Physicochemical understanding of the selectivity at an ion selective electrode of the liquid membrane type and relation between the selectivity and distribution ratios in the ion-pair extraction

The potentials, E-ISE, and selectivity coefficients, K-ij(pot), at an ion selective electrode (ISE) of liquid membrane type were related quantitatively to distribution ratios, D, of the primary ion, i(z), and the interfering ion, j(z), between an aqueous, W, and an organic, O, solutions, taking into account that all the E-ISE, K-ij(pot) and D terms could be expressed physicochemically by using standard Galvani potential differences, E-o, (or standard free energies, Delta G(tr)(o)) for transfers of i(z) and j(z) from water, W, to the ISE membrane, M, or organic solution, O, stability constants, K-st, of complexes of i(z) and j(z) with an ionophore in M or O and formation constants of ion-pairs, K-ip, of i(z)- and j(z)-species with coexisting counter ions in M or O. The equations theoretically derived for K-ij(pot) indicated that the K-ij(pot) in Nicolsky-Eisenman equation was a simple function of D in i(z) and j(z) when K-ij(pot) at an ISE was determined under the conditions recommended by IUPAC for the separate solution and the fixed solution methods. Though the derived equations were verified experimentally by employing a Na+-ISE composed of a nitrobenzene membrane containing dibenzo-18-crown-6 as an example, they are considered to be generally applicable to the estimation of K-ij(pot) at liquid membrane type ISE, (C) 1998 Elsevier Science B.V. All rights reserved.