To a suspension of 18 g of calcium 3-methylsulfonyl-4-(2-acetylamino-5-chloro-4-hydroxyphenylazo)phenylsulfonate, 25 ml of dimethylacetamide, and 54 ml of acetonitrile was added dropwise to 18 ml of phosphorus oxychloride at room temperature with stirring. After completion of the addition, the reaction was allowed to proceed at 60° C. for 3 hours, and the reaction mixture was cooled to room temperature and then poured in 1 l of ice water. The mixture was stirred at a temperature of 10° C. or less for 1 hour and the resulting crystalline precipitate was collected by filtration, washed with water and air-dried.
