EFFECT OF SIDE-ARM VARIATION IN DIBENZO-16-CROWN-5 COMPOUNDS ON THE POTENTIOMETRIC SELECTIVITY FOR SODIUM-ION

The influence of attaching one or two pendent groups to the central ca rbon of the three-carbon bridge in dibenzo-16-crown-5 upon the potenti ometric response of the ionophore to Li+, Na+, and Kt in solvent polym eric membranes has been assessed. Side-arm variation includes alkyl, O CH3, OCH2CO2H, OCH2-CO2C2H5, and OCH2C(O)N(C2H5)(2) units. The Na+/Kselectivity of the electrodes is increased by enhanced oxygen basicity in the sidearm of the ionophore. Attachment of an alkyl group to the polyether ring carbon, which bears an oxygen-containing sidearm, produ ces a substantial increase in the Na+/K+ selectivity due to preorganiz ation of the binding site. The Na+/Li+ selectivity responds to side-ar m variation within the ionophore quite differently than does the Na+/K + selectivity.