Sudan

SUDAN, neutral azocolours of different composition, designated by the numbers I, II, and III. All S. are insoluble in water, soluble in alcohol, acetone, and even more easily in fats and lacquers, which is why they are used in alcoholic solutions as reagents for fat. The most commonly used is S. III (amidoazobenzolazo-/?-naphthol), introduced into histological technique by Daddi in 1897 for the detection of fat and fat-like substances in tissues. A saturated solution in 70% alcohol is used, which colors neutral fats red and lipoid substances red-orange or red-yellow; fatty acids, according to W. Rosenthal, are not colored at all by S. III, according to Handwerk, on the contrary, are colored a garnet-red color, while neutral fats take a more orange shade. For staining, material, preferably fixed in formalin, is taken, although completely fresh material can also be used; over-fixation in the fixative is not recommended, as the staining is weak. Depending on the material, staining lasts from several minutes to a day. According to Handwerk, prolonged staining should be avoided, as even 70% alcohol extracts a little fat from tissues, especially in small droplets. To accelerate staining, Herxheimer recommends using an alkaline solution (absolute alcohol 70.0, 10% solution of caustic soda 20.0, distilled water 10.0, S. III to saturation), which colors the preparation in 2–3 minutes.