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4-Nitrophenol is phenol with a nitro group opposite the hydroxy group on the benzene ring: positions 1 and 4. Its chemical formula is ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{N}{\mathrm{O}}_3$, and the alternative name p-nitrophenol describes the same substitution pattern.^[ref]

Removing the phenolic proton produces its conjugate base, 4-Nitrophenolate. This change matters in optical enzyme assays: alkaline phosphatase can release 4-Nitrophenol from 4-Nitrophenyl phosphate, and the alkaline mixture favours the strongly absorbing anionic form. A spectrophotometer follows product-associated absorbance; an enzyme-free reference blank distinguishes the reaction signal from background.^[ref]

A 1980 reference-material study measured a molar absorption coefficient of $18380\pm 90\mathrm{L}\mathrm{mo}{\mathrm{l}}^{-1}\mathrm{c}{\mathrm{m}}^{-1}$ at $401\mathrm{nm}$ and $25{}^{\circ }\mathrm{C}$ under its specified alkaline conditions.^[ref] For an illustrative blank-corrected absorbance $A=0.3676$ and optical path $l=1\mathrm{cm}$ under matching conditions, the Beer-Lambert law gives $c=A/(\epsilon l)=20\mu \mathrm{mol}{\mathrm{L}}^{-1}$. Applying that coefficient at a different wavelength, pH or reaction composition needs validation: colour alone does not fix a concentration.