[OH:1][C:2]1[CH:11]=[CH:10][C:9]2[C:4](=[CH:5][C:6]([OH:12])=[CH:7][CH:8]=2)[C:3]=1[N:13]=O.[S:15]([O-:18])([OH:17])=[O:16].[Na+].S(=O)(O)[O-].Cl>O>[OH:1][C:2]1[CH:11]=[C:10]([S:15]([OH:18])(=[O:17])=[O:16])[C:9]2[C:4](=[CH:5][C:6]([OH:12])=[CH:7][CH:8]=2)[C:3]=1[NH2:13]In the second reaction stage, the 2,7-dihydroxy-1-nitrosonaphthalene (I) which is generally still moist with water and which reacts in the tautomeric oxime form (Ia), is first suspended, according to Boninger, Chem. Ber. 27, 3050 (1894), in commercially-available 37% aqueous sodium hydrogen sulfite solution and stirred at about 20°-25° C. until it is completely dissolved. The bisulfite addition compound (IIa) thus formed is not isolated, but the brown reaction solution is acidified with hydrochloric acid and heated to about 25°-60° C. 2,7-Dihydroxy-1-aminonaphthalene-4-sulfonic acid (II) is formed by reduction of the nitroso group and aromatization of the cycloaliphatic ring system. The aminonaphtholsulfonic acid (II) is isolated by filtering off the resulting light-gray crystals with suction, washing these on a suction filter first with water and then with methanol and, if desired, drying them. The reaction product thus prepared is used without further purification, if appropriate still moist, for the next reaction stage. The yield of reaction product II is about 90-95% of theory. ##STR6##
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