1-Hydroxynaphthalene-7-sulfonic acid

1-Hydroxynaphthalene-7-sulfonic acid [20191-62-2]. (8-hydroxynaphthalene-2- sulfonic acid), C10H8O4S, Mr 224.2, is very soluble in water, and most of its metal salts are also readily soluble. It couples with diazotized 4- nitroaniline to give the 2,4-disazo compound. Nitration gives 2,4-dinitro-1-naphthol-7-sulfonic acid, which was known as Flavianic acid when used as a yellow dye for wool.

1-Naphthol is sulfonated with concentrated sulfuric acid at 130℃ to give a mixture of 2,7- and 4,7-disulfonic acids. This mixture is difficult to separate, but dilution with water and heating the resulting dilute sulfuric acid solution at 120℃ for 30 min result in selective desulfonation in the 2- and 4-positions.