Selectivity of calix[4]arene-crown-6 for cesium ion in ISE: effect of the conformation

A new series of calix-crowns were used in ion-selective electrodes with two solvents (dibutyl sebacate and o-nitrophenyl octyl ether). As ionophores, the 1,3-dimethoxy and the three different conformers of the 1,3-diisopropoxycalix[4]arene-crown-6 (i.e., 1,3-alternate, partial cone, and cone) were considered. Their behavior toward cesium ion was studied in terms of detection limit (DL), sensitivity (S, mV/pCs), and selectivity toward protons, ammonium, alkali, and alkaline earth metal ions. The lowest DL value was obtained for membranes containing the 1,3-diisopropoxy derivative in the 1,3-alternate conformation. To correlate the analytical results with the structural properties of the ligand and with the nature of the polymeric membrane, a multifactor ANOVA with interactions was carried out on selectivities toward univalent and divalent ions. As for the alkali metal ions, a highly significant negative correlation (p < 0.01) between the pK Cs/M pot values and the ionic radius was found; in addition, for the smaller ions H +, Li +, and Na +, the four ionophores showed higher differences in selectivity than for the larger ions K + and Rb + as well as for alkaline earth metal ions. Better results than those previously reported with other ligands in terms of DL (5 × 10 -7 M) and pK Cs/M pot (4.46) were found for the 1,3-diisopropoxycalix-[4]arene-crown-6 in the 1,3-alternate conformation.