Carrier Antibiotics and Model Compounds as Components of Selective Ion-sensitive Electrodes

The ion selectivity exhibited by different types of membrane electrodes is rationalized by a theoretical treatment of the membrane potential. An equation, which is admirably suited for the interpolation of e.m.f.-values, is derived for the electrode response of membrane electrodes of the electrically neutral ligand type (antibiotics, model compounds) to mixed solutions of monovalent and divalent cations. In contrast to the classical Nicolsky equation it describes the contribution of interfering ions to the e.m.f. by an activity-independent selectivity term. Electrically neutral ligands selective for calcium ions to be used as components for liquid membrane electrodes were designed and prepared. The corresponding electrodes are in all respects superior to the calcium ion liquid membrane electrodes described so far. In the past decade, a great number of selective ion-sensitive electrodes have been designed and have found application in all branches of routine analysis'4. Usually, a membrane electrode assembly (shown schematically in Figure 1) is employed. The e.m.f. of this electrochemical cell is related to the ion activities in the sample solution by the extended Nicolsky equation E = E0 + 'ln [a1 + E Kt?t(a/'J ] (1) z1F E: cell potential (e.m.f.) E0: constant reference potential a1: activity of a primary ion ?j in the sample solution a: activity of an interfering ion JZj in the sample solution KtPt: selectivity factor, characteristic of a given membrane RT/F: Nernst factor The theoretical treatment of the membrane potentialt has led to an understanding of the ion specificity exhibited by different electrode types, the t The so-called membrane potential is defined as the electrical potential difference between the two outside solutions (Figure 1) and may be broken down into a boundary potential and a diffusion potential, i.e. internal( membrane potential. For all practical purposes, the membrane potential can be regarded as the only activity-dependent contribution to the cell potential.