Phosphoric acid is an inorganic acid with the chemical formula H3PO4. Often called orthophosphoric acid, it is a common laboratory and industrial chemical. Its systematic IUPAC name is trihydroxidooxidophosphorus. For basic reference and further data see chemical compound resources.

Chemical characteristics and behavior

H3PO4 is a triprotic acid, meaning it can lose up to three protons (H+). In aqueous solution it establishes an equilibrium among three conjugate bases: dihydrogen phosphate (H2PO4−), hydrogen phosphate (HPO4 2−) and phosphate (PO4 3−). Its solutions are colorless, miscible with water, and the acid is often handled as an aqueous solution rather than a pure substance. Structural and speciation details are discussed in many chemistry sources such as reference texts.

Production and historical notes

Two main industrial routes yield phosphoric acid. The wet process treats phosphate rock with sulfuric acid to produce a liquid phosphoric acid and gypsum as a byproduct; this method supplies most of the material used for fertilizers. A thermal or "white" process oxidizes elemental phosphorus to make a purer acid for specialty applications. Historically, the development of phosphorus chemistry and fertilizer demand in the 19th and 20th centuries drove large-scale production improvements; general summaries appear at industry overviews.

Major uses

  • Fertilizers: a principal intermediate for ammonium phosphates and other phosphate fertilizers.
  • Food and beverage: used as an acidulant and flavoring agent (often labeled E338) in some soft drinks and processed foods.
  • Industrial: rust removal, pH adjustment, metal treatment, and as a precursor in many phosphate chemicals.
  • Laboratory and specialty: used in buffer solutions, wet chemical analysis, and some dental products.

For practical examples and regulatory details consult technical and safety guides, e.g. safety and use information.

Concentrated phosphoric acid is corrosive and can cause burns on contact; inhalation or ingestion of concentrated solutions is hazardous. Environmentally, phosphate runoff from agriculture and wastewater can contribute to eutrophication of freshwater systems. Chemically, orthophosphoric acid is distinct from related phosphorus oxyacids such as phosphorous acid (H3PO3) and polyphosphoric acids, which have different oxidation states and reactivity.

Overall, phosphoric acid is a versatile, widely produced compound with important roles in agriculture, food, industry and research. When handling or applying it, follow appropriate safety guidance and environmental best practices to minimize harm.