Table 1.
| Ca/P molar ratio | Compound | Formula | Solubility at 25 °C, −log(Ks) | Solubility at 25 °C, g/L | pH stability range in aqueous solutions at 25 °C |
|---|---|---|---|---|---|
| 0.5 | Monocalcium phosphate monohydrate (MCPM) | Ca(H2PO4)2·H2O | 1.14 | ~18 | 0.0–2.0 |
| 0.5 | Monocalcium phosphate anhydrous (MCPA) | Ca(H2PO4)2 | 1.14 | ~17 | [c] |
| 1.0 | Dicalcium phosphate dihydrate (DCPD), mineral brushite | CaHPO4·2H2O | 6.59 | ~0.088 | 2.0–6.0 |
| 1.0 | Dicalcium phosphate anhydrous (DCPA), mineral monetite | CaHPO4 | 6.90 | ~0.048 | [c] |
| 1.33 | Octacalcium phosphate (OCP) | Ca8(HPO4)2(PO4)4·5H2O | 96.6 | ~0.0081 | 5.5–7.0 |
| 1.5 | α-Tricalcium phosphate (α-TCP) | α-Ca3(PO4)2 | 25.5 | ~0.0025 | [a] |
| 1.5 | β-Tricalcium phosphate (β-TCP) | β-Ca3(PO4)2 | 28.9 | ~0.0005 | [a] |
| 1.0–2.2 | Amorphous calcium phosphate (ACP) | CaxHy(PO4)z·nH2O, n = 3–4.5; 15–20% H2O | [b] | [b] | ~5–12 [d] |
| 1.5–1.67 | Calcium-deficient hydroxyapatite (CDHA)[e] | Ca10-x(HPO4)x(PO4)6-x(OH)2-x[f] (0 < x < 1) | ~85.1 | ~0.0094 | 6.5–9.5 |
| 1.67 | Hydroxyapatite (HA, HAp or OHAp) | Ca10(PO4)6(OH)2 | 116.8 | ~0.0003 | 9.5–12 |
| 1.67 | Fluorapatite (FA or FAp) | Ca10(PO4)6F2 | 120.0 | ~0.0002 | 7–12 |
| 1.67 | Oxyapatite (OA or OAp) | Ca10(PO4)6O | ~69 | ~0.087 | [a] |
| 2.0 | Tetracalcium phosphate (TTCP or TetCP), mineral hilgenstockite | Ca4(PO4)2O | 38–44 | ~0.0007 | [a] |
[a] These compounds cannot be precipitated from aqueous solutions.
[b] Cannot be measured precisely. However, the following values were found: 25.7 ± 0.1 (pH = 7.40), 29.9 ± 0.1 (pH = 6.00), 32.7 ± 0.1 (pH = 5.28). The comparative extent of dissolution in acidic buffer is: ACP >> α-TCP >> β-TCP > CDHA >> HA > FA.
[c] Stable at temperatures above 100 °C.
[d] Always metastable.
[e] Occasionally CDHA is named as precipitated HA.
[f] In the case x = 1 (the boundary condition with Ca/P = 1.5), the chemical formula of CDHA looks as follows: Ca9(HPO4)(PO4)5(OH).