= 4-Nitrophenyl phosphate
\b[4-Nitrophenyl phosphate] is a <phosphate monoester> in which a phosphate oxygen joins the aromatic group of <4-Nitrophenol>. The neutral acid has formula $\mathrm{C_6H_6NO_6P}$; a salt or hydrated reagent has a different complete formula. The “4” places the <nitro group> opposite the oxygen-linked substituent on the ring.<4-Nitrophenyl phosphate in PubChem>[CID378, structure, molecular formula and description]{ref}
<Alkaline phosphatase> can hydrolyze <4-Nitrophenyl phosphate>, releasing <4-Nitrophenol> and inorganic <phosphate>. In the alkaline assay mixture the colored anionic product permits measurement with a <spectrophotometer> at $405\,\mathrm{nm}$. The manufacturer's calculation subtracts a blank and specifies a coefficient of $18.5\,\mathrm{mM^{-1}cm^{-1}}$ under its stated conditions.<Diethanolamine Assay for the Enzymatic Assay of Alkaline Phosphatase by MilliporeSigma>[§4, reaction; §§7.4–7.5, blank subtraction and coefficient]{ref}
For an original endpoint example, suppose blank-corrected <absorbance> is $0.370$ through an <optical path length> of $0.50\,\mathrm{cm}$, with that coefficient applicable. The <Beer-Lambert law> gives
$$
c=\frac{A}{\varepsilon\ell}
=\frac{0.370}{18.5\times0.50}\,\mathrm{mM}
=0.040\,\mathrm{mM}.
$$
This is $40\,\mathrm{\mu M}$ product. For an initial substrate concentration of $100\,\mathrm{\mu M}$, one-to-one conversion with no other product source would mean 40-percent conversion. These numbers are an illustrative model, not measured data from the cited assay.
The interpretation requires a suitable <blank correction>, known <optical path length> and stable measurement conditions. Changing <pH> changes the product's ionization and optical response. An endpoint measures accumulated product; estimating catalytic activity requires a time-dependent measurement and the corresponding calibration.
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