AbstractAttenuated total reflectance infrared spectroscopy (ATR‐IR) has been used to study the cation permselectivity of liquid‐membrane ion‐selective electrodes (ISEs). All spectroscopic evidence was found to agree with the interpretation of permselectivity as being due to complete exclusion of counter‐ions from the boundary phase. Compared to ATR‐IR, optical second harmonic generation (SHG) has an even more pronounced surface sensitivity. It has been demonstrated to be a promising new technique for the study of ISE response mechanisms, as it allows one to observe the actual phase boundary. On the other hand, further potentiometric evidence for the influence of ionic sites on the emf response was found, both by a study of membranes free of ionic sites as well as by the investigation of leaching processes at poly(viny1 chloride) membranes. Finally, the use of a photoresponsive crown ether ionophore allowed the determination ofemfresponse characteristics in relation to the ionophore concentration without any changes in the membrane co
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