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

Potentiometric selectivities of 11 dibenzo-16-crown-5 compounds for al kali metal, alkaline earth metal, and ammonium ions have been determin ed in soh ent polymeric membrane electrodes. The ionophores bear one o r 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. Attach ment of a propyl group to the ring carbon that bears an extended, oxyg en-containing side arm increases the selectivity for Na+ relative to l arger alkali metal and alkaline earth metal cations. For a given side arm, a linear relationship is obtained when the enhancement in Na+ sel ectivity produced by attachment of a geminal propyl group is plotted a gainst the diameter of the interference ion. Potentiometric responses of the dibenzo-16-crown-5 compounds are rationalized in terms of the c rown ether ring size and the oxygen basicity, conformational positioni ng, and rigidity of the side arm.