Calcium pyrophosphate (Ca2P2O7) is a chemical compound, an insoluble calcium salt containing the pyrophosphate anion. There are a number of forms reported: an anhydrous form, a dihydrate, Ca2P2O7·2H2O and a tetrahydrate, Ca2P2O7·4H2O. Deposition of dihydrate crystals in cartilage are responsible for the severe joint pain in cases of calcium pyrophosphate deposition disease (pseudo gout) whose symptoms are similar to those of gout.[1] Ca2P2O7 is commonly used as a mild abrasive agent in toothpastes,[citation needed] because of its insolubility and nonreactivity toward fluoride.[2]
 | |
| Names | |
|---|---|
| IUPAC name Calcium diphosphate | |
Other names
| |
| Identifiers | |
3D model (JSmol) | |
| ChEBI | |
| ChemSpider | |
| ECHA InfoCard | 100.029.292 |
| E number | E450(vi) (thickeners, ...) |
| MeSH | Calcium+pyrophosphate |
PubChem CID | |
| UNII | |
CompTox Dashboard (EPA) | |
| |
| |
| Properties | |
| Ca2O7P2 | |
| Molar mass | 254.053 g/mol |
| Appearance | White powder |
| Density | 3.09 g/cm3 |
| Melting point | 1,353 °C (2,467 °F; 1,626 K) |
| insoluble | |
| Solubility | soluble in HCl, nitric acids |
Refractive index (nD) | 1.585 |
| Hazards | |
| NFPA 704 (fire diamond) | |
| Flash point | Non-flammable |
| Related compounds | |
Other anions | Calcium phosphate |
Other cations | Magnesium pyrophosphate Sodium pyrophosphate |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). | |
Preparation
Crystals of the tetrahydrate can be prepared by reacting sodium pyrophosphate, Na4P2O7 with calcium nitrate, Ca(NO3)2, at carefully controlled pH and temperature:[3]
- Na4P2O7(aq)+2 Ca(NO3)2(aq)→ Ca2P2O7·4 H2O + 4 NaNO3
The dihydrate, sometimes termed CPPD, can be formed by the reaction of pyrophosphoric acid with calcium chloride:[citation needed]
- CaCl2 + H4P2O7(aq) → Ca2P2O7·2 H2O + HCl.
The anhydrous forms can be prepared by heating dicalcium phosphate:[citation needed]
- 2 CaHPO4 → Ca2P2O7 + H2O
At 240-500 °C an amorphous phase is formed, heating to 750 °C forms β-Ca2P2O7, heating to 1140 - 1350 °C forms the α-Ca2P2O7.
Structure of anhydrous and hydrated forms
The stable tetrahydrate was originally reported to be rhombohedral but is now believed to be monoclinic. Additionally there is an unstable monoclinic form.[3]
The dihydrate is triclinic, with hydrogen bonding between the two water molecules and hydrogen bonds to the O atoms on the anion.[citation needed] An hexagonal dihydrate has also been reported.[4]
The anhydrous form has 3 polymorphs, α-, β-, and metastable γ[5] (Tα/β=1140ºС[6]). The high temperature form α- is monoclinic (P21/n, a=12.66(1)Å, b=8.542(8)Å, c=5.315(5)Å, Z=4, ρα=2.95 g/cm3), with 8 coordinate calcium, the lower temperature form β- is tetragonal (P41, a=b=6.684Å, c=24.144Å, V=915.40Å3, Z=8, ρβ=3.128 g/cm3), with calcium in four different coordination environments, 2 that are 7 coordinate, one eight and one 9. In both the pyrophosphates are essentially eclipsed.[7][8][9]