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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.

Hydrogenphosphate, $\mathrm{HP}{\mathrm{O}}_4^{2-}$, is the doubly charged anion produced by removing a proton from dihydrogenphosphate. It is therefore the conjugate base of dihydrogenphosphate, and it is also the conjugate acid of fully deprotonated orthophosphate, $\mathrm{P}{\mathrm{O}}_4^{3-}$. These species share the same single-phosphorus, four-oxygen framework.^[ref]

The remaining proton allows hydrogenphosphate to participate on either side of an acid-base reaction:In the first reaction, hydrogenphosphate accepts a proton; in the second, it donates one. For a simple accounting example, one mole of $\mathrm{N}{\mathrm{a}}_2\mathrm{HP}{\mathrm{O}}_4$ supplies one mole of phosphate units and two moles of sodium counterions when dissolved. Subsequent proton transfers can change how those phosphate units are distributed between chemical species without changing the total amount of phosphorus. A stated concentration of total inorganic phosphate must therefore be distinguished from the concentration of hydrogenphosphate itself.

Orthophosphate denotes the uncondensed, single-phosphorus phosphate unit. Its fully deprotonated anion is $\mathrm{P}{\mathrm{O}}_4^{3-}$: one phosphorus surrounded by four oxygen atoms. In aqueous biochemical usage, inorganic phosphate, ${\mathrm{P}}_{\mathrm{i}}$, can include protonated forms such as hydrogenphosphate, $\mathrm{HP}{\mathrm{O}}_4^{2-}$, and dihydrogenphosphate, ${\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_4^{-}$. Their proportions depend on pH; the name alone does not specify a unique protonation state.^[ref]

The contrast with pyrophosphate is connectivity. Pyrophosphate joins two phosphorus centres through an oxygen bridge; hydrolysis breaks that bridge to yield two separate orthophosphate units. One charge-balanced representation isThe exact proton balance changes with the chosen chemical species, but two phosphorus atoms remain two phosphorus atoms. M. Kunitz's 1952 isolation of inorganic pyrophosphatase from Saccharomyces cerevisiae provided a purified enzyme that specifically catalysed conversion of inorganic pyrophosphate to orthophosphate, making this distinction experimentally measurable.^[ref]