In a 6-l. wide-mouthed bottle, fitted with a mechanical stirrer and a separatory funnel, are placed 200 g. (1.39 moles) of α-naphthol melting at 91° or higher (Note 1) and 910 cc. of 10 per cent sodium hydroxide solution (2.5 moles). The mixture is stirred for ten minutes, at the end of which time it has become a homogeneous solution.

While the naphthol is dissolving, a solution of benzenediazonium chloride is prepared. In a 3-l. wide-mouthed bottle are placed 128 g. (125 cc., 1.38 moles) of aniline and 1.5 kg. of cracked ice (Note 2), and upon this mixture while it is stirred (by hand) is poured 410 cc. of concentrated hydrochloric acid (31.5 per cent; sp. gr. 1.16). Some crystals of aniline hydrochloride separate at this point. A solution of 100 g. (1.45 moles) of sodium nitrite in 200 cc. of water is now run in slowly from a 500-cc. separatory funnel. The tip of the stem of the separatory funnel should dip well below the surface of the liquid. The mixture is stirred (by hand if desired) and a drop is tested from time to time with starch-iodide paper. This is best done by diluting the test-drop on a watch glass with about 1 cc. of water and then placing a drop of this solution on the starch-iodide paper. The sodium nitrite solution is added until there is a test for nitrous acid which is permanent for five minutes. The diazotization is then complete; all but 2–5 per cent of the nitrite is required.

To the alkaline solution of α-naphthol is now added 1 kg. of cracked ice, the stirrer is started and the diazonium solution run in through a separatory funnel, the stem of which dips below the surface of the solution. The stirring must be uniform and efficient during the addition (Note 3). The addition of the diazonium solution should take ten minutes. Ice is added if necessary to keep the solution below 10°. The mixture is stirred for one-half hour after the addition of the diazonium solution and then allowed to stand for three hours at 7–10° (Note 4). It is then filtered on a Büchner funnel, washed with 1 l. of water and sucked as dry as possible.

The moist benzeneazo-α-naphthol is next transferred to a 5-l. wide-mouthed bottle or a 2-gallon (8-l.) crock equipped with a mechanical stirrer and containing 3 l. of 10 per cent sodium hydroxide solution. The mixture is stirred for ten minutes to insure solution of the dye, and then filtered. The insoluble solid is stirred up with 500 cc. of 10 per cent sodium hydroxide, filtered again and washed with water. The solid, which weighs 10–90 g., consists mainly of the 2,4-disazo derivative (m. p. 197–198°), together with any benzeazo-β-naphthol that may be present.

To the deep-red solution is added 550 g. of sodium hydrosulfite (of at least 80 per cent purity); the mixture is stirred for five minutes and then for five thirty-second periods during one-half hour (Note 5). The solution slowly loses its red color (a thin layer of aniline collects on the surface), and the temperature rises from 25–50°. At the end of one-half hour the solution should no longer be red; it is then rapidly cooled to 20° in a freezing bath and by the addition of about 500 g. of ice.

To the solution is now added, with stirring, 625 cc. (6.3 moles) of concentrated (31.5 per cent; sp. gr. 1.16) hydrochloric acid (Note 6) or its equivalent of acid of approximately the same strength. A cream-colored precipitate of the aminonaphthol forms. It is rapidly filtered off on a Büchner funnel (Note 7) and washed with one portion (200 cc.) of water about half saturated with sulfur dioxide. The precipitate soon darkens on exposure to air and is therefore rapidly transferred to a 2-gallon (8-l.) crock containing 2 l. of water and 340 cc. of concentrated hydrochloric acid. The soft lumps are broken up with a cork stopper set on the end of a stirring rod. Steam is passed into the mixture for forty-five minutes, and during the last thirty minutes the mixture is stirred mechanically. At the end of this time the amine will have passed completely into solution. An insoluble impurity weighing 25–30 g. is removed by filtering with suction. The filtrate is cooled to 25° and if it is not clear is filtered again. To the clear solution at 25° is added 1200 cc. of concentrated hydrochloric acid with stirring, when a light gray precipitate of the aminonaphtholhydrochloride immediately forms. The solution is allowed to stand for two to three hours and is then filtered with suction. The hydrochloride is sucked as free from mother liquor as possible, washed very thoroughly with hydrochloric acid (1 volume acid to 1 volume of water), and dried on a porous plate. The liquors are concentrated, under reduced pressure, to a small volume, and a second crop is thus obtained. The product is of a light purplish tint (Note 8) and weighs 175–200 g. (65–74 per cent of the theoretical amount, based on the quantity of α-naphthol employed) (Note 9).