Table 1 |
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| Existing calcium orthophosphates and their major properties (Dorozhkin 2009, 2011) | |||||
| 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 | approximately 18 | 0.0 to 2.0 |
| 0.5 | Monocalcium phosphate anhydrous (MCPA or MCP) | Ca(H2PO4)2 | 1.14 | approximately 17 | a |
| 1.0 | Dicalcium phosphate dihydrate (DCPD), mineral brushite | CaHPO4· 2H2O | 6.59 | approximately 0.088 | 2.0 to 6.0 |
| 1.0 | Dicalcium phosphate anhydrous (DCPA or DCP), mineral monetite | CaHPO4 | 6.90 | approximately 0.048 | a |
| 1.33 | Octacalcium phosphate (OCP) | Ca8(HPO4)2(PO4)4· 5H2O | 96.6 | approximately 0.0081 | 5.5 to 7.0 |
| 1.5 | α-Tricalcium phosphate (α-TCP) | α-Ca3(PO4)2 | 25.5 | approximately 0.0025 | b |
| 1.5 | β-Tricalcium phosphate (β-TCP) | β-Ca3(PO4)2 | 28.9 | approximately 0.0005 | b |
| 1.2 to 2.2 | Amorphous calcium phosphates (ACP) | CaxHy(PO4)z· nH2O, n = 3 to 4.5%; 15 to 20% H2O | c | b | approximately 5 to 12 d |
| 1.5 to 1.67 | Calcium-deficient hydroxyapatite (CDHA or Ca-def HA)e | Ca10–x(HPO4)x(PO4)6–x(OH)2–x (0 < x < 1) | approximately 85 | approximately 0.0094 | 6.5 to 9.5 |
| 1.67 | Hydroxyapatite (HA, HAp or OHAp) | Ca10(PO4)6(OH)2 | 116.8 | approximately 0.0003 | 9.5 to 12 |
| 1.67 | Fluorapatite (FA or FAp) | Ca10(PO4)6 F2 | 120.0 | approximately 0.0002 | 7 to 12 |
| 1.67 | Oxyapatite (OA, OAp or OXA)f | Ca10(PO4)6O | approximately 69 | approximately 0.087 | b |
| 2.0 | Tetracalcium phosphate (TTCP or TetCP), mineral hilgenstockite | Ca4(PO4)2O | 38 to 44 | approximately 0.0007 | b |
a Stable at temperatures above 100°C.
b These compounds cannot be precipitated from aqueous solutions.
c 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.
d Always metastable.
e Occasionally, it is called “precipitated HA”.
f Existence of OA remains questionable.
Dorozhkin Progress in Biomaterials 2012 1:1 doi:10.1186/2194-0517-1-1
