Off-white paste. 1-Hydroxynaphthalene-3,6-disulfonic acid [578-85-8]. (4-hydroxynaphthalene-2,7- disulfonic acid), RG acid, Violet acid, C10H8O7S2, Mr 304.3: the free acid ( pKa 8.56) and its alkalimetal salts are readily soluble in water, but the acid sodium salt can be precipitated from concentrated solution with salt. Sulfonation with oleum at 125℃ gives a tetrasulfonic acid, and amination with ammonia and ammonium chloride at 160℃ gives a mixture of 3-amino1-hydroxynaphthalene-6-sulfonic acid and 6- amino-1-hydroxynaphthalene-3-sulfonic acid. Alkali fusion at 170℃ gives mainly 1,6-dihydroxynaphthalene-3-sulfonic acid, and at 250℃ yields 1,3,6-trihydroxynaphthalene. Diazo coupling takes place in the 2-position.
Naphthalene-1,3,6-trisulfonic acid is heated with 50 % sodium hydroxide at 180 C for 15 h in an autoclave. After dilution with water, the product is salted out as the monosodium salt and purified by washing with brine.
