It is deep blue dye that is the product of the Berthelot's reaction, a common test for ammonia.

[2] The indophenol group, with various substituents in place of OH and various ring substitutions, is found in many dyes used in hair coloring and textiles.

[3] Indophenol is used in hair dyes, lubricants, redox materials, liquid crystal displays, fuel cells and chemical-mechanical polishing.

[1][4] In the Berthelot test (1859), a sample suspected of containing ammonia is treated with sodium hypochlorite and phenol.

Dichlorophenol-indophenol (DCPIP), a form of indophenol, is often used to determine the presence of vitamin C (ascorbic acid).