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]
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 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]
^ abChristoffersen, Margaret R.; Balic-Zunic, Tonci; Pehrson, Søren; Christoffersen, Jørgen (2000). "Growth and precipitation of a monoclinic calcium pyrophosphate tetrahydrate indicating auto-inhibition at pH7". Journal of Crystal Growth. 212 (3–4): 500–506. Bibcode:2000JCrGr.212..500C. doi:10.1016/S0022-0248(00)00231-1.
^Mandel, Gretchen S.; Renne, Kathleen M.; Kolbach, Ann M.; Kaplan, Wayne D.; Miller, Jay D.; Mandel, Neil S. (1988). "Calcium pyrophosphate crystal deposition disease: Preparation and characterization of crystals". Journal of Crystal Growth. 87 (4): 453–462. Bibcode:1988JCrGr..87..453M. doi:10.1016/0022-0248(88)90093-0.
^Parodi, J. A.; Hickok, R. L.; Segelken, W. G.; Cooper, J. R. (1965). "Electronic Paramagnetic Resonance Study of the Thermal Decomposition of Dibasic Calcium Orthophosphate". Journal of the Electrochemical Society. 112 (7): 688. Bibcode:1965JElS..112..688P. doi:10.1149/1.2423665.
^Webb, N. C. (1966). "The crystal structure of β-Ca2P2O". Acta Crystallographica. 21 (6): 942–948. doi:10.1107/S0365110X66004225.
