Azo Dyes

AZO DYES, the name for dyes that contain the -N=N- group in their composition. They are formed by the combination of diazotized primary aromatic amines with aromatic alcohols, phenols, or amines. Azo dyes of the simplest composition are constructed according to the type R-N=N-R-OH or R-N=N-R-NH₂. Azo dyes find wide application in the textile industry; they are used to dye and print cotton, wool, and silk yarns and fabrics. The azo dyes used for dyeing cotton fabrics are insoluble in water and therefore cannot be used directly for dyeing. On the fiber, however, they produce coloration through the combination of their two constituent parts. In connection with this, dyeing is carried out in two phases: 1) impregnation with phenol or naphthol, 2) treatment with an unstable diazo compound, which is prepared right there at the factory. To form the diazo compound, an amine (e.g., paranitraniline) is taken and treated in the cold with nitrous acid (diazotized). Nitrous acid is formed by the interaction of its salt, called a nitrite, with a mineral acid (for example, HCl). A diazo solution is prepared (e.g., for a red dye) as follows: paranitraniline is stirred in hot water (66% sulfuric acid is added for dissolution) and cooled with ice. At the same time, nitrite is stirred with ice and water in a tub, and both solutions are gradually mixed. Upon completion of the reaction, the mixture is passed through a sieve. Then it is stored in an earthenware jar surrounded by ice. The fabric to be dyed is first impregnated on a padding machine with a naphtholate solution, dried, and passed through the padding machine again for impregnation with the diazo solution. The dyed fabric then goes to be washed. In a sanitary-hygienic sense, both azo dyes and the dyeing of fabrics with them are of interest in the sense that during work—mainly during the preparation of the diazo solution—the release and volatilization of nitrogen oxides into the air occurs (see Nitrogen). The preparation of diazo solutions is usually carried out in dyeing factories in very poorly maintained, primitively equipped premises; ventilation systems are either completely absent or are irrationally designed. During chemical analysis of the air in these premises at a number of our dyeing factories, significant amounts of nitric acid were detected, expressed in tenths of a milligram per liter of air. Poisoning by other ingredients included in the composition of azo dyes, for example, aniline, is also possible. Preventive measures: isolation of the room for the preparation of diazo solutions, mechanization of the process, equipment with proper exhaust ventilation; in dye works—installation of supply and exhaust ventilation.

N. Rovenbaum.

Of the azo dyes in microtechnique, the following are especially commonly used: Chrysoidin (2-amido-azo-benzene)—a basic red-brown dye; Bismarck brown (3-amido-azobenzene)—a basic dark brown dye (for vital staining). In histology, oxyazo dyes are used as plasma stains—the yellow Orange G and the red Bordeaux; as a reagent for fat—Sudan III.