Table 3.
Properties of the biologically relevant calcium orthophosphate. (Reproduced with permission from Prof. Epple, M. [121])
| Ca/P ratio | Compound | Formula | Solubility at 25oC, –log(Ksp) | Solubility at 37 oC, –log(Ksp) | pH stability range in aqueous solution at 25 oC |
|---|---|---|---|---|---|
| 0.5 | monocalcium phosphate monohydrate (MCPM) | Ca(H2PO4)2·H2O | 1.14 | no data | 0.0–2.0 |
| 0.5 | monocalcium phosphate anhydrate | Ca(H2PO4)2 | 1.14 | no data | [d] |
| 1.0 | diacalcium phosphate dehydrate (DCPD, “brushite”) | CaHPO4·2H2O | 6.59 | 6.63 | 2.0–6.0 |
| 1.0 | diacalcium phosphate anhydrate (DCPA, “monetite”) | CaHPO4 | 6.90 | 7.02 | [d] |
| 1.33 | octacalcium phosphate (OCP) | Ca8(HPO4)2(PO4)4·5H2O | 96.6 | 95.9 | 5.5–7.0 |
| 1.5 | α-tricalcium phosphate (α-TCP) | α-Ca3(PO4)2 | 25.5 | 25.5 | [b] |
| 1.5 | β-tricalcium phosphate (β -TCP) | β-Ca3(PO4)2 | 28.9 | 29.5 | [b] |
| 1.2-2.2 | amorphous calcium phosphate (ACP) | Cax(PO4)y·nH2O | [c] | [c] | [c] |
| 1.5–1.67 | Calcium-deficient hydroxyapatite (CDHA) | Ca10-x(HPO4)x(PO4)6-x(OH)2-x (0 < x < 1) | ≈ 85.1 | ≈ 85.1 | ≈ 6.5–9.5 |
| 1.67 | hydroxyapatite | Ca10(PO4)6(OH)2 | 116.8 | 117.2 | 9.5–12 |
| 2.0 | tetracalcium phosphate (TTCP) | Ca4(PO4)2O | 38–44 | 37–42 | [b] |
The solubility is given as the logarithm of the ion product of the given formulae (excluding hydrate water) with concentrations in mol/L. [b] these compounds cannot be precipitated from aqueous solution). [c] cannot be measured precisely. However, the following values were reported: 25.7 ± 0.1 (pH 7.40), 29.9 ± 0.1 (pH 6.00), 32.7 ± 0.1 (pH 5.28). [d] Stable at temperatures above 100 °C. [e] Always metastable. The composition of a precipitate depends on the solution pH values and composition [121].