ALKALI-METAL, ALKALINE-EARTH METAL, AND AMMONIUM ION SELECTIVITIES OF DIBENZO-16-CROWN-5 COMPOUNDS WITH FUNCTIONAL SIDE ARMS IN ION-SELECTIVE ELECTRODES

Potentiometric selectivities of 11 dibenzo-16-crown-5 compounds for alkali metal, alkaline earth metal, and ammonium ions have been determined in soh ent polymeric membrane electrodes. The ionophores bear one or two pendent groups on the central carbon of the three-carbon bridge in the polyether ring. Side-arm variation includes OCH3, OCH2CH2OCH3, OCH2CO2C2H5, OCH2C(O)N(C2H5)(2), and OCH2C(O)N(C5H11)(2) units. Attachment of a propyl group to the ring carbon that bears an extended, oxygen-containing side arm increases the selectivity for Na+ relative to larger alkali metal and alkaline earth metal cations. For a given side arm, a linear relationship is obtained when the enhancement in Na+ selectivity produced by attachment of a geminal propyl group is plotted against the diameter of the interference ion. Potentiometric responses of the dibenzo-16-crown-5 compounds are rationalized in terms of the crown ether ring size and the oxygen basicity, conformational positioning, and rigidity of the side arm.