Contribution of the diffusion potential to the membrane potential and to the ion-selective electrode response

A generalized quasi-thermodynamic theory of the solvent polymeric ion-selec tive membranes containing ion-exchanging sites (charged carrier) and a co-e xchanger (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 laye r. A computer simulation of the potentiometric signal and of its selectivity i s fulfilled. A surprising result of the simulations is that though the valu e of diffusion potential achieves sometimes several tens of mV, it is almos t the same for different profiles assumed. If being profile-independent, th e diffusion potential occurs also rime-independent. Therefore the usually f ast response of ISEs actually does not mean that only boundary potentials m atter. The variability of selectivity coefficients to a large extent is als o caused by diffusion potential, Artificial elimination of the diffusion potential with ionic additives to m embranes based on calcium dioctylphenylphosphate allows one to eliminate th e peculiarities like Ca/pH potential dips and also to stabilize selectivity coefficients.