N-Benzoyl H acid

4-Benzamido-5-hydroxynaphthalene-2,7-disulfonic acid [117-46-4], N-benzoyl H acid, is produced by reaction of H acid with benzoyl chloride in aqueous alkaline sodium carbonate solution at 35℃ for 5 h, followed by heating at 95℃ for 2 h to hydrolyze the Obenzoyl derivative (yield 80 %). It is used to produce C.I. Reactive Red 4 and C.I. Direct Green 13.