A mixture of 145 ml. of nitric acid (sp. gr. 1.42) and 400 ml. of water in a 2-l. beaker is cooled to 30° in a slush of ice and water, and 350 g. (1.46 moles) of pure, anhydrous 1-amino-2-naphthol-4-sulfonic acid (Note 1) is weighed into a separate 2-l. beaker. The beaker is removed from the bath; a 10-g. portion of the 1-amino-2-naphthol-4-sulfonic acid is stirred into the solution, and the liquid is then allowed to become entirely motionless. Generally, oxidation starts during this addition; if not, 2 ml. of concentrated nitric acid is poured carefully down the side of the beaker without stirring. Oxidation commences in 1–2 minutes and the mixture turns yellow (Note 2).

The beaker is replaced in the ice bath, and 20–25 g. of 1-amino-2-naphthol-4-sulfonic acid is stirred into the mixture by hand. A second portion of 20–25 g. is added immediately and stirred. The mixture begins to froth and is covered with a layer of 100 ml. of ether which serves as an efficient subsident (Note 3). The remainder of the 1-amino-2-naphthol-4-sulfonic acid is added in 20–25 g. portions during the course of 3–4 minutes, the mixture being stirred well after each addition. Oxides of nitrogen are freely evolved, and a stiff yellow-orange paste is formed. The temperature is maintained between 25° and 30° by vigorous stirring and by controlling the rate of addition of the compound (Note 4). The oxidation is complete within 3–4 minutes after the last addition and gas is then no longer evolved. The thick mass is stirred until the temperature has dropped to 5–10° and then 175 ml. of saturated ammonium chloride solution (30°) is added.

After the mixture has been cooled to 0°, the ammonium 1,2-naphthoquinone-4-sulfonate is collected on a 20-cm. Büchner funnel and as much of the mother liquor is removed as possible by pressing the cake with a porcelain spatula or glass stopper. The product is washed with three equal portions of a cold mixture of 150 ml. of saturated ammonium chloride solution and 100 ml. of water. The wash solution is removed as completely as possible, and the product is washed twice with 50-ml. portions of ethanol, followed by 300 ml. of ether in small portions (Note 5). The ammonium 1,2-naphthoquinone-4-sulfonate is spread out in a thin layer and dried to constant weight at 35–40°. An orange, microcrystalline product of bright appearance is thus obtained. The yield is 350–365 g. (94–98%). The ammonium salt is of good quality and is sufficiently pure for many purposes. No satisfactory method has been devised for its further purification, but it can be converted into a pure potassium salt as follows.

Seventeen hundred milliliters of water containing 0.3 ml. of liquid bromine is heated in a 2-l. Erlenmeyer flask to 50° on a steam bath (Note 6). The flask is removed, 50 g. of ammonium 1,2-naphthoquinone-4-sulfonate is added, and the mixture is shaken for a few minutes until solution is complete. Three grams of Norit is added, and the solution is stirred for 2–3 minutes. It is then filtered by suction and the clear orange filtrate is transferred to a 4-l. Erlenmeyer flask. Four hundred milliliters of saturated potassium chloride solution (30°) is added rapidly in one portion, and the flask is allowed to stand undisturbed. Orange crystals of potassium salt begin to separate immediately, and after standing for 30 minutes the contents of the flask are cooled to 0° in an ice-salt bath and the potassium 1,2-naphthoquinone-4-sulfonate is collected on a 15-cm. Büchner funnel. The solid is washed with 150 ml. of cold, dilute potassium chloride solution (30 ml. of saturated potassium chloride solution added to 120 ml. of water), and then with 150 ml. of ethanol in small portions followed by 300 ml. of ether. The potassium salt is dried to constant weight at 40–50°. The yield is 48–50 g. (90–92% based on the ammonium salt used). The product consists of orange needles free of colored decomposition products but still containing traces of ammonium salts. An ammonium-free compound is obtained by a second crystallization. Seventeen hundred milliliters of water containing 0.2 ml. of liquid bromine is heated to 60° in a 2-l. Erlenmeyer flask on a steam bath; the flask is removed, and 50 g. of the potassium salt is added. The salt dissolves rapidly; the solution is filtered with suction; and the clear orange filtrate is transferred to a 4-l. Erlenmeyer flask. Three hundred milliliters of saturated potassium chloride solution is added rapidly in one portion, and the flask is allowed to stand undisturbed. Orange needles of potassium salt appear in a moment or two, and after standing for 30 minutes the mixture is cooled to 0° in an ice-salt bath. The crystals are collected, washed, and dried as outlined above. The salt thus obtained weighs 45–49 g. (90–98% based on the potassium 1,2-naphthoquinone-4-sulfonate used). This product compared favorably with the sodium salt prepared by the more elaborate borax process¹ with respect to color, colored decomposition products and ammonia content (Note 7) and (Note 8).