= Hydrogenphosphate
{title2=$\mathrm{HPO_4^{2-}}$}
<Hydrogenphosphate>, $\mathrm{HPO_4^{2-}}$, 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>, $\mathrm{PO_4^{3-}}$. These species share the same single-<phosphorus>, four-<oxygen> framework.<Phosphate ions in ChEBI>[CHEBI:43474; formula, net charge and conjugate-acid/base relations]{ref}
The remaining <proton> allows <hydrogenphosphate> to participate on either side of an <acid-base reaction>:
$$
\mathrm{HPO_4^{2-}+H^+\rightleftharpoons H_2PO_4^-},\qquad
\mathrm{HPO_4^{2-}\rightleftharpoons PO_4^{3-}+H^+}.
$$
In the first reaction, <hydrogenphosphate> accepts a <proton>; in the second, it donates one. For a simple accounting example, one mole of $\mathrm{Na_2HPO_4}$ 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.
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