phosphate

phosphate

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

Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

phosphate

[′fä‚sfāt]

(chemistry)

Generic term for any compound containing a phosphate group (PO₄^3-), such as potassium phosphate, K₃PO₄.

Generic term for a phosphate-containing fertilizer material.

(mineralogy)

A mineral compound characterized by a tetrahedral ionic group of phosphate and oxygen, PO₄^3-.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Phosphate

 

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 anion, are soluble in water; of the disubstituted and trisubstituted orthophosphates, which contain the anions and , 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

→Me_n+2P_nO_3n+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 Me_n+2P_nO_3n+1, or

and cyclic metaphosphates can be described by the formula Me_nP_nO_3n, 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:

H_n+2P_nO_3n+1 + nNH₃ = (NH₄)_nH₂P_nO_3n+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 Me_nR, P_nO_n(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.

L. V. KUBASOVA

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.

E. E. NIFANTEV

REFERENCES

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

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.