Dihydrogenphosphate (${\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_4^{-}$)

Dihydrogenphosphate, ${\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_4^{-}$, is the singly charged anion obtained when phosphoric acid loses one proton. It is one protonation state of orthophosphate: one phosphorus atom remains connected to four oxygen atoms. Dihydrogenphosphate is the conjugate base of phosphoric acid and the conjugate acid of hydrogenphosphate.^[ref]

For example, reaction with hydroxide can transfer a proton from dihydrogenphosphate to the added base:The total electric charge is $-2$ on either side, and the number of phosphorus atoms is unchanged. This is an acid-base reaction, rather than the breaking of a pyrophosphate bridge. Together, dihydrogenphosphate and hydrogenphosphate can form a buffer solution, accepting added base or acid through interconversion. Their relative amounts at chemical equilibrium depend on pH and the relevant dissociation constant; a total phosphate measurement alone does not determine the amount of dihydrogenphosphate.