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Ion-selective electrodes (ISE) based on conventional plasticized PVC membranes with solid inner contact (SC) have so far had unsatisfactory lower detection limits. Here it is shown that electropolymerization of pyrrole in the presence of potassium hexacyanoferrate(II)/(III) on Pt results in adequate inner contact for SC-ISE. The nanomolar lower detection limit achieved with the Pb 2+ -selective PVC membranes investigated is comparable with values obtained with optimized internal solutions of liquid-contact ISE.

The potentiometric response behavior of Ca 2þ -selective poly(vinyl chloride) (PVC) and polyurethane (PU) membranes with different inner contacts has been compared. Evidence for the formation of a water film between membrane and internal contact and, hence, a less than optimal lower detection limit have been found for membranes directly in contact with bare Au or with polypyrrole generated by electropolymerization in the presence of KCl. A significantly better behavior is shown by membranes with polypyrrole prepared in the presence of potassium hexacyanoferrate. Best performances have been obtained with solvent-cast poly(3-octylthiophene) as the internal contacting layer. As compared with the PVC membranes, those with PU had a significantly worse performance throughout.

From electrochemical measurements at the interface of two immiscible electrolytes, the current at controlled potential is usually a linear function of the ion concentration in the aqueous phase. Surprisingly, a linear relationship between the current and the logarithm of the sample ion activity is found for corresponding measurements on ion-selective electrode membranes. Here, a theoretical explanation for the apparent contradiction between the behavior of the two kinds of system is given. Experimental results obtained with conventional ion-selective PVC membranes as well as with membranes based on PVC free membrane matrices are presented.

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