= Dihydrogenphosphate
{title2=$\mathrm{H_2PO_4^-}$}
<Dihydrogenphosphate>, $\mathrm{H_2PO_4^-}$, 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>.<Phosphate ions in ChEBI>[CHEBI:39745; definition, formula, net charge and conjugate-acid/base relations]{ref}
For example, reaction with <hydroxide> can transfer a <proton> from <dihydrogenphosphate> to the added <base>:
$$
\mathrm{H_2PO_4^-+OH^-\rightleftharpoons HPO_4^{2-}+H_2O}.
$$
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>.
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