81-05-0

2-Aminonaphthalene-5-sulfonic acid [81-05-0]. (6-aminonaphthalene-1-sulfonic acid), Dahl’s acid, Dressel acid, D acid, C10H9NO3S, Mr 223.24, crystallizes from water as needles although only 0.4 % soluble at 100℃. The sodium, calcium, and barium salts are moderately soluble in water. Bromination in acetic acid gives the 1-bromo derivative. Sulfonation with oleum gives a mixture of 2-aminonaphthalene-5,7-disulfonic acid together with some 1,5-disulfonic acid. Fusion with potassium hydroxide at 260℃ yields 6-amino-1-naphthol.

Tobias acid is added to a mixture of sulfuric acid and oleum, and the reaction is completed by stirring for 12 h at 25℃. The resulting sulfonation mass, containing mainly 2-aminonaphthalene-1,5-disulfonic acid, is poured into water and heated at 105℃ for 3 h to desulfonate and precipitate the product which, after cooling, is filtered off; the yield is 95 %.