hydrogenphosphate

Hydrogen phosphate refers to the hydrogen phosphate ion, HPO4^2−, a polyatomic anion 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, the HPO4^2−/H2PO4− system is an important component of the phosphate buffer system, with pKa values such that the second dissociation H2PO4− ⇌ HPO4^2− + H+ has a pKa of about 7.2 at 25 °C, providing significant buffering capacity near neutral pH. The full triplet of dissociation constants for phosphoric acid is commonly cited as pKa1 ≈ 2.15, pKa2 ≈ 7.21, and pKa3 ≈ 12.35, reflecting the stepwise removal of protons from H3PO4.

Structure and properties: phosphorus in HPO4^2− is tetrahedrally coordinated to four oxygen atoms. In the ion, one oxygen is bonded as an OH group, while the remaining oxygens bear negative character through resonance, giving an overall charge of 2−. The ion forms a variety of salts with common cations such as sodium, potassium, and ammonium.

Occurrence and preparation: hydrogen phosphate is produced by partial neutralization of phosphoric acid. It occurs in many phosphate salts used commercially, including disodium hydrogen phosphate (Na2HPO4) and monosodium phosphate (NaH2PO4), which correspond to the HPO4^2− and H2PO4− species, respectively.

Applications: the HPO4^2− ion is used as a buffering agent in biochemical and clinical laboratories, and in various industrial and agricultural contexts as part of phosphate fertilizers and water-treatment formulations. It participates in biological phosphate metabolism and mineral buffering processes.