orthophosphates

Orthophosphates are the simplest phosphate-containing species derived from phosphoric acid, consisting of the orthophosphate ion PO4^3− and its protonated forms. In water, the orthophosphate system includes H3PO4, H2PO4−, HPO4^2− and PO4^3−, with acid–base equilibria characterized by pKa values of about 2.15, 7.20 and 12.35. Consequently, at acidic pH the species are mainly H3PO4 or H2PO4−, near neutral pH the bis- and monoanions H2PO4− and HPO4^2− predominate, and at strongly basic pH PO4^3− dominates.

Orthophosphates form salts with alkali and alkaline earth metals, e.g., Na3PO4, Na2HPO4, NaH2PO4, K3PO4, and so on. When polyvalent cations such as Ca2+ or Mg2+ are present, orthophosphate can precipitate as calcium phosphate or magnesium phosphate. They are distinct from condensed phosphates, which contain P–O–P linkages.

Occurrences include minerals such as apatite in geology and as nutrient phosphate in biology, where phosphate is essential for energy metabolism, nucleic acids, and signaling. In industry and laboratories, orthophosphates serve as buffering agents (phosphate buffers in biology and chemistry), fertilizers, food additives, and water-treatment chemicals.

Production is typically by neutralization of phosphoric acid (H3PO4) with bases to yield various salts, such as NaH2PO4, Na2HPO4, and Na3PO4, or by neutralization with Ca(OH)2 to form calcium phosphate. Environmental concerns include eutrophication; wastewater treatment commonly removes orthophosphates to limit algal blooms.