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salt or ester of phosphoric acidphosphoric acid,
any one of three chemical compounds made up of phosphorus, oxygen, and hydrogen (see acids and bases). The most common, orthophosphoric acid, H3PO4, is usually simply called phosphoric acid.
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, H3PO4. Because phosphoric acid is tribasic (having three replaceable hydrogen atoms), it forms monophosphate, diphosphate, and triphosphate salts in which one, two, or three of the hydrogens of the acid are replaced, respectively. Because replaceable hydrogens remain in monophosphates and diphosphates, they are sometimes called acid phosphates. The most important inorganic phosphate is calcium phosphate, Ca3(PO4)2. It makes up the larger part of phosphate rock, a mineral that is abundantly distributed throughout the world. Since calcium phosphate is only slightly soluble in water, it is not very suitable as a source of the phosphorus necessary for plant life; however, by treating it with sulfuric acid the soluble calcium acid phosphate known as superphosphatesuperphosphate
or superphosphate of lime,
Ca(H2PO4)2, is a compound produced by treating rock phosphate with sulfuric acid or phosphoric acid, or a mixture of the two.
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 of lime is formed. Other important inorganic phosphates include ammonium phosphate, important as a fertilizer; trisodium phosphate, used in detergents and for softening water; and disodium phosphate, used to some extent in medicine and in preparing baking powders. Various acid phosphates, e.g., those of calcium, magnesium, and sodium, are sometimes present in carbonated beverages. Microcosmic salt, used in certain bead tests in chemical analysis, is sodium ammonium phosphate. Organic phosphates play an important role in metabolism. For example, in the metabolism of sugars (which have hydroxyl groups, -OH, in their molecules), phosphate esters are often formed as an intermediate compound. Formation of these esters is called phosphorylation. Nucleotides are phosphate esters that play an important role in the conservation and use of the energy released in the metabolism of foods in the body; adenosine triphosphateadenosine triphosphate
(ATP) , organic compound composed of adenine, the sugar ribose, and three phosphate groups. ATP serves as the major energy source within the cell to drive a number of biological processes such as photosynthesis, muscle contraction, and the synthesis of
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 is an important nucleotide. DNA and RNA (see nucleic acidnucleic acid,
any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis.
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) are complex polymeric organic phosphates.



a salt or ester of phosphoric acid.

Phosphate salts. Phosphate salts are classified as orthophosphates or polymeric phosphates. The latter are divided into polyphosphates, which have a straight-chain phosphate-anion structure, metaphosphates, which have a ring-shaped (cyclic) phosphate-anion, and ultraphosphates, which have a reticulated, branched phosphate-anion structure. Phosphates also include extremely stable compounds—boron phosphates, BPO4, and aluminum phosphates, AlPO4 (although it would be more correct to regard them as mixed anhydrides—P2O5 and B2O3, and P2O5 and Al2O3).

ORTHOPHOSPHATES. Orthophosphates, which are salts of orthophosphoric acid, H3PO4, occur in monosubstituted, disubstituted, or trisubstituted form. Monosubstituted orthophosphates, which contain the Phosphate anion, are soluble in water; of the disubstituted and trisubstituted orthophosphates, which contain the anions Phosphate and Phosphate, respectively, only the salts of alkali metals and ammonium are soluble. Trisubstituted orthophosphates, with the exception of triammonium phosphate, (NH4)3PO4·3H2O, are thermally stable; tricalcium phosphate markedly dissociates only at temperatures above 2000°C (dissociation improves in a vacuum): Ca3(PO4)2 = 3CaO + P2O5. The heating of monosubstituted and disubstituted orthophosphates induces their dehydration with the release of structural water and the formation of polymeric (straight-chain or cyclic) phosphates as follows:

(n – 2)MeH2PO4 + 2Me2HPO4

→Men+2PnO3n+1 + (n – l)H2O

where n (here and below) is the degree of polymerization.

