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Segmented sandwich membrane method of studying stoichiometry and stability constants of ion-ionophore complexes in ion-selective membranes is considered in detail. The experimental data (reported earlier in Russian) concerning complexes of various ions with valinomycin, with H+-selective neutral ionophore hexabutyltriamidophosphate, and with anion-binding neutral ionophore p-hexyl trifluoroacetylbenzoate is presented in a compact form. Advantages of titration technique in the sandwich membrane method (the presence of an internal criterion of reliability, and the possibility of direct determination of complex stoichiometry coefficients) are specially addressed. Biases of the estimates of the constants caused by ion-pair formation in real membranes and by diffusion potential are analyzed by means of computer simulations. The possibility of revealing two coexisting complexes with different compositions is also discussed.

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A generalized quasi‐thermodynamic theory of the solvent polymeric ion‐selective membranes containing ion‐exchanging sites (charged carrier) and a co‐exchanger (ionic additives) and in contact with two mixed aqueous solutions of electrolytes IX2 and JX is developed. To account for diffusion potential it is proposed to assume various profiles of species in the diffusion layer. A computer simulation of the potentiometric signal and of its selectivity is fulfilled. A surprising result of the simulations is that though the value of diffusion potential achieves sometimes several tens of mV, it is almost the same for different profiles assumed. If being profile‐independent, the diffusion potential occurs also time‐independent. Therefore the usually fast response of ISEs actually does not mean that only boundary potentials matter. The variability of selectivity coefficients to a large extent is also caused by diffusion potential. Artificial elimination of the diffusion potential with ionic additives to membranes based on calcium dioctylphenylphosphate allows one to eliminate the peculiarities like Ca/pH potential dips and also to stabilize selectivity coefficients.

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