calcium pyrophosphate
{{Chembox
| Watchedfields = changed
| verifiedrevid = 476998565
| ImageFile = Calcium pyrophosphate.png
| IUPACName=Calcium diphosphate
| OtherNames={{Unbulleted list|Diphosphoric acid, calcium salt (1:2)|Dicalcium diphosphate|Dicalcium pyrophosphate}}
|Section1={{Chembox Identifiers
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 23034
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = X69NU20D19
| 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
| InChIKey = JUNWLZAGQLJVLR-XBHQNQODAN
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 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
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = JUNWLZAGQLJVLR-UHFFFAOYSA-J
| CASNo=7790-76-3
| CASNo_Ref = {{cascite|correct|CAS}}
| PubChem=24632
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 32598
| SMILES = [Ca+2].[Ca+2].[O-]P([O-])(=O)OP([O-])([O-])=O
| MeSHName=Calcium+pyrophosphate
}}
|Section2={{Chembox Properties
| Formula=Ca2O7P2
| MolarMass= 254.053 g/mol
| Appearance= White powder
| Density=3.09 g/cm3
| MeltingPtC=1353
| BoilingPt=
| Solubility= insoluble
| SolubleOther = soluble in HCl, nitric acids
| RefractIndex = 1.585
}}
|Section3={{Chembox Hazards
| NFPA-H = 2
| NFPA-F = 0
| NFPA-R = 0
| NFPA-S =
| FlashPt = Non-flammable
}}
|Section8={{Chembox Related
| OtherAnions = Calcium phosphate
| OtherCations = Magnesium pyrophosphate
Sodium pyrophosphate
}}
}}
Calcium pyrophosphate refers to any member of a series of inorganic compound with the formula {{chem2|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 are responsible for the severe joint pain in cases of calcium pyrophosphate deposition disease (pseudo gout) whose symptoms are similar to those of gout.{{EMedicine|article|330936|Calcium Pyrophosphate Deposition Disease}} Ca2P2O7 is commonly used as a mild abrasive agent in toothpastes because of its insolubility and nonreactivity toward fluoride.{{Ullmann|author1=Klaus Schrödter|author2=Gerhard Bettermann|author3=Thomas Staffel|author4=Friedrich Wahl|author5=Thomas Klein|author6=Thomas Hofmann|title=Phosphoric Acid and Phosphates|doi=10.1002/14356007.a19_465.pub3|year=2012}}
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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:
: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:{{cn|date=September 2023}}
:CaCl2 + H4P2O7(aq) → Ca2P2O7·2 H2O + HCl.
The anhydrous forms can be prepared by heating dicalcium phosphate:
: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.
The dihydrate is triclinic, with hydrogen bonding between the two water molecules and hydrogen bonds to the O atoms on the anion.{{cn|date=September 2023}} An hexagonal dihydrate has also been reported.
The anhydrous form has 3 polymorphs, α-, β-, and metastable γ{{Cite journal|last1=Parodi|first1=J. A.|last2=Hickok|first2=R. L.|last3=Segelken|first3=W. G.|last4=Cooper|first4=J. R.|date=1965|title=Electronic Paramagnetic Resonance Study of the Thermal Decomposition of Dibasic Calcium Orthophosphate|url=https://iopscience.iop.org/article/10.1149/1.2423665|journal=Journal of the Electrochemical Society|language=en|volume=112|issue=7|pages=688|doi=10.1149/1.2423665|bibcode=1965JElS..112..688P}} (Tα/β=1140ºС{{Cite journal|last1=Hill|first1=W L|last2=Reynolds|first2=D S|last3=Hendbicks|first3=S B|last4=Jacob|first4=K D|date=1945-02-01|title=Nutritive Evaluation of Defluorinated Phosphates and Other Phosphorus Supplements. I. Preparation and Properties of the Samples|url=https://doi.org/10.1093/jaoac/28.1.105|journal=Journal of AOAC International|volume=28|issue=1|pages=105–118|doi=10.1093/jaoac/28.1.105|issn=0095-9111}}). 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.{{Cite journal|last=Calvo|first=C.|date=1968-07-01|title=Crystal structure of .alpha.-calcium pyrophosphate|url=https://doi.org/10.1021/ic50065a019|journal=Inorganic Chemistry|volume=7|issue=7|pages=1345–1351|doi=10.1021/ic50065a019|issn=0020-1669}}
References
{{reflist | refs=
- {{cite journal |doi=10.1016/S0022-0248(00)00231-1 |title=Growth and precipitation of a monoclinic calcium pyrophosphate tetrahydrate indicating auto-inhibition at pH7 |journal=Journal of Crystal Growth |volume=212 |issue=3–4 |pages=500–506 |year=2000 |last1=Christoffersen |first1=Margaret R. |last2=Balic-Zunic |first2=Tonci |last3=Pehrson |first3=Søren |last4=Christoffersen |first4=Jørgen |bibcode=2000JCrGr.212..500C }}
- {{cite journal |doi=10.1016/0022-0248(88)90093-0 |title=Calcium pyrophosphate crystal deposition disease: Preparation and characterization of crystals |journal=Journal of Crystal Growth |volume=87 |issue=4 |pages=453–462 |year=1988 |last1=Mandel |first1=Gretchen S. |last2=Renne |first2=Kathleen M. |last3=Kolbach |first3=Ann M. |last4=Kaplan |first4=Wayne D. |last5=Miller |first5=Jay D. |last6=Mandel |first6=Neil S. |bibcode=1988JCrGr..87..453M }}
- {{cite journal |doi=10.1149/1.2423665 |title=Electronic Paramagnetic Resonance Study of the Thermal Decomposition of Dibasic Calcium Orthophosphate |journal=Journal of the Electrochemical Society |volume=112 |issue=7 |pages=688 |year=1965 |last1=Parodi |first1=J. A. |last2=Hickok |first2=R. L. |last3=Segelken |first3=W. G. |last4=Cooper |first4=J. R. |bibcode=1965JElS..112..688P }}
- {{cite journal |doi=10.1107/S0365110X66004225 |title=The crystal structure of β-Ca2P2O |journal=Acta Crystallographica |volume=21 |issue=6 |pages=942–948 |year=1966 |last1=Webb |first1=N. C. }}
}}
{{Calcium compounds}}
{{DEFAULTSORT:Calcium Pyrophosphate}}