sentences:  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−1 cm−1 in strong alkali).In contrast, 4-nitrophenol has a weak absorbance at 405 nm (ε = 0.2 mM−1 cm−1).