Hydrogenphosphate ($\mathrm{HP}{\mathrm{O}}_4^{2-}$)

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.