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 Original Research Article

The potentials, EISE, and selectivity coefficients, Kpotij, at an ion selective electrode (ISE) of liquid membrane type were related quantitatively to distribution ratios, D, of the primary ion, iz, and the interfering ion, jz, between an aqueous, W, and an organic, O, solutions, taking into account that all the EISE, Kpotij and D terms could be expressed physicochemically by using standard Galvani potential differences, E0, (or standard free energies, ΔG0tr) for transfers of iz and jz from water, W, to the ISE membrane, M, or organic solution, O, stability constants, Kst, of complexes of iz and jz with an ionophore in M or O and formation constants of ion-pairs, Kip, of iz- and jz-species with coexisting counter ions in M or O. The equations theoretically derived for Kpotij indicated that the Kpotij in Nicolsky–Eisenman equation was a simple function of D in iz and jz when Kpotijat 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 Kpotij at liquid membrane type ISE.