PO4

PO4 commonly refers to the phosphate group and the phosphate ion. In inorganic chemistry, the orthophosphate ion is PO4^3-, a tetrahedral species with phosphorus at the center bonded to four oxygens. It exhibits resonance among the P–O bonds and carries a formal −3 charge. Phosphoric acid, H3PO4, can lose protons to form dihydrogen phosphate (H2PO4^−) and hydrogen phosphate (HPO4^2−), with successive pKa values of about 2.15, 7.20, and 12.35, respectively. At neutral pH, HPO4^2− and H2PO4^− are the predominant species, with PO4^3− present in smaller amounts.

Phosphate is ubiquitous in nature and biology. It is a key nutrient for all living organisms and a central component of nucleic acids (DNA and RNA), ATP, and phospholipids. In geology, phosphate minerals such as apatite form important phosphate reservoirs. In aqueous systems, phosphate balance affects soil fertility and water quality; excessive phosphate from agriculture and industry can contribute to eutrophication.

Common salts and minerals include sodium phosphate (Na3PO4), disodium phosphate (Na2HPO4), monosodium phosphate (NaH2PO4), and calcium phosphate minerals used in bones and teeth. In industry, phosphates are employed as fertilizers, buffering agents, water-softening salts, and food additives.

Isotopically, phosphorus-31 is NMR-active and widely used in spectroscopy to study phosphate-containing compounds and metabolic pathways. Overall, the PO4 group is defined by a tetrahedral phosphate center, versatile protonation states, and broad relevance across chemistry, biology, and environmental science.