N[CH2:2][C:3]1[C:12]2[C:7](=[CH:8][C:9]([S:13]([OH:16])(=[O:15])=[O:14])=[CH:10][CH:11]=2)[CH:6]=[CH:5][CH:4]=1.[OH-:17].[Na+].[Cr](O[Cr]([O-])(=O)=O)([O-])(=O)=[O:20].[Na+].[Na+]>O>[C:2]([C:3]1[C:12]2[C:7](=[CH:8][C:9]([S:13]([OH:16])(=[O:15])=[O:14])=[CH:10][CH:11]=2)[CH:6]=[CH:5][CH:4]=1)([OH:20])=[O:17]118.6 g (0.5 mol) of 1-aminomethylnaphthalene-6-sulphonic acid (100%) are stirred with 375 ml of water and about 50 g of 45% strength NaOH; the pH value of the mixture is brought to 7-7.5. After addition of 149 g of sodium dichromate (0.5 mol), the mixture is heated at 240° C. in an autoclave for 15 hours. After cooling, the green chromium hydroxide residue is filtered off and washed with hot water. The filtrate is acidified to pH 2 with hydrochloric acid (30% strength) under the influence of heat and then cooled to 20° C. The 1-carboxynaphthalene-6-sulphonic acid which precipitates out in the form of its sodium salt is filtered off and washed with 100 ml of saturated NaCl solution.
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