6357-93-3

4,6-Dihydroxynaphthalene-2-sulfonic acid [6357-93-3]. (2,8-dihydroxynaphthalene6-sulfonic acid), dioxy G acid, C10H8O5S, Mr 240.2, is readily soluble in water, but the sodium salt is less so and can be salted out.

Dioxy G acid is used as an intermediate in the production of 2-amino-8-hydroxynaphthalene-6-sulfonic acid (g acid) and its Nsubstituted derivatives by means of the Bucherer reaction with ammonia or primary amines. It is also used as a coupling component for azo dyes such as C.I. Mordant Black 56.

The potassium salt of 2-hydroxynaphthalene-6,8-disulfonic acid is heated with 70 % caustic liquor for 3 h at 230℃ and then at 240℃ for completion of the reaction. The melt is diluted with water and poured into excess hydrochloric acid. After sulfur dioxide has been expelled, salt is added and the batch is cooled to precipitate the product, which is filtered off and washed with brine; the yield is 80 %.