Abstract: Compounds of the formula ##STR1## in which R.sub.1 signifies an optionally substituted hydrocarbon radical,R.sub.2 signifies hydrogen, halogen, nitro, a linear or branched optionally substituted alkyl radical such as haloalkyl, an optionally substituted cycloalkyl radical, alkoxy, alkylthio, an optionally substituted aryl radical, an acyl radical of an aliphatic or aromatic sulphonic or carboxylic acid, an ester or amide group of a sulphonic acid, an ester or amide group of a carboxylic acid, an ester group of a carbamic acid, unsubstituted or mono- or disubstituted amino or an acylamino group,The ring A may be further substituted, and may as R.sub.2 bear a condensed cycloalkyl radical, but ring A and any substituent on it is free from carboxylic or sulphonic acid groups and from groups in the ortho-position to the azo group which are capable of the formation of metal complexes,Provided that the total number of carbon atoms in R.sub.