HPO42PO43

HPO42PO43 is not a single chemical but a shorthand reference to two related phosphate anions that occur in aqueous solution: hydrogen phosphate (HPO4^2−) and phosphate (PO4^3−). These species arise from the stepwise deprotonation of phosphoric acid (H3PO4) and are part of the phosphate buffer system that dominates phosphate chemistry in water and biological contexts.

Structure and relationship: In both HPO4^2− and PO4^3−, phosphorus is tetrahedrally coordinated to four oxygen atoms. In hydrogen phosphate, one oxygen is protonated (P–OH), giving a −2 charge. In phosphate, all oxygens are deprotonated, yielding a −3 charge. The two species interconvert with changing pH via the successive loss of protons: H2PO4− ⇌ HPO4^2− + H+ and HPO4^2− ⇌ PO4^3− + H+. The pKa values for these steps are commonly cited as about 7.2 (H2PO4− to HPO4^2−) and 12.3 (HPO4^2− to PO4^3−), though exact values depend on temperature and ionic strength.

In solution, the relative abundance of HPO4^2− and PO4^3− is governed by pH. Around physiological pH (approximately 7.4), HPO4^2− is typically the predominant species, with PO4^3− forming a smaller fraction; as pH increases, the proportion of PO4^3− grows.

Occurrence and roles: Phosphate species are central to biology and environmental chemistry. They participate in energy transfer (ATP), nucleic acids, and signaling. In water and soils, they buffer pH, interact with calcium and other cations to influence mineralization (such as calcium phosphate formation), and affect nutrient availability.

Notes: The notation HPO42PO43 is informal; correct chemistry uses HPO4^2− and PO4^3− with their respective charges. The two species are related by pH-dependent equilibria and are frequently discussed together in the context of phosphate buffering.