ION-EXCHANGE DIFFUSION IN ELECTROMEMBRANES AND ITS DESCRIPTION USING THE MAXWELL-STEFAN FORMALISM

Exchange diffusion experiments of the exchanging ion pairs Cl-/Br- and Cl-/SO42- in an anion exchange membrane and of Na+/H+ in three cation exchange membranes have been carried out using a dialysis cell. The t ime dependent concentrations in the cell are measured with precision d ensimetry based on the principle of the vibrating tube. Thermodynamic ion exchange equilibria between the membrane phase and the electrolyte solution have also been measured for the three systems Cl-/Br-, Cl-/S O42-, and Na+/H+ using atomic absorption spectrometry and ion chromato graphy as analytical methods. The experimental data of exchange diffus ion and exchange equilibria are used to study the diffusion velocity o f the mobile ions in the membrane on the basis of the Maxwell-Stefan f ormalism of multicomponent diffusion processes. In frame of the soluti on-diffusion model equations are derived for the ion exchange diffusio n process which allow to determine diffusion coefficients of the excha nging ions in the membrane and which account also for frictional effec ts between the ions expressed by a coupling diffusion coefficient. The method developed can generally be applied to study transport processe s in electromembrane systems. (C) 1997 Elsevier Science B.V.