= Phosphate triester
A \b[phosphate triester] is a <phosphoric ester> in which three organic groups are attached through <oxygen> to one <phosphorus> center. A general connectivity formula is
$$
\mathrm{O=P(OR)(OR')(OR'')}.
$$
Unlike a neutral-acid <phosphate monoester> or <phosphate diester>, this representation contains no remaining P–OH group. The organic groups may nevertheless contain their own ionizable sites; “triester” does not guarantee that every molecule in the class is electrically neutral.
<Trimethyl phosphate> supplies a concrete example: $\mathrm{O=P(OCH_3)_3}$ has formula $\mathrm{C_3H_9O_4P}$. Its three methyl groups each contribute one carbon and three hydrogens, while the central unit and linking oxygens contribute one phosphorus and four oxygens.<Trimethyl phosphate in PubChem>[CID10541, molecular formula, structure and SMILES]{ref}
An original connectivity comparison replaces one $\mathrm{-OCH_3}$ group by $\mathrm{-OH}$. The resulting fragment, $\mathrm{O=P(OCH_3)_2(OH)}$, is a diester because only two organic groups remain attached through oxygen. Merely removing a proton from that O–H group changes charge without restoring a third ester linkage.
The suffix counts esterification at one phosphorus center. Three separate <phosphate monoesters> elsewhere on a large molecule do not constitute one <phosphate triester>. This local counting rule helps distinguish a change in connectivity from a change in <pH>, protonation or total phosphorus content.
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