23 ноября, 2015 
Pokraskin (o-Tolidine, o-Dianitidine)
A solution is made by heating to 70°C. a mixture of 18.4 grams (0.1 mole) of technically pure benzidine, 23 cc. 30 per cent hydrochloric acid, and 150 cc. water. (If a perfectly clear solution is desired, hydrochloric acid which is free from sulfuric acid must be used.) The solution is cooled to 30-40°, and 50 grams of ice is added, causing part of the hydrochloride to separate. An additional 23 cc. hydrochloric acid, diluted with water, is added with good stirring, and then 70 cc. of a 20 per cent nitrite solution is added over a period of 10 seconds. The temperature is about 10-12° and the solution should become clear in 1 minute. At lower temperatures, the last traces of benzidine sulfate disappear only after 8 to 10 minutes. The solution is tested with Congo red and starch-iodide paper. The solution is almost neutral, but no diazoamino compounds are formed as in the case of aniline.
Tolidine and dianisidine must not be boiled, but should be dissolved below 40°C. In the plant, the solutions are allowed to stand overnight with half of the hydrochloric acid, and the rest of the acid and the ice are added on the next day.
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