Calcium pyrophosphate refers to any member of a series of

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

with the formula . They are white solids that are insoluble in water. They contain the

pyrophosphate In chemistry, pyrophosphates are phosphorus oxyanions that contain two phosphorus atoms in a linkage. A number of pyrophosphate salts exist, such as disodium pyrophosphate () and tetrasodium pyrophosphate (), among others. Often pyrophosphates a ...

anion, although sometimes they are referred to as

phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...

s. 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 Calcium pyrophosphate dihydrate (CPPD) crystal deposition disease, also known as pseudogout and pyrophosphate arthropathy, is a rheumatologic disease which is thought to be secondary to abnormal accumulation of calcium pyrophosphate dihydrate c ...

(pseudo gout) whose symptoms are similar to those of

gout Gout ( ) is a form of inflammatory arthritis characterized by recurrent attacks of pain in a red, tender, hot, and Joint effusion, swollen joint, caused by the deposition of needle-like crystals of uric acid known as monosodium urate crysta ...

. Ca₂P₂O₇ is commonly used as a mild abrasive agent in

toothpaste Toothpaste is a paste or gel dentifrice that is used with a toothbrush to clean and maintain the aesthetics of Human tooth, teeth. Toothpaste is used to promote oral hygiene: it is an abrasive that aids in removing dental plaque and food from th ...

s because of its insolubility and nonreactivity toward fluoride. __TOC__

Preparation

Crystals of the tetrahydrate can be prepared by treating a solution of

sodium pyrophosphate Tetrasodium pyrophosphate, also called sodium pyrophosphate, tetrasodium phosphate or TSPP, is an inorganic compound with the chemical formula, formula Na4P2O7. As a salt, it is a white, water-soluble solid. It is composed of pyrophosphate anion an ...

with

calcium nitrate Calcium nitrate are inorganic compounds with the formula Ca(NO3)2(H2O)x. The anhydrous compound, which is rarely encountered, absorbs moisture from the air to give the tetrahydrate. Both anhydrous and hydrated forms are colourless salts. Hydrated ...

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 Pyrophosphoric acid, also known as diphosphoric acid, is the inorganic compound with the formula H4P2O7 or, more descriptively, HO)2P(O)sub>2O. Colorless and odorless, it is soluble in water, diethyl ether, and ethyl alcohol. The anhydrous aci ...

with

calcium chloride Calcium chloride is an inorganic compound, a Salt (chemistry), salt with the chemical formula . It is a white crystalline solid at room temperature, and it is highly soluble in water. It can be created by neutralising hydrochloric acid with cal ...

: :CaCl₂ + H₄P₂O₇(aq) → Ca₂P₂O₇·2 H₂O + HCl. The anhydrous forms can be prepared by heating

dicalcium phosphate Dicalcium phosphate is the calcium phosphate with the formula CaHPO4 and its dihydrate. The "di" prefix in the common name arises because the formation of the HPO42– anion involves the removal of two protons from phosphoric acid, H3PO4. It is al ...

: :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. An hexagonal dihydrate has also been reported. The anhydrous form has 3 polymorphs, α-, β-, and metastable γ (T_α/β=1140ºС). 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, ρ_α=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, ρ_β=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.

References

{{DEFAULTSORT:Calcium Pyrophosphate Calcium compounds Pyrophosphate salts Phosphates ja:ピロリン酸カルシウム