phosphoric acid, any one of three chemical compounds made up of phosphorus , oxygen, and hydrogen (see acids and bases ). The most common, orthophosphoric acid, H 3PO 4, is usually simply called phosphoric acid. Two molecules of it are formed by adding three molecules of water, H 2O, to one molecule of phosphorus pentoxide (phosphoric anhydride, P 2O 5). 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, H 4P 2O 7; at still higher temperatures metaphosphoric acid, HPO 3, 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.
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