Table 1. Existing calcium orthophosphates and their major properties29,30.
| 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 or MCP) |
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 or DCP), 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] |
| 1.2–2.2 |
Amorphous calcium phosphates (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 or Ca-def HA)[e] |
Ca10-x(HPO4)x(PO4)6-x(OH)2-x (0 < x < 1) |
~85 |
~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, OAp or OXA)[f] |
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).31 The comparative extent of dissolution in acidic buffer is: ACP > > α-TCP > > β-TCP > CDHA > > HA > FA.32[c] Stable at temperatures above 100°C. [d] Always metastable. [e] Occasionally, it is called “precipitated HA (PHA).” [f] Existence of OA remains questionable.