Einflusse der Protonierung und der Selbstinhibition im elektrochemischen Verhalten von Phenylhydroxylamin

Abstract

From a polarographic study of the reduction of phenylhydroxylamine, it was shown that the reducible form of the compound is not the uncharged molecule, but the mono- and diprotonated cations. Due to the kinetics of the first and second protonation reactions, respectively, taking place presumably as heterogeneous reactions with the depolarizer adsorbed at the electrode, the polarographic wave height decreases twice at two different pH regions with decreasing acidity. The value of the acid dissociation constant pKa(I) of the singly charged cation protonated at the nitrogen atom was found from absorption spectra to be 2.19. The second dissociation constant pKa(II) corresponding to the location of another proton at the oxygen atom was derived from the pH dependence of the polarographic half-wave potentials to be approximately 1.1 From the latter as well as from a logarithmic analysis of the currents it has been concluded that side reactions take place parallel with the reduction to aniline: benzidine, in strongly acid solutions, and hydrazobenzene, in weakly acid solutions, are generated through dimerization of the intermediate radicals, both compounds being strongly adsorbed and hindering the electrode process. The presence of hydrazobenzene at the electrode was proved directly. © 1970 Elsevier Sequoia S.A., Lausanne.