To a filtered solution of 18.2 g of 5-nitroanthranilic acid in 300 ml of water and 22 ml of 5N sodium hydroxide is added 6.9 g of sodium nitrite. The resulting, slightly warm solution is poured over a rapidly stirred mixture of 34.0 ml of concentrated hydrochloric acid, 50 ml of acetic acid, 350 g of ice and 100 ml of water in a 2 liter beaker. This mixture is stirred in an ice bath during preparation of a solution of the sodium salt of 7-amino-1-naphthol-3-sulfonic acid (gamma acid). Thus, to a solution of 28 g of 85% pure 7-amino-1-naphthol-3-sulfonic acid in 50 ml of water and 24 ml of 5N sodium hydroxide is added 30 g of sodium acetate trihydrate in 30 ml of water. The resulting solution is added to the rapidly stirred diazonium salt solution and the mixture is stirred for approximately 5 minutes then is heated to 90° C. and allowed to stand overnight at room temperature. The mixture is filtered and the product is washed with a small amount of water. The product if then dissolved in 1300 ml of hot water and filtered through a steam heated, sintered funnel. To the filtrate maintained at 80° C. is added 140 g of sodium formate. The mixture is cooled to room temperature and is filtered. The product is washed with a 10% aqueous solution of sodium formate followed by ethanol to give 2-(2-amino-8-hydroxy-6-sulfo-1-naphthylazo)-5-nitrobenzoic acid, 6'-sodium salt.
