4-Nitrophenol can be used as a pH indicator.  A solution of 4-nitrophenol appears colorless below pH 5.4 and yellow above pH 7.5. This color-changing property makes this compound useful as a pH indicator. 
The yellow color of the 4-nitrophenolate form (or 4-nitrophenoxide) is due to a maximum of absorbance at 405 nm (ε = 18.3 to 18.4 mM cm in strong alkali). In contrast, 4-nitrophenol has a weak absorbance at 405 nm (ε = 0.2 mM cm).
The isosbestic point for 4-nitrophenol/4-nitrophenoxide is at 348 nm, with ε = 5.4 mM cm.