Titrimetric Determination of N,N′-Disubstituted p-Phenylenediamines

Three types of p-phenylenediamine derivatives used extensively as antioxidants and antiozonants in rubber compounds are N,N′-di-sec-alkyl-, N-sec-alkyl-N′-aryl-, and N,N′-diaryl-p-phenylenediamines. Available methods for these compounds are based mainly on a colorimetric determination of their colored oxidation products, the Wurster's salts or the p-quinonediimines. The deep coloration of the Wurster's salts was used as the basis of a spot test for the rapid detection of these diamines in rubber compounds. This test permits one to differentiate the three types of p-phenylenediamines. An investigation of the neutralization of p-phenylenediamines in nonaqueous solvents showed that N,N′-di-sec-alkyl- and N-sec-alkyl-N′-aryl-p-phenylenediamines could be determined readily in this way. The N,N′-diaryl derivatives did not give satisfactory titration curves because of their low basicity. By the addition of chloranil, however, diaryl-p-phenylenediamines were oxidized to the Wurster's salts with the formation of one equivalent of base which could be neutralized by acid. A similar procedure was applicable to the determination of N,N′-diaryl-p-quinonediimines, or mixtures of these diamines and diimines.