaPO 4 ): Occurrence and properties

“…The water content of OCP crystals is *20 % that of DCPD and this is partly responsible for its lower solubility. The latest data on OCP structure (Davies et al 2012) and solubility (Pan and Darvell 2009c) are available.…”

Calcium orthophosphates

“…The water content of OCP crystals is about 20% that of DCPD, and this is partly responsible for its lower solubility. New data on OCP solubility have been published recently in reference 181.…”

“…For example, addition of Ag + , 501 - 503 Zn 2+ , 503 , 972 and Cu 2+ , 503 , 972 , 973 has been used for imparting antimicrobial effect, while radioactive isotopes of 90 Y, 504 , 153 Sm 181 , 505 - 507 Re 505 have been incorporated into HA bioceramics and injected into knee joints to treat rheumatoid joint synovitis 504 , 505 , 507 . More to the point, apatites were found to incorporate individual molecules, such as water, oxygen and carbon dioxide 83 …”