Influence of Anions on Electrochemical Properties of Polypyrrole-Modified Electrodes

The influence of anions ClO₄^–, NO₃^–, Cl^–, SO₄^2–, and DDS^– (dodecyl sulfate) on the cyclic voltammetric response of polypyrrole-modified electrodes is studied. The change in the film composition is examined by electron probe microanalysis. It is established that essential changes in the shape of voltammograms take place during cycling if the anions are not sufficiently freely mobile in the polymer film and insertion of cations from the solution is necessary to guarantee electroneutrality of the system. Some differences between the mobility of Cl^– ions and ClO₄^– or NO₃^– ions are in good agreement with the results of semi-empirical quantum chemical calculations showing that the interaction of Cl^– and Br^– ions with pyrrole oligomers is stronger than that of NO₃^– or ClO₄^– ions. Nevertheless, it is established that the peak current determined from voltammograms increases linearly with the increase of the scan rate with very high correlation coefficient. It means that it is possible to describe the behavior of ClO₄^–, NO₃^– and Cl^– ions in the framework of the model of free ions. The redox behavior of the PPy films doped with anions of low mobility such as SO₄^2– and DDS^– depends essentially on the nature of cations in the test solution. It is found that the mobility of cations increases in the row Li⁺ < Na⁺ < K⁺ < Cs⁺. The mobility of DDS^– ions in the PPy in ethanolic solution is significantly higher and their electrochemical properties are quite similar to PPy|Cl or NO₃^– film in aqueous solution.