HPO42

HPO4^2−, commonly called hydrogen phosphate, is the hydrogen phosphate ion, a dianion derived from phosphoric acid (H3PO4). It is the conjugate base of dihydrogen phosphate (H2PO4−) and the conjugate acid of phosphate (PO4^3−). In aqueous solution its relative abundances depend on pH, forming part of the biological phosphate buffer system.

structurally, phosphorus in HPO4^2− is tetrahedrally coordinated by four oxygen atoms. In hydrogen phosphate one oxygen bears a proton, forming an OH group, while the remaining oxygens carry negative character that is delocalized through resonance. This gives the ion multiple resonance forms and a overall 2− charge distributed over the oxygens.

HPO4^2− occurs in many inorganic salts, such as disodium hydrogen phosphate (Na2HPO4). It can be prepared by partial neutralization of phosphoric acid with base; for example, reacting H3PO4 with two equivalents of NaOH yields Na2HPO4. In solution, HPO4^2− participates in the phosphate buffer system: it can accept a proton to form H2PO4− (pKa2 ≈ 7.2) or donate a proton to form PO4^3− (pKa3 ≈ 12.3). Thus it is amphiprotic.

Applications of HPO4^2− include use as a buffering agent in foods, medicines, and biology; as a source of phosphate in fertilizers; and in water treatment and detergent formulations. It readily forms salts with metal cations, such as calcium hydrogen phosphate (CaHPO4), which are relevant in scales, deposits, and mineralization studies. Safety handling follows standard inorganic phosphate guidelines; while relatively low toxicity, proper care and labeling are advised in laboratory and industrial contexts.