Cinétique d'échange d'ions. Influence de la sélectivité et effet de l'interdiffusion dans les films de Nernst. Application à l'échange d'ions H⁺/[Math] dans une membrane sulfonique

Faculté des Sciences et des Techniques, Laboratoire de Chimie Macromoléculaire, ERA 471, BP67, 76130 Mont Saint-Aignan, France.

Résumé

Nous présentons une analyse théorique de la cinétique de l'échange d'ions compensateurs dans une membrane quand le phénomène est contrôlé par les films de Nernst. Nous prenons en compte les potentiels d'interdiffusion dans les films et, par l'intermédiaire des isothermes d'échange, la sélectivité. La confrontation avec les résultats expérimentaux est tout à fait satisfaisante quand les solutions d'électrolytes ne sont pas trop concentrées. Ceci est illustré ici par l'échange d'ions H⁺/[Math] dans une membrane sulfonique initialement sous forme [Math], relativement hydrophile et de capacité assez faible.

Abstract

Film controlled ion-exchange rates have been considered. The influence of membrane selectivity and of interdiffusion potentials has been regrouped in a unified treatment, as an extension of the Adamson-Grossman and Schlogl-Helfferich treatments. The concentrations at the film-membrane interface have been expressed in function of membrane and bulk solution concentrations, the shape of ion-exchange isotherm and diffusive properties. Analytical solutions valid even with non constant selectivity coefficients were obtained by decomposing the isotherm into linear segments.

Calculations show that the initial exchange rate is little dependant on selectivity. However, if the membrane composition covers the whole isotherm during the exchange process, two different limiting rate laws are predictable according to the relative affinities of the entering and leaving exchanged ions toward the membrane.

When the affinity of the leaving ion is very high the exchange rate quickly declines but the contrary is true with high entering-ion affinity.

The rate at wich H⁺ ions are taken up by a sulfonic membrane initially in the [Math] form was experimentally studied. The hydrophilic membrane of slightly lower capacity than the most usual ion exchangers had an affinity slightly in favor of H⁺ ions. The kinetic behaviour of this system compared to the limiting laws is an intermediate one and agrees well with the theory. At low electrolyte concentrations the rate of exchange was proportional to this concentration. At higher concentrations (10^—1 N) the rate was too high to allow accurate measurements.