phosphoric acid

phosphoric 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. 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.