4-(benzoylamino)-5-hydroxynaphthalene-1,7-disulphonic acid

4-Benzamido-5-hydroxynaphthalene-1,7-disulfonic acid [6361-49-5], N-benzoyl K acid, is produced by reacting purified K acid with benzoyl chloride in aqueous alkaline sodium carbonate solution at 35 – 40 ℃ for 18 h, followed by heating for 1 h at 90℃ to hydrolyze the O-benzoyl derivative (yield 80 %). It is used to produce C.I. Acid Red 133, C.I. Acid Red 157, and C.I. Mordant Red 21.