= Pyrophosphate
{title2=$\mathrm{P_2O_7^{4-}}$}
<Pyrophosphate> is the two-<phosphorus> member of the condensed <phosphate> family, with a <phosphorus>–<oxygen>–<phosphorus> bridge joining its two phosphate units. The fully deprotonated <anion> is $\mathrm{P_2O_7^{4-}}$; its acid is $\mathrm{H_4P_2O_7}$. The name also occurs in salts and partially protonated forms, so an actual <chemical solution> must be described with its <pH> and <protonation> state.
Cleaving the bridge by <hydrolysis> produces two separate <phosphate> units. One balanced representation is
$$
\mathrm{P_2O_7^{4-}+H_2O\longrightarrow 2HPO_4^{2-}}.
$$
Both the atoms and the total charge balance: the two products together carry charge $-4$. The equation distinguishes chemical cleavage from merely releasing an intact <pyrophosphate> <ion> from a larger molecule.
A classic experiment isolated crystalline <inorganic pyrophosphatase> from yeast and tested its activity on <pyrophosphate>. The <enzyme> accelerated <hydrolysis> and required activating <metal> <ions>, including <magnesium>. Its failure to hydrolyze the tested organic compounds containing related phosphate linkages demonstrated <reaction substrate> specificity: recognizing free <pyrophosphate> is not equivalent to attacking every <phosphorus>–<oxygen>–<phosphorus> linkage.<Crystalline inorganic pyrophosphatase isolated from baker's yeast by M. Kunitz>[abstract, catalytic specificity and activating ions]{ref}
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