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4-Nitrophenolate is the conjugate base formed when 4-Nitrophenol loses its phenolic proton. Its chemical formula is ${\mathrm{C}}_6{\mathrm{H}}_4\mathrm{N}{\mathrm{O}}_3^{-}$ and its net electric charge is $-1$. The acid and base share the same substituted benzene framework; removing a proton changes the charge and electronic structure without removing the nitro group.^[ref]

The distinction is experimentally useful because alkaline 4-Nitrophenol assays exploit the strongly absorbing anionic form. A 1980 reference-material study measured the molar absorption coefficient under specified alkaline conditions at $401\mathrm{nm}$ and $25{}^{\circ }\mathrm{C}$. Matching the final reaction conditions mattered to the spectrophotometric analysis.^[ref]

An ideal dilute-solution calculation explains why changing pH can change colour without forming more product. Write $c_{\mathrm{T}}=[\mathrm{HA}]+[{\mathrm{A}}^{-}]$ for total acid-plus-base concentration. The Henderson-Hasselbalch equation gives the anionic fractionAt $\mathrm{pH}=\mathrm{p}{K}_a$, half is anionic; two pH units higher, $\alpha =100/101\approx 0.9901$. Thus a hypothetical fixed total $20\mu \mathrm{M}$ contains $10\mu \mathrm{M}$ anion at the first condition and about $19.80\mu \mathrm{M}$ at the second. These are relative-to-pKa examples, not an asserted universal numerical pKa. Where the protonated form absorbs weakly, absorbance tracks the anionic concentration more directly than the unchanged total.