= Phosphate monoester
A \b[phosphate monoester] is a <phosphate ester> with one organic group joined to <phosphorus> through an <oxygen> atom. <Nomenclature of Phosphorus-Containing Compounds of Biochemical Importance by IUPAC and IUBMB>[§1, phosphoric esters, and §4, phosphodiesters]{ref} A neutral-acid representation is $\mathrm{RO-P(=O)(OH)_2}$. Ionization changes the O–H groups and charge without changing the count of ester linkages. “Mono” counts the one esterified group at that <phosphate> unit, not the number of carbon atoms in $R$.
For example, an ethyl group in $\mathrm{CH_3CH_2O-P(=O)(OH)_2}$ contains two carbons, but the compound is still a <phosphate monoester>. A <phosphate diester> instead has two organic groups linked through oxygen to the same <phosphorus>, represented as $\mathrm{RO-P(=O)(OH)-OR'}$. This structural counting distinguishes the classes without requiring a particular <pH>.
<Hydrolysis> of a <phosphate monoester> releases the corresponding hydroxyl-containing compound and inorganic <phosphate>. <Alkaline phosphatase> is an <enzyme> activity classified by that reaction, with broad <substrate (biochemistry)> specificity.<Enzyme Nomenclature by the IUBMB>[EC3.1.3.1, Reaction, systematic name and Comments]{ref}
An experimentally useful example is <4-Nitrophenyl phosphate>. Its <hydrolysis> releases <4-Nitrophenol>, whose colored anionic form permits an optical assay. That readout follows cleavage of the ester linkage; it does not count every possible phosphorus-containing compound in the sample.<Diethanolamine Assay for the Enzymatic Assay of Alkaline Phosphatase by MilliporeSigma>[§4, reaction; §§7.4–7.5, absorbance measurement and calculation]{ref}
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