Cesium-ion selective electrodes based on calix[4]arene dibenzocrown ethers

Four calix[4]arene dibenzocrown ether compounds have been prepared and evaluated as Cs⁺-selective ligands in solvent polymeric membrane electrodes. The ionophores include 25,27-bis(1-propyloxy)calix[4]arene dibenzocrown-6 1, 25,27-bis(1-alkyloxy)calix[4]arene dibenzocrown-7s 2 and 3, and 25,27-bis(1- propyloxy)calix[4]arene dibenzocrown-8 4. For an ion-selective electrode (ISE) based on 1, the linear response concentration range is 1 x 10^-1 to 1 x 10^-6 M of Cs⁺. Potentiometric selectivities of ISEs based on 1-4 for Cs⁺ over other alkali metal cations, alkaline earth metal cations, and NH₄/⁺ have been assessed. For 1-ISE, a remarkably high Cs⁺/Na⁺ selectivity was observed, the selectivity coefficient (K(Cs,Na/Pot)) being ca. 10^-5. As the size of crown ether ring is enlarged from crown-6 (1) to crown-7 (2 and 3) to crown-8 (4), the Cs⁺ selectivity over other alkali metal cations, such as Na⁺ and K⁺, is reduced successively. Effects of membrane composition and pH in the aqueous solution upon the electrode properties are also discussed.