with diazonium salts of the general formula: ##STR5## wherein R3 is an unsubstituted phenyl radical or a phenyl radical substituted in the 2-position by a carboxyl group and A'??? is a conventional counterion, by alkaline coupling into formazanes of the general formula: ##STR6## wherein R and R2' have the meanings given in general formula (II) and R3' has the meaning given in general formula (III)