Calcium Phosphate Solubility: The Need for Re-Evaluation

Abstract

The determination of the solubility of calcium phosphates by the conventional large excess of solid method has been demonstrated to be inappropriate. The problem lies in incongruent dissolution, leading to phase transformations, and lack of detailed solution equilibria: all calculations have been based on simplifications, which are only crudely approximate. The absolute solid-titration approach shows excellent reliability and reproducibility. Using solid titration, the true solubility isotherm of hydroxyapatite (HAp) has been found to lie substantially lower than previously reported. In addition, contrary to wide belief, dicalcium phosphate dihydrate (DCPD) is not the most stable phase below pH ∼4.2, where calcium-deficient HAp is less soluble. The misunderstanding here arises from the metastability of DCPD, which nucleates much more easily than HAp at low pH. Such results indicate that the Ca−P system is in need of complete reappraisal. The solid-titration method can be extended to other complex systems.

Calcium orthophosphates (CaPO 4 ): Occurrence and properties

“…Using an absolute solid-titration approach, the true solubility isotherm of HA was found to lie substantially lower than previously reported. In addition, contrary to a wide belief, DCPD appeared not to be the most stable phase below pH *4.2, where CDHA was less soluble (Pan and Darvell 2009a).…”

“…6 (Elliott 1994;Amjad 1997;Brown 1992;Martin and Brown 1997;McDowell et al 1977;Chow 2009;Ishikawa 2010). However, in 2009, the classic solubility data of CaPO 4 (Elliott 1994;Amjad 1997;Brown 1992;Martin and Brown 1997;McDowell et al 1977;Chow 2009;Ishikawa 2010) were mentioned to be inappropriate (Pan and Darvell 2009a). According to the authors of the latter study, all previous solubility calculations were based on simplifications, which were only crudely approximate.…”

“…In medicine, TTCP is widely used for preparation of various self-setting CaPO 4 formulations (Dorozhkin 2013b; Moseke and Gbureck 2010); however, to the best of my knowledge, there is no commercial bone-substituting product consisting solely of TTCP. For the comprehensive information on TTCP, the readers are referred to a special review (Moseke and Gbureck 2010), while the spectra (Jillavenatesa and Condrate 1997) and solubility (Pan and Darvell 2009a) of TTCP are well described elsewhere.…”

“…A higher mineral to collagen ratio typically yields stronger, but more brittle, bones (Turner and Burr 1993;Currey 2004;Currey et al 2004). For example, bones from the leg of a cow have a relatively high concentration of CaPO 4 (for support), whereas bones from the antler of a deer have a relatively high concentration of collagen (for flexibility) (Pan and Darvell 2009a). It is interesting to note that bones exhibit several physical properties such as piezoelectricity (Anderson and Eriksson 1970) and pyroelectricity (Lang 1966).…”

About the Genetic Mechanisms of Apatites: A Survey on the Methodological Approaches

“…Many papers dealing with the solubility of HAp have been published during the years, being the HAp stability of crucial importance mainly for biomedical applications. The values of the solubility product vary over a range of 10 11 , according to Van Wazer [76] and many other authors [77][78][79][80]. A unique value of solubility is controversial.…”