Abstract: Diphenylamine has been found to be an excellent coupling agent for the identification of aromatic amines. Also, due to the facts that the coupling reaction can take place in an acidic medium and that the reagent itself is rather stable, the effect of hydrogen ion concentration and the change of redox potentials to the coloration can also be tested. The mechanism of the formation and the change of the coloration are discussed, the wrong conclusion appearing in the literature having been pointed out. This paper also deals with the properties of the produced dyes-derivatives of Tropaeolin ○○ type——as the acid-base, adsorption and redox indicator. From a practical point of view none of these compounds seems to have any distinct advantage over the existing indicators in acid-base titrations. Interesting is the behavior of them as an adsorption indicator. In direct titration of chloride in a weakly acidic medium, the property is similar to bromophenol blue and metanil yellow. It is concluded that in the field of oxidation-reduction indicators concerning the reversible color change in titrations with sodium nitrite in an acidic medium, the using of them as the internal indicator for diazotization titration warrants further study.