[N+:1]([C:4]1[CH:9]=[CH:8][C:7]([N:10]=[N:11][C:12]2[C:21]3[C:16](=[CH:17][CH:18]=[CH:19][CH:20]=3)[C:15]([N:22]([CH3:24])[CH3:23])=[CH:14][CH:13]=2)=[CH:6][CH:5]=1)([O-])=O.O.O.O.O.O.O.O.O.O.S([O-])(O)(=O)=O.[Na+].CO>O>[NH2:1][C:4]1[CH:9]=[CH:8][C:7]([N:10]=[N:11][C:12]2[C:21]3[C:16](=[CH:17][CH:18]=[CH:19][CH:20]=3)[C:15]([N:22]([CH3:24])[CH3:23])=[CH:14][CH:13]=2)=[CH:6][CH:5]=1A mixture of 15 g of 1-(4-nitro-phenylazo)-4-(dimethylamino)naphtalene, 75 ml sodiumhydrogen sulphate nonahydrate (50% wt in water) and 80 ml of methanol was heated at reflux for 3 h. The reaction mixture was then cooled to room temperature and poured into 300 ml of water. The brownish precipitate was filtered off, washed with water (200 ml) and cold methanol-water (100 ml/100 ml) to give 12 g of nearly pure 1-(4-amino-phenylazo)-4-(dimethylamino)naphtalene as a brownish solid.
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