Influence of galvanostatic polarizations on potential vs. logarithm of ion activity dependences of all-solid-state ionselective electrodes with conducting polymer ion-to-electron transducer was studied. As a model system K þ -sensor with polypyrrole solid contact and poly(vinyl chloride) based membrane containing valinomycin was chosen. The influence of the lipophilic salt included to the membrane composition was of special interest. Qualitatively different behavior was observed for higher (above 10 À4 M) and lower KCl concentration in the sample solution. In the former case the responses were stable in time. If the membrane contains hydrophilic cations, galvanostatic polarization causes a parallel shift of the linear plot: potential vs. logarithm of activity, owing to ohmic potential drop. On the other hand, in the presence of highly lipophilic cations in the membrane, for anodic polarization, the potential dependence on electrolyte concentration disappears due to ion exchange inhibition and/or possible extraction of electrolyte anions. In dilute solutions more pronounced influence of time and current polarity on recorded potentials was observed. Theoretical calculations show that the observed potential drifts can be essentially ascribed to concentration profile changes of K þ ions in the solution, next to the membrane surface. Spontaneous release of K þ ions, which can also affect the concentration profile, can be compensated by cathodic polarization, resulting in improved detection limit, compared to open circuit conditions.
Amperometric responses of all‐solid‐state ion‐selective electrodes, recorded under potentiostatic conditions, were studied on example of potassium‐selective sensors with polypyrrole solid contact, at potential corresponding to reduction of the solid contact material and accompanying transfer of potassium ions across the membrane. Selective and stable in time linear dependences of current vs. logarithm of analyte concentration were recorded, resulting from high membrane resistance and changing membrane potential. The influence of experimental parameters as applied potential or thickness of the membrane was discussed. Advantages of the amperometric mode compared to potentiometric one relate to possibility of tailoring analytical parameters (sensitivity, magnitude of the signal) as well as over one order of magnitude decrease of the detection limit. The latter effect is achieved due to externally forced incorporation of potassium ions from the solution to the membrane, compensating their spontaneous release to the sample solution. A method of experimental setup simplification was proposed, with application of two‐electrode system, which can be used in the absence of external polarization source. The required driving force for the current flow was assured by spontaneous oxidation process occurring at the second electrode, coupled with reduction of the solid contact material of the ion‐selective electrode. In this case also stable in time calibration plots can be recorded.
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