Apatite Formation: Why It May Not Work as Planned, and How to Conclusively Identify Apatite Compounds

Table 2

Saturation index relative to various phases of interest and considering as initial conditions the ionic concentrations of blood plasma reported in Table 1.

Selected phases	𝑝𝐾sp (25°C)	Reference	Saturation index^(c) for blood; plasma conditions (kJ/mol)
Hydroxyapatite (HA) Ca10(PO4)6(OH)2	116.8^(a)	[26]	−15.673
Octacalcium phosphate (OCP) Ca8(PO4)4(HPO4)2⋅5H2O	96.6^(b)		−1.540
Monetite (DCPA) CaHPO4	6.9		24.875
Brushite (DCPD) CaHPO4⋅2H2O	6.59		25.381
Calcite c-CaCO3	8.48	[27]	−2.377
Vaterite v-CaCO3	7.91		−0.788
Magnesium hydrogenphosphate MgHPO4⋅3H2O	18.18	[28]	3.171

^(a)Expressed relative to HA formula in Ca₁₀.
^(b)Expressed relative to OCP formula in Ca₈.
^(c)Saturation index (Δ𝐺_𝑠) [29] calculated as: Δ𝐺_𝑠=−(𝑅𝑇/𝑁)Ln(𝐼𝑃/𝐾_sp) where 𝑁 is the number of ions involved, IP is the ion product, and 𝐾_sp is the solubility product. A negative value of Δ𝐺_𝑠 means that the medium is supersaturated with respect to the concerned phase.