Study of the Behavior of Calix[4]arene-Based Sodium-Selective Electrodes by Means of ANOVA

Two ethylamide and two butyl ester calix[l]arene ionophores differing by the presence of a tert-butyl group in the upper lipophilic rim were used to construct liquid membrane ISEs with two solvents (dibutyl sebacate and dibutyl phthalate). Their behavior toward sodium ion was tested in terms of detection limit (DL), sensitivity (S, mV/pNa), and selectivity toward protons and alkali and alkaline earth metal ions. In order to clarify the role of the structural parameters in the behavior of the ionophores, each of these response variables was tested with a multifactor ANOVA with interactions, using four classification factors: (i) nature of the polar part of the lower rim, (ii) presence of the tert-butyl group in the upper rim, (iii) dielectric constant of the organic solvent, and, for selectivity, (iv) nature of the interfering ion. It was found that amides show a higher selectivity toward other alkali metal ions and a lower DL and S than the corresponding esters. Furthermore, the presence of the tert-butyl group in the upper rim increases both S and the selectivity toward alkali metals and decreases the DL, especially for esters; however, for calcium ion, it increases the selectivity of amide membranes and decreases that of the ester membranes. The major effects of a higher dielectric constant of the solvent are decreases in the selectivity toward alkaline earth ions and the DL and S of amide calixarenes. Hypotheses are proposed for the interpretation of the data.