In a 3-l. round-bottomed flask fitted with a reflux condenser are placed 218 g. (1 mole) of coarsely powdered technical o-nitroaniline-p-sulfonic acid, and a hot mixture of 775 cc. of concentrated sulfuric acid (sp. gr. 1.84) and 950 cc. of water (Note 1). Heat is applied and the mixture is refluxed gently for one hour after solution is practically complete (total about three hours). The resulting dark solution is allowed to cool and is poured slowly into 12 l. of cold water in a crock (Note 2).

After cooling, the dense orange-yellow precipitate is filtered with suction. This crop of crystals after air-drying weighs about 70 g. and melts at 68–70°. The filtrate is returned to the crock and made slightly alkaline with 50 per cent sodium hydroxide solution (about 2.25 l.). It is then made barely acid to litmus with sulfuric acid. This neutralization will produce considerable heat, and the mixture should be thoroughly cooled before filtering. This second crop of crystals weighs about 22–25 g. and melts at 69–70.5°. The total crude yield is 90–95 g.

The two crops of crystals may be combined for recrystallization from boiling water, using 1 l. for each 9 g. The hot solution is filtered quickly, and the filtrate is cooled thoroughly and filtered with suction. The crystals are dried at 50°. The product melts at 69–71°. An alternative method of purification is to dissolve the crude product in about 250 cc. of 95 per cent alcohol, filter the solution, dilute with 1 l. of hot water, and warm on a steam bath if a precipitate forms. On cooling this solution, the product separates in orange-brown needles, and is filtered and air-dried. The yield is about 78 g. (56 per cent of the theoretical amount), m.p. 69.5–70.5°.