Phosphoric ester
= Phosphoric ester
= Phosphate ester
{synonym}
A \b[phosphoric ester] is an <ester> of <phosphoric acid>: an organic group is joined through <oxygen> to <phosphorus>. Replacing one, two or three of the acid's O–H hydrogens by organic groups gives a <phosphate monoester>, <phosphate diester> or <phosphate triester>. A <phosphoric anhydride> instead contains an acid-derived linkage, such as P–O–P, rather than an additional P–O–C ester linkage.<Nomenclature of Phosphorus-Containing Compounds of Biochemical Importance by IUPAC and IUBMB>[§§1–3, phosphoric esters, phosphate terminology and phosphoric anhydrides]{ref}
For a structural counting example, compare
$$
\mathrm{CH_3O-P(=O)(OH)_2}
\quad\text{and}\quad
\mathrm{(CH_3O)_2P(=O)(OH)}.
$$
The first has one organic O–P attachment; the second has two. Removing an O–H proton changes the charge but leaves that attachment count unchanged. Conversely, adding a second separate monoester group elsewhere on a large <molecule> does not turn the first phosphorus center into a diester.
This distinction matters in biochemical measurements. <4-Nitrophenyl phosphate> is a <phosphate monoester> whose cleavage can be followed optically; the released inorganic <phosphate> is a different chemical species. An <alkaline phosphatase> assay detects a specified conversion under specified conditions, rather than counting all phosphorus-containing material indiscriminately.<Diethanolamine Assay for the Enzymatic Assay of Alkaline Phosphatase by MilliporeSigma>[§4, hydrolysis reaction; §§7.4–7.5, optical rate measurement]{ref}