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]
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IUPAC name
Calcium diphosphate
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Other names
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Identifiers | |
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3D model (JSmol)
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ChEBI |
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ChemSpider |
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ECHA InfoCard | 100.029.292 |
E number | E450(vi) (thickeners, ...) |
MeSH | Calcium+pyrophosphate |
PubChem CID
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UNII |
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CompTox Dashboard (EPA)
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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 |
Refractive index (nD)
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1.585 |
Hazards | |
NFPA 704 (fire diamond) | |
Flash point | Non-flammable |
Related compounds | |
Other anions
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Calcium phosphate |
Other cations
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Magnesium pyrophosphate Sodium pyrophosphate |
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
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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]
The dihydrate, sometimes termed CPPD, can be formed by the reaction of pyrophosphoric acid with calcium chloride:[citation needed]
The anhydrous forms can be prepared by heating dicalcium phosphate:[citation needed]
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.
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]