Potentiometric selectivity of p-doped polymer films

It was observed previously that anion-exchanging polymer films doped with common anions such as Cl-, NO3- are rather inselective. The reasons for observed lack of selectivity are still obscured. In this paper, the potentiometric selectivity of polymer films is discussed. This interpretation is based on the previously presented model for potentiometric responses of p-doped polymer layers, and on the theory of selectivity of solid state ion-selective electrodes. Both the differences and similarities between the two systems are indicated. The equations given are used for the calculations of selectivity coefficients for polypyrrole films in presence of nitrate, chloride and thiocyanate ions. In the absence of a diffusion potential inside the membrane, the observed selectivity coefficients can be interpreted in terms of the formation of hypothetical sparingly soluble salts between polymer solution anions. It was shown that this applies to the nitrate interference on PPyCl membranes. This system is analogous to bromide interference on an AgCl membrane. According to the theoretical consideration, the K-Cl,NO3(poly) was equal, within the range of experimental error, to 1/K-NO3,Cl(poly). Although the system is modelled with idealising assumptions the experimental results are in good accordance with prediction of the theory. (C) 2000 Elsevier Science B.V. All rights reserved.