Preconcetration and voltammetric behavior of chromium(VI) at Pt electrodes modified with poly(4-vinylpyridine)

Abstract

The electrochemical behavior of Cr(VI) incorporated into a polymer film is complicated by the irreversibility of the Cr(VI)/Cr(III) couple and the tendency for Cr(III) to form a passivating layer on surfaces. The former problem requires that in cyclic experiments the Cr(VI) concentration in the polymer must be sustained by diffusion from the solution phase; hence, the polymer film must be thinner than in investigations of reversible couples. Such films of poly(4-vinylpyridine) can be prepared by adsorption onto a Pt substrate. After protonation of the film, Cr(VI) was incorporated by an ion-exchange mechanism. Passivation was prevented by performing the cyclic voltammetric experiments in an electrolyte of 0.15 M NaF buffered at pH 3.5. Apparently, labile Cr(III)-fluoride complexes were formed that allowed ejection of almost all of Cr(III) from the film during positive-going potential excursions and at the positive limit of the interrupted cyclic experiments. At high Cr(VI) concentrations, steady state cyclic voltammograms were bulk diffusion-controlled over the range 10 to 1000 mV s −1, whereas at low concentrations (10 −6 M) the cathodic peaks had the characteristic of the reduction of a surface layer. That the described system permitted the study of an irreversible system at an ionomer-coated electrode is important to the development of methodology in electroanalytical chemistry since it may permit an analog of stripping voltammetry to be performed on irreversible couples.