148-25-4

4,5-Dihydroxynaphthalene-2,7-disulfonic acid [148-25-4]. (1,8-dihydroxynaphthalene-3,6-disulfonic acid), chromotropic acid, C10H8O8S2, Mr 320.3: the disodium salt is readily soluble in water. Alkali fusion yields 1,3,8-trihydroxynaphthalene-6-sulfonic acid. Diazo coupling occurs in the 3- and 6-positions.

Chromotropic acid is used as an analytical reagent and as a coupling component for a wide range of azo dyes, e.g., C.I. Acid Violet 6, C. I. Mordant Blue 13, and C.I. Direct Blue 84.

ChEBI: A naphthalenesulfonic acid that is naphthalene-2,7-disulfonic acid substituted by hydroxy groups at positions 4 and 5.

2-Hydroxynaphthalene-3,6,8- trisulfonic acid total liquor is concentrated and heated with caustic soda liquor to 158℃ for 20 – 30 h . The melt is diluted with water and poured into excess hydrochloric acid. Salt is added, sulfur dioxide expelled, and the mixture cooled to precipitate the product as the disodium salt in 88 % yield.
An alternative process is based on direct fusion (caustic liquor at 228℃ under pressure) of 1-aminonaphthalene-3,6,8-trisulfonic acid to chromotropic acid without isolating the intermediate oxy Koch acid.