orthophosphate

Orthophosphate refers to the simplest form of phosphate in inorganic chemistry, the phosphate ion PO4^3−. In aqueous solution its speciation depends on pH, with H2PO4− and HPO4^2− as the common protonated forms and PO4^3− becoming dominant at higher pH. The term is sometimes used to distinguish the uncondensed orthophosphate unit from condensed phosphates such as pyrophosphate or polyphosphates.

Orthophosphate occurs naturally in minerals such as apatite and in soluble salts derived from phosphoric acid. It is a major source of phosphorus in soils and natural waters and is released by weathering and mineralization. It is also a common component of commercial fertilizers, which can supply plants with readily available phosphate.

Biologically, orthophosphate is an essential nutrient. Phosphate groups are ubiquitous in biology, forming parts of ATP, nucleic acids (DNA and RNA), phospholipids, and many enzymes. Organisms take up inorganic phosphate and rapidly incorporate it into metabolic and structural molecules, making phosphate a critical element in energy transfer and genetic information.

Chemical and environmental properties are governed by several acid–base equilibria. The pKa values for phosphoric acid are about 2.15, 7.20, and 12.35, so at neutral pH the dominant species are H2PO4− and HPO4^2−. Orthophosphate readily forms complexes and insoluble precipitates with polyvalent cations such as calcium, iron, and aluminum, and it is subject to cycling between dissolved and particulate forms. In water management, orthophosphate is monitored because it can contribute to eutrophication; in some drinking-water systems, small amounts are added as corrosion inhibitors to form protective scales on pipes. It is important to distinguish orthophosphate from condensed phosphates, which have different chemical behavior and biological availability.