Diffusion potential is induced by coupled transport of ions through liquid membrane: nonlinear response to pH change in a reverse permeation system

Abstract

In a cation exchange liquid membrane-aqueous alkali metal chloride system, diffusional flux of alkali metal ion driven by proton was observed. A supported liquid membrane formed on a Teflon filter by impregnating it with stearic acid-doped 1-octanol was used. The internal aqueous phase contained KCl and HCl, and the external aqueous phase also contained KCl. The initial concentrations of K + ions of both phases were 1×10 −1 mol dm −3 for all the measurements. The concentration of HCl in the internal solution was kept at 1×10 −2 mol dm −3. The pH of the external solution was changed successively with HCl, appropriate buffer solution, or KOH. The pH dependence of membrane potential showed hysteresis loop in the range from neutral to alkaline pH, where reverse ion permeation was observed after the flux had been measured in the system with the external solution of an alkaline pH (pH 13). In the acidic range below neutral pH, the hysteresis of the membrane potential as well as reverse ion permeation was not observed. To elucidate the correlation between the appearance of hysteresis loop and the reverse ion permeation driven by proton across the membrane, the time course of the membrane potential in response to pH change was investigated. In the pH range where reverse permeation phenomena appeared, the time dependence of the membrane potential in nonsteady-state showed biphasic behavior. From the time course curve of the membrane potential, the total membrane potential was divided into the Donnan potential and the diffusion potential. From these findings, it was demonstrated that the diffusion potential was generated within the membrane only in the alkaline range where reverse ion permeation occurred. Analyzing the diffusional flux, the diffusion coefficient of potassium ion in the membrane was obtained taking the Donnan potential into account to be much greater than that in the membrane solvent. As a result of comparison of the diffusional fluxes measured by atomic absorption spectrometry and solution conductometry, the flux of the potassium ion was found to be significantly greater than that of the hydrogen ion in the opposite direction, especially at extremely high pH region. This implies the flows of hydroxide ions and neutralization reaction within the membrane facilitate the reverse ion permeation process of potassium ions.