Influence of a Classical Chelate Effect due to a Covalent Linkage of Ionophore Subunits on the Performance of Corresponding Ion Selective Electrodes

Abstract

To investigate the influence of a covalent interconnection of ionophore subunits (e.g. to obtain so-called bis(crown) compounds) on the ion selectivity of corresponding electrodes, a theoretical treatment of the underlying chelate effect is presented. Entropic contributions given by standard-state and statistical terms, as well as enthalpic contributions resulting from a ligand preorganization, are discussed. The latter contributions are shown to reflect the intrinsic chelate effect, as redefined by Frausto da Silva, and to be decisive for a possible enhancement of the potentiometric selectivity of ion selective electrodes based on such ionophores. An experimental study of the ion selectivities as well as of the alkali-ion complex stabilities of covalently bridged and non-bridged ligands with Li⁺ selectivity uncovers no significant effects that could be attributed to a preorganization of these ligands.