1-hydroxynaphthalene-2-sulphonic acid

1-Hydroxynaphthalene-2-sulfonic acid [567-18-0]. Baum’s acid, C10H8O4S, Mr 224.23, is sparingly soluble in water and is precipitated from aqueous solution by hydrochloric acid. Coupling with diazo compounds and nitrosation both take place in the 4-position. Sulfonation gives 1-naphthol-2,4-disulfonic acid and nitration gives 2,4-dinitro-1-naphthol.

The reaction of 1-naphthol with oleum in the cold yields a mixture of the 2- and 4- sulfonic acids; the former may be separated from the aqueous quenched reaction mixture by salting out the potassium salt after removing unreacted 1-naphthol.