Calcium phosphate

Calcium phosphate is the name given to a family of minerals containing calcium ions (Ca2+) together with orthophosphates (PO43-), metaphosphates or pyrophosphates (P2O74-) and occasionally hydrogen or hydroxide ions.

It is the principal form of calcium found in bovine milk. Seventy percent of bone is made up of hydroxyapatite, a calcium phosphate mineral (known as bone mineral). Tooth enamel is composed of almost ninety percent hydroxyapatite.

Chemical Properties
Unlike most other compounds calcium phosphate is increasingly insoluble at higher temperatures. Thus heating causes precipitation.

In milk it is found in higher concentrations than would be possible at the normal pH because it exists in a colloidal form in micelles bound to casein protein with magnesium, zinc and citrate - collectively referred to as colloidal calcium phosphate (CCP)

Uses
It is used in the production of phosphoric acid and fertilizers, for example in the Odda process. Overuse of certain forms of calcium phosphate can lead to nutrient-containing surface runoff and subsequent adverse effects upon receiving waters such as algal blooms and eutrophication.

Calcium phosphate is used in baking as a raising agent, with E number E341. It is also used in cheese products.

Tricalcium phosphate is also used as a nutritional supplement and occurs naturally in cow milk, although the most common and economical forms for supplementation are calcium carbonate (which should be taken with food) and calcium citrate (which can be taken without food). Hydroxyapatite (e.g. calcium hydrogen phosphate) as a food supplement has not been currently studied well, so its usage as a supplement is discouraged.

It is used in a variety of dental products for remineralization and as a diluent in some medications where it will give the tablet a grey colour in the absence of additional colouring agents.

Another practical application of the compound is its use in gene transfection of cells. It is not too well understood, but the calcium phosphate precipitate DNA and form a complex that is thought to help the DNA enter the cell.

Calcium phosphates

 * Calcium dihydrogen phosphate, E341(i): Ca(H2PO4)2
 * Calcium hydrogen phosphate, E341(ii): CaHPO4
 * Tricalcium phosphate (or tricalcic phosphate), E341(iii): Ca3(PO4)2, sometimes referred to simply as calcium phosphate or calcium orthophosphate
 * Hydroxyapatite Ca5(PO4)3(OH)
 * Apatite Ca10(PO4)6(OH, F, Cl, Br)2
 * Octacalcium phosphate Ca8H2(PO4)6.5H2O

In addition to the above, of the compounds occurring in the CaO-H2O-P4O10 phase diagram, Ca4P2O9 (probably Ca3(PO4)2.CaO) is notable.

Compendial status

 * British Pharmacopoeia