All phosphorus compounds occurring in nature are orthophosphates. Water-soluble orthophosphates are commercially prepared by first extracting orthophosphoric acid from natural phosphates, primarily apatites, and then adding hydroxides, ammonia, chlorides, or carbonates to the acid; for example,

H3PO4 + NH3 = NH4H2PO4

H3PO4 + KCl = KH2PO4 + HCl

Poorly soluble orthophosphates of heavy metals, such as Ag and Cu, are formed as a result of exchange reactions; for example,

2Na2HPO4 + 3AgNO3 = Ag3PO4 + 3NaNO3 + NaH2PO4

POLYMERIC PHOSPHATES. Straight-chain polyphosphates can be described by the formula Men+2PnO3n+1, or

and cyclic metaphosphates can be described by the formula MenPnO3n, or

The properties of polymeric phosphates depend on the nature of the cation, the phosphate-anion structure, the degree of polymerization, the structure, and other factors. Thus, for example, the solubility of straight-chain polyphosphates generally decreases with increased degree of polymerization. However, it can be increased by the modification of the polyphosphates: for example, by altering the melt’s rate of cooling.

Polymeric phosphates, both straight-chain and cyclic, are usually prepared by the dehydration of monosubstituted and disubstituted orthophosphates or by the neutralization of the corresponding polyphosphoric acid or metaphosphoric (cyclic) acid:

Hn+2PnO3n+1 + nNH3 = (NH4)nH2PnO3n+1

Sometimes these processes coincide, for example, during the high-temperature ammoniation of orthophosphoric acid in the preparation of ammonium polyphosphates. On a commercial scale, these processes are used to obtain sodium pyrophosphate and sodium tripolyphosphate (Na4P2O7 and Na5P3O10, respectively) and, to a lesser extent, potassium pyrophosphate, potassium tripolyphospate, and polymeric metaphosphates (sodium phosphate glass, potassium metaphosphate).

The best-studied cyclic metaphosphates are trimetaphosphate, tetrametaphosphate, hexametaphosphate, and octametaphos-phate.

ULTRAPHOSPHATES. Ultraphosphates are compounds of the general formula MenR, PnOn(5+R)/2, where R = Me2O/P2O5. As a rule, they are amorphous, glasslike, hygroscopic substances that are readily hydrolized in air to yield polyphosphates and metaphosphates. In the presence of a large quantity of water, metaphosphates may undergo hydrolysis owing to the complete splitting of the P—O—P bond into orthophosphates. The ultraphosphates of calcium, magnesium, manganese, and certain lanthanides that are isolated in crystalline form are usually not hygroscopic. Ultraphosphates are formed from the thermal dehydration of a mixture of orthophosphates with phosphoric acid or with phosphoric anhydride, that is, given the condition O < Me2O/P2O5 < l.

USES. Calcium, ammonium, and potassium phosphates are widely used as phosphorus fertilizers. The manufacture of phosphate feeds, such as defluorinated phosphates, dicalcium phosphate dihydrate, disodium phosphate, and phosphates of urea, H3PO4 (NH2)2CO, increased in trie 1970’s. Sodium and potassium phosphates, especially tripolyphosphates, are used as components of liquid and powdered detergents. They are also used in drilling operations as surface-active agents, in the cement and textile industries, and in the preparation of wool and cotton for bleaching and dyeing. Some phosphates, for example, (NH4)2HPO4, are used in the food industry to improve dough flakiness. Certain phosphates, such as BPO4, are used as catalysts in organic synthesis reactions. The phosphates of mainly alkali metals are constituents of enamels, glazes, glasses, refractory materials (for example, antipyrenes), and soft abrasives; they are also used in the phosphatization of metals (Mg, Fe, Zn). The crystals of monosubstituted potassium and ammonium phosphates are used as ferroelectric and piezoelectric materials. Phosphates are used in the pharmaceutical industry for the production of medicinal preparations, such as phosphacol and adenosine triphosphate, and for the production of toothpastes.


Phosphate esters. The best-known phosphoric-acid esters are monosubstituted, disubstituted, and trisubstituted orthophosphates—ROP(O)(OH)2, (RO)2P(O)OH, and (RO)3PO, respectively, where R is an alkyl, aryl, or heterocyclic residue. They are obtained by the interaction of POCI3 with alcohols:

Other methods are also used.

Phosphate esters are used as pesticides, additives to oils, and extractants. Certain organic phosphates, such as nucleic acids and adenosine phosphates, carry out important functions in living organisms.



Prodan, E. A., L. I. Prodan, and N. F. Ermolenko. Tripolifosfaty i ikh primenenie. Minsk, 1969.
See also references under


Generic term for any compound containing a phosphate group (PO43-), such as potassium phosphate, K3PO4.
Generic term for a phosphate-containing fertilizer material.
A mineral compound characterized by a tetrahedral ionic group of phosphate and oxygen, PO43-.


any salt or ester of any phosphoric acid, esp a salt of orthophosphoric acid
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