1 INTRODUCTIONRapid and precise methods to obtain the diffusion coefficients of counter-ions are im-portant for the characterization of ion exchange membranes.Many theoreticaldescriptions of ion transport in ion exchange membranes have been developed by usingthe principles of irreversible thermodynamics,or the Nernst-Planck equations.Fick’s law can also be used for the description of the transport of ions with equaldiffusivity.However,for counter-ions of different diffusivities,Nerst-Planck
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