Calcium pyrophosphate
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| Names | |
|---|---|
| IUPAC name
Calcium diphosphate
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| Other names
Diphosphoric acid, calcium salt (1:2)
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| Identifiers | |
7790-76-3  |
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| ChEBI | CHEBI:32598 |
| ChemSpider | 23034 |
| Jmol 3D model | Interactive image |
| MeSH | Calcium+pyrophosphate |
| PubChem | 24632 |
| UNII | X69NU20D19 |
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| 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 |
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Refractive index (nD)
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1.585 |
| Vapor pressure | {{{value}}} |
| Related compounds | |
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Other anions
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Calcium phosphate |
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Other cations
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Magnesium pyrophosphate Sodium pyrophosphate |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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| Infobox references | |
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.[2]
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) +2Ca(NO3)2(aq) → Ca2P2O7·4H2O + 2Na2NO3
The dihydrate, sometimes termed CPPD, can be formed by the reaction of pyrophosphoric acid with calcium chloride:[2]
- CaCl2 + H4P2O7(aq) → Ca2P2O7·2H2O + HCl.
The anhydrous forms can be prepared by heating dicalcium phosphate:[2]
- 2CaHPO4 → 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.[2] An hexagonal dihydrate has also been reported.[4]
The anhydrous form has 3 polymorphs, α-, β-, and a metastable γ. The high temperature form α- is monoclinic, with 8 coordinate calcium, the lower temperature form β- is tetragonal, with calcium in four different coordination environments, 2 that are 7 coordinate, one eight and one 9. In both the pyrophosphates are essentially eclipsed.[5][6]
References
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