Calcium pyrophosphate

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Calcium pyrophosphate
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Names
IUPAC name
Calcium diphosphate
Other names
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  • Diphosphoric acid, calcium salt (1:2)
  • Dicalcium diphosphate
  • Dicalcium pyrophosphate
Identifiers
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3D model (JSmol)
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MeSH Calcium+pyrophosphate
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  • InChI=1S/2Ca.H4O7P2/c;;1-8(2,3)7-9(4,5)6/h;;(H2,1,2,3)(H2,4,5,6)/q2*+2;/p-4 checkY
    Key: JUNWLZAGQLJVLR-UHFFFAOYSA-J checkY
  • InChI=1/2Ca.H4O7P2/c;;1-8(2,3)7-9(4,5)6/h;;(H2,1,2,3)(H2,4,5,6)/q2*+2;/p-4
    Key: JUNWLZAGQLJVLR-XBHQNQODAN
  • [Ca+2].[Ca+2].[O-]P([O-])(=O)OP([O-])([O-])=O
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
1.585
Hazards
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2
0
0
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).

Template:Chembox Footer/trackingTemplate:Short description

Calcium pyrophosphate refers to any member of a series of inorganic compound with the formula Page Module:Chem2/styles.css has no content.Ca2P2O7(H2O)n. They are white solids that are insoluble in water. They contain the pyrophosphate anion, although sometimes they are referred to as phosphates. The inventory includes an anhydrous form, a dihydrate (Ca2P2O7·2H2O), and a tetrahydrate (Ca2P2O7·4H2O). Deposition of dihydrate crystals in cartilage is 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 because of its insolubility and nonreactivity toward fluoride.[2]

In 2025, a natural analogue of synthetic β-Ca2P2O7 was found in a super-deep diamond from the Chapadão plateau, Mato Grosso, Brazil.  It has been named grahampearsonite to recognize the contributions to diamond research of Graham Pearson.[3].

Preparation

Crystals of the tetrahydrate can be prepared by treating a solution of sodium pyrophosphate with calcium nitrate with careful control of pH and temperature:[4]

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:[2]

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.[4]

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.[5]

The anhydrous form has three polymorphs: α-, β-, and metastable γ[6] (Tα/β=1140ºС[7]). 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, two that are 7 coordinate, one 8 and one 9. In both the pyrophosphates are essentially eclipsed.[8][9][10]

References

  1. ^ Calcium Pyrophosphate Deposition Disease at eMedicine
  2. ^ a b Script error: No such module "Template wrapper".
  3. ^ Page Module:Citation/CS1/styles.css has no content.Wang, Yanjuan; Nestola, Fabrizio; Day, Maxwell C.; Jalowitzki, Tiago; Novais Rodrigues, Eduardo; Gervasoni, Fernanda; Cedeño, Daniel Grings; da Silva, Sebastião William; Fuck, Reinhardt Adolfo; Nava, Jacopo; Santello, Lisa; Pamato, Martha G.; Novella, Davide; Innocenzi, Francesca; Curtolo, Andrea; La Fortezza, Mattia; Qu, Kai (December 3, 2025). "Grahampearsonite, Ca2P2O7, a new mineral found in a super-deep diamond from Juína, Brazil". American Mineralogist. doi:10.2138/am-2025-10072 – via CrossRef.
  4. ^ a b Page Module:Citation/CS1/styles.css has no content.Christoffersen, 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.
  5. ^ Page Module:Citation/CS1/styles.css has no content.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.
  6. ^ Page Module:Citation/CS1/styles.css has no content.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.
  7. ^ Page Module:Citation/CS1/styles.css has no content.Hill, W L; Reynolds, D S; Hendbicks, S B; Jacob, K D (1945-02-01). "Nutritive Evaluation of Defluorinated Phosphates and Other Phosphorus Supplements. I. Preparation and Properties of the Samples". Journal of AOAC International. 28 (1): 105–118. doi:10.1093/jaoac/28.1.105. ISSN 0095-9111.
  8. ^ Page Module:Citation/CS1/styles.css has no content.Calvo, C. (1968-07-01). "Crystal structure of .alpha.-calcium pyrophosphate". Inorganic Chemistry. 7 (7): 1345–1351. doi:10.1021/ic50065a019. ISSN 0020-1669.
  9. ^ Page Module:Citation/CS1/styles.css has no content.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.
  10. ^ Page Module:Citation/CS1/styles.css has no content.Webb, N. C. (1966). "The crystal structure of β-Ca2P2O". Acta Crystallographica. 21 (6): 942–948. doi:10.1107/S0365110X66004225.

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