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, H ₃ PO ₄ , is usually simply called phosphoric acid. Two molecules of it are formed by adding three molecules of water, H ₂ O, to one molecule of phosphorus pentoxide (phosphoric anhydride, P ₂ O 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 ₄ P ₂ O 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.

phosphoric acid May be one of three types: orthophosphoric acid (H₃PO₄), metaphosphoric acid (HPO₃), or pyrophosphoric acid (H₄P₂O₇). Orthophosphoric acid and its salts are used as acidity regulators and in acid‐fruit‐flavoured beverages.

phosphoric acid Group of acids. Tetraoxophosphoric acid (H₃PO₄, formerly orthophosphoric acid) is a colourless liquid obtained by the action of sulphuric acid on phosphate rock (calcium phosphate); it is used in dental adhesives, flavoured syrups, fertilizers, soaps, detergents, and anticorrosive coatings for metals. Metaphosphoric acid (HPO₃) is obtained by heating tetraoxophosphoric acid; it is used as a dehydrating agent. Heptaoxodiphosphoric (H₄P₂O₇, formerly pyrophosphoric acid) is formed by moderately heating tetraoxophosphoric acid or by reacting phosphorus pentoxide (P₂O₅) with water; it is used as a catalyst and in metallurgy.