Calix [4]pyrrole liquid membrane electrodes display a potentiometric response towards the neutral form of phenol derivatives. They do not respond towards the corresponding anionic forms. The mechanism of the generation of the potentiometric signal relies on: (1) The creation of supramolecular complex calix[4]pyrroleÀnitrophenol at the organic/aqueous interface; (2) The increasing of the acidity of OH group from nitrophenol molecules due to formation of hydrogen bond between calix[4]pyrroleÀnitrophenol at the interface; (3) The dissociation of the OH group from calix[4]pyrroleÀnitrophenol complex and the ejection of H þ to the aqueous layer adjusted to the membrane organic phase boundary. The sensitivity and selectivity of the calix[4]pyrrole liquid membrane electrodes are governed by the lipophilicity and acidity of the phenolic guests. Also, the affinity to create a hydrogen bond between the host and the target molecule is a parameter which has a strong influence on the phenomena studied.

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This work presents a study on the generation of an ''anionic'' potentiometric response of a liquid polymeric membrane based on calix[4]pyrrole towards neutral chloro-and fluorophenol isomers. The results show that a calix[4]pyrrole membrane is able to recognize chloro and fluoro derivatives of phenols, and discriminate between ortho-, meta-, and para-isomers of chlorophenol. The mechanism of molecular ANALYTICAL LETTERS recognition relies on the formation of a supramolecular complex between a phenolic guest and the calix[4]pyrrole host occurring at the organic/aqueous interface, which leads to a change of charge separation. The sensitivity and selectivity of the sensor studied depend on the acidity and lipophilicity of the phenolic analytes. The selected matrix effect on the potentiometric measurements has been checked.

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