The term azo compounds involves the compounds of general formula R-N=N-R´ where the nitrogen atoms of the azo group N=N are bound with carbon atoms of the groups R and R´. From among the large variety of such combinations practically important are those in which both groups (R, R´) or at least one of them have aromatic character. Extraordinarily important are compounds having hydroxy and/or amino groups in positions conjugated with the azo group since they are intensively coloured, can be widely modified to obtain various physico-chemical properties, and are relatively easily available.