phosphoric acid

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phosphoric acid,

any one of three chemical compounds made up of phosphorusphosphorus
[Gr.,=light-bearing], nonmetallic chemical element; symbol P; at. no. 15; at. wt. 30.97376; m.p. 44.1°C;; b.p. about 280°C;; sp. gr. 1.82 at 20°C;; valence −3, +3, or +5.
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, oxygen, and hydrogen (see acids and basesacids and bases,
two related classes of chemicals; the members of each class have a number of common properties when dissolved in a solvent, usually water. Properties
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). The most common, orthophosphoric acid, H3PO4, is usually simply called phosphoric acid. Two molecules of it are formed by adding three molecules of water, H2O, to one molecule of phosphorus pentoxide (phosphoric anhydride, P2O5). It occurs as rhombic crystals or as a viscous liquid; both are deliquescent. The crystals melt at about 42°C;. It has specific gravity 1.834 at 18°C;, is soluble in alcohol, and is very soluble in water. It is a tribasic acid and forms orthophosphate salts with either one, two, or all three of the hydrogens replaced by some other positive ion. When it is heated to about 225°C;, it dehydrates to form pyrophosphoric acid, H4P2O7; at still higher temperatures metaphosphoric acid, HPO3, is formed. Salts of pyrophosphoric acid are pyrophosphates; salts of metaphosphoric acid are metaphosphates. Phosphoric acid is prepared commercially by heating calcium phosphate rock with sulfuric acid; purer grades may be prepared by treating red phosphorus with nitric acid. It is used in pickling and rust-proofing metals, in acidifying jellies and beverages, and in preparing phosphate salts.

Phosphoric Acid


any of the oxygen-containing acids of phosphorus that are hydration products of phosphoric anhydride. A distinction is made between orthophosphoric acid (often simply called phosphoric acid) and condensed phosphoric acids. Orthophosphoric acid (H3PO4), which is formed on the dissolution of P4O10 (or P2O5) in water, is the most important acid and the acid that has been most thoroughly studied.

Orthophosphoric acid occurs as colorless hygroscopic crystals having a density of 1.87 g/cm3 and a melting point of 42.35°C; the crystal hydrate H3PO4· 1/2H2O, with a melting point of 29.32°C, is known to exist. The density of the widely used 85-percent strength H3PO4 is 1.685 g/cm3 at 25°C; the viscosity is 47 × 10–3 millinewton · sec/m2, and the specific heat in the temperature range 20°–120°C is 2,064.1 joules/(kg · °K) [0.493 calorie/(g · °C)]. H3PO4 is miscible with water in any ratio. The dissociation constants at 25°C are K1 = 7 × 10–3, K2 = 10–8, and K3 = 4 × 10–13. Orthophosphoric acid is tribasic and is of medium strength. It forms three series of salts, called phosphates. When heated in solution, the acid undergoes dehydration, and condensed phosphoric acids are formed.

Orthophosphoric acid is prepared commercially by treating phosphate rock with sulfuric acid and by burning elemental phosphorus. With the former, natural phosphates are broken down by sulfuric and phosphoric acids:

Ca5F(PO4)3 + 5H2SO4 + nH3PO4 = (n + 3)H3PO4 + 5CaSO4 + HF

Filters are then used to separate the acid formed from insoluble CaSO4. With the latter method, phosphorus is first burned to form phosphoric anhydride: P4 + 5O2 = P4O10; the anhydride is then subjected to hydration: P4O10 + 6H20 = 4H3PO4. Commercial orthophosphoric acid is of great importance in the manufacture of phosphorus and compound fertilizers and commercial phosphates; it is also widely used in phosphating metals and as a catalyst in organic synthesis. Food-grade phosphoric acid is used in the preparation of nonalcoholic beverages, medicines, and dental cements.

Condensed (polymeric) phosphoric acids are divided into poly-phosphoric acids, in which the phosphate anion has a linear structure and the general formula Hn+2PnO3n+1; metaphosphoric acids, where the anion has a cyclic structure and the general formula (HPO3)n; and ultraphosphoric acids, which have a branched, reticulate structure. Polyphosphoric acids have the greatest practical value. The most thoroughly investigated of the polyphosphoric acids is diphosphoric (pyrophosphoric) acid (H4P2O7), which has two crystalline forms (melting points, 54.3°C and 71.5°C). Pyrophosphoric acid is tetrabasic, with the following dissociation constants at 18°C: K1 = 1.4 × 10–1, K2 = 1.1 × 10–2, K3 = 2.1 × 10–7, and K4 = 4.1 × 10–10. Triphosphoric and tetraphosphoric acids are obtained in the form of dilute solutions. The existence of phosphoric acids that are more highly condensed, containing up to 12 atoms per chain, has been established by paper chromatography. Polyphosphoric acids are polyelectrolytes. Cyclic metaphosphoric acids, for example, H3P3O9 and H4P4O12, are strong. Ultraphosphoric acids have not been thoroughly investigated.

Condensed phosphoric acids are obtained by the dehydration of orthophosphoric acids, by the hydration of phosphoric anhydride with the appropriate quantity of water, and by ion exchange from the corresponding condensed phosphates. They are used primarily in the manufacture of highly concentrated phosphorus fertilizers, as catalysts in organic synthesis and in the preparation of petroleum products, and in the preparation of various polyphosphates.


Kratkaia khimicheskaia entsiklopediia, vol. 5. Moscow, 1967.
Van Wazer, J. Fosfor i ego soedineniia, vol. 1. Moscow, 1962. (Translated from English.)
Postnikov, N. N. Termicheskaia fosfornaia kislota. Moscow, 1970.
Kopylev, B. A. Tekhnologiia ekstraktsionnoi fosfornoi kisloty. Leningrad, 1972.


phosphoric acid

[fä′sfȯr·ik ′as·əd]
(inorganic chemistry)
H3PO4 Water-soluble, transparent crystals, melting at 42°C; used as a fertilizer, in soft drinks and flavor syrups, pharmaceuticals, water treatment, and animal feeds and to pickle and rust-proof metals. Also known as orthophosphoric acid.