Preparation of 2-nitrophenol-4,6-disulfonic acid
94 grams of pure phenol is melted at 50°, and 300 grams of 100% sulfuric acid is added to it, with good stirring, over a period of 10 minutes. The mixture is heated to 100°C. for 1 hour, then cooled to 50°, and 300 grams of oleum containing 60 % of sulfur trioxide, is added. The mixture is again heated at 100° for 1 hour, then cooled in an ice-salt bath to —5°, and a mixture of 100 grams of 65 % nitric acid and 100 grams of 100 % sulfuric acid is added dropwise, the temperature being kept between —5 and 0°. The nitration requires about 3 hours, and must not be carried out too rapidly or picric acid will be formed. After 10 hours, the reaction mixture is poured into a mixture of 500 grams of ice and 1 liter of water, and the 2-nitrophenol-4,6-disulfonic acid is salted out with 300 grams of sodium chloride. The total volume is now about 2 liters. The sodium sulfonate is precipitated as a light yellow solid which is filtered off after 10 hours and washed on the filter with 200 ml. saturated salt solution. The yield of moist press cake is about 380 to 400 grams.
The fundamental processes of dye chemistry, by H. E. Fierz-David, 153-154, 1949