In a 12-l. round-bottomed flask (Note 1) fitted with a mechanical stirrer is placed 500 g. (3.5 moles) of technical β-naphthol dissolved in a warm solution of 140 g. (3.5 moles) of sodium hydroxide in 6 l. of water. The solution is cooled to 0° in an ice and salt bath, and 250 g. (3.5 moles) of powdered technical sodium nitrite is added. Stirring is started, and 1100 g. (833 cc., 4.6 moles) of sulfuric acid (sp. gr. 1.32) is added from a dropping funnel at such a rate that the whole is added in one to one and one-half hours, the temperature being kept at 0°. During the reaction, crushed ice is added from time to time to maintain the temperature at 0°; about 1 kg. is generally used (Note 2). After all the sulfuric acid has been added, the solution should react acid to Congo red paper. The mixture is stirred one hour longer at the low temperature, and then the nitroso-β-naphthol, which has gradually separated during the reaction, is filtered with suction and washed thoroughly with water. The product is at first light yellow in color, but after three to four days it gradually changes to a dark brown. The moisture content seems to have some effect on the color.

After the product has been air-dried for about four days, the yield is about 665 g.; it melts at 97°. A sample of this partially dried product, on drying in a vacuum desiccator over sulfuric acid for twenty hours, loses about 10 per cent of its weight, and the melting point is 106°. By longer drying under ordinary conditions, the melting point of 106° is reached (Note 3). The total yield of dry product is about 595 g. (99 per cent of the theoretical amount).

This product is satisfactory for most purposes. It may be obtained in a crystalline condition, however, by recrystallizing from hot ligroin (b.p. 60–90°). About 2 g. of nitroso-β-naphthol will dissolve in 15 cc. of boiling ligroin. The product is not very soluble in cold ligroin, so that nearly all is recovered.