In a 12-l. round-bottomed flask, fitted with a mechanical stirrer, is placed 1045 cc. of concentrated commercial hydrochloric acid (sp. gr. 1.138). The flask is surrounded with a freezing mixture of ice and salt, and, when the contents are at 0°, stirring is started and 500 g. of cracked ice is added, or more ice can be added and the external cooling dispensed with; then 372 g. (364 cc., 4 moles) of aniline, also cooled to 0°, is run in during five minutes. The mixture is treated with 500 g. more of cracked ice, and a cold solution (0°) of 290 g. (4 moles) of technical sodium nitrite dissolved in 600 cc. of water is allowed to run in slowly (twenty to thirty minutes) from a separatory funnel, the end of which is drawn to a small tip and reaches nearly to the bottom of the flask. During this addition, the stirrer is operated rather vigorously, and the temperature is kept as near 0° as possible by the frequent addition of cracked ice (about 1 kg.).

In the meantime, a sodium sulfite solution is prepared by dissolving 890 g. (20 moles) of sodium hydroxide, of about 90 per cent purity, in about 1 l. of water and then diluting to 6 l. A few drops of phenolphthalein solution are added and sulfur dioxide passed in, first until an acid reaction is indicated and then for two or three minutes longer. During the addition of the sulfur dioxide, the solution is cooled with running water. On account of the strong alkaline solution, the original color produced by the phenolphthalein is very faint, but this slowly increases until it becomes deep just before the acid point is reached. It is best to remove a small sample of the liquid from time to time, dilute with three or four volumes of water, and add a drop more of phenolphthalein (Note 1).

The sodium sulfite solution is placed in a 12-l. flask and cooled to about 5°. Approximately 500 g. of cracked ice is added, and then, with mechanical stirring, the diazonium salt solution is run in as rapidly as possible (Note 2). The mixture becomes a bright orangered. The flask is now warmed to about 20° on a steam bath, until the solid sodium sulfite, which has separated while cooling, redissolves. The total amount of liquid is now about 10 l. One-half of this is poured into another 12-l. flask, and both halves are warmed on the steam bath to 60–70°, until the color becomes quite dark (thirty to sixty minutes). Sufficient hydrochloric acid (300–400 cc.) is now added (Note 3) to each flask to make the solutions acid to litmus. The heating is continued and the color gradually becomes lighter until, after four to six hours, the solutions have become nearly colorless; they may be heated overnight, if desired.

To the hot solutions is now added about one-third of their volume of concentrated hydrochloric acid (2 l. to each portion), and the mixtures are cooled, first in running water, then in a freezing mixture, to 0° (Note 4). The phenylhydrazine hydrochloride precipitates in the form of slightly yellowish or pinkish crystals which may be filtered off and dried (Note 5).

The free base is liberated by adding to the phenylhydrazine hydrochloride 1 l. of a 25 per cent solution of sodium hydroxide. The phenylhydrazine separates and is taken up with benzene (two 300-cc. portions). The combined extractions are well dried with 200 g. of solid sodium hydroxide (Note 6), poured off, and distilled. Most of the benzene may be distilled under ordinary pressure, and the remainder, and any low-boiling impurities, under diminished pressure. The pure phenylhydrazine boils at 137–138°/18 mm., and is obtained as a pale yellow liquid (Note 7). It can be crystallized on cooling in an ice bath; the crystals melt at 23°. The crude phenylhydrazine from two lots of aniline (744 g.) is best distilled at one time and gives 695–725 g. of pure product (80–84 per cent of the theoretical amount).