= Orthophosphate
<Orthophosphate> denotes the uncondensed, single-<phosphorus> <phosphate> unit. Its fully deprotonated <anion> is $\mathrm{PO_4^{3-}}$: one <phosphorus> surrounded by four <oxygen> atoms. In aqueous biochemical usage, inorganic <phosphate>, $\mathrm{P_i}$, can include protonated forms such as <hydrogenphosphate>, $\mathrm{HPO_4^{2-}}$, and <dihydrogenphosphate>, $\mathrm{H_2PO_4^-}$. Their proportions depend on <pH>; the name alone does not specify a unique protonation state.<Phosphate ions in ChEBI>[CHEBI:18367, orthophosphate synonym and formula; CHEBI:35780, 43474 and 39745, phosphate-ion family and conjugate-acid/base relations]{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 is
$$
\mathrm{P_2O_7^{4-}+H_2O\longrightarrow 2HPO_4^{2-}}.
$$
The exact proton balance changes with the chosen chemical species, but \b[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.<Crystalline inorganic pyrophosphatase isolated from baker's yeast by M. Kunitz>[Journal of General Physiology 35, pp. 423–450; abstract, specific hydrolysis of pyrophosphate into orthophosphate]{ref}
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