Sulfuric Acid → 4-((4-Aminophenyl)diazenyl)benzenesulfonic acid

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173 g of sulfanilic acid were diazotized in a conventional manner in 1000 ml of ice-water at pH 1.3 in the presence of 60 g of concentrated sulfuric acid using 69 g of sodium nitrite. The stirred suspension was rendered less acid with 100 g of sodium bicarbonate and admixed with 220 g of sodium anilinomethanesulfonate. The coupling was completed overnight. The pH of the mixture was 5.7. Then 100 g of amidosulfuric acid and 100 g of concentrated sulfuric acid diluted with a little ice were added, and the batch was heated to 95° C. and stirred at that temperature for 15 min. Thereafter the batch was cooled down to room temperature and the precipitate was filtered off with suction. After washing with 3000 ml of water, the filter cake was dried at 80° C. to leave 256 g of 4'-aminoazobenzene-4-sulfonic acid having a purity of 98.6%.

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