Novel cesium-selective electrodes based on lipophilic 1,3-bisbridged cofacial-calix[6]crowns

Five bisbridged calix[6]crowns have been investigated as Cs+ ionophore in PVC membrane electrodes. As ionophores, three 1,3-bisbridged calix[6]crown-4-ethers(I -III), 1,3-bisbridged calix[6]crown-5-ether(IV), and 1,3-bisbridged calix[6]crown-6-ether(V) have been evaluated. The membranes all give good Nernstian response in the concentration range from 1 x 10(-7) to 1 x 10(-1) M of cesium ion. The best detection limits (-log a(cs+) = 7.08-7.36) are obtained for electrode membranes containing 1,3-bisbridged cofacial-calix[6]crown-4-ethers(I-III), and the values are the lowest compared with those reported previously. The highest selectivity coefficients [pK(Cs+j)(pot) = 5.53(Cs/Na), 3.74(Cs/K), 2.63(Cs/Rb)] are obtained for the membrane of 1,3-bisbridged calix[6]crown-4-ether(II), and these values are also the highest compared with previous reports for Cs+-ISEs. The highest selectivity towards cesium ion is attributed to the geometrically cofacial positions of two crown-ethers in calix[6]crowns in order to provide the complex of cesium ion and eight oxygens of cofacial crowns.