Basic ion transport characteristics of a homogeneous amphoteric ion exchange membrane (Kanegafuchi Chem. Ind. 1.0-PA-29) were studied by measuring the membrane potentials and conductances. Concentration cell systems of NaCl, KCl, CaCl2, and SrCl2 with a concentration range of 10−1–10−3 mol dm−3 were studied and the electroconductive membrane permeabilities were estimated from the electrochemical data. Generally, Cl− is slightly more permeable than cations under the present experimental conditions, except the result that K+ is more permeable at low concentrations. Unlike the usual uniform cation exchange membrane of which transport characteristics to ions strongly depend upon the number of ionic charges, a “1.0-PA-29” amphoteric membrane exhibits different transport properties for the NaCl and KCl systems. In the NaCl system, the membrane was constantly anionic selective, while in the KCl system, biphasic ion selectivities were observed depending upon the external KCl concentration. Fundamental electrochemical properties of the membrane systems were discussed by examining the dependencies of membrane permeability on external electrolyte concentrations and on differences in ionic species. The present result suggests that the ionic size is one of the critical factors determining the transport characteristics across amphoteric ion exchange membrane.
Transport properties of a cation-exchange membrane-aqueous sodium chloride solution system have been studied. The membrane used was a sulfonated styrene–divinyl benzene copolymer, Neosepta C66-5T. The salt concentration was kept at 10−1 mol dm−3 on one side of the membrane and varied from 10−3 to 10−2 mol dm−3 on the other. The six elements of a conductance matrix have been experimentally determined according to the previous theory and the membrane properties were discussed in terms of conductance elements. All the elements of the conductance matrix were positive within the concentration range studied. This would be characteristic of a cation-exchange membrane. The electrical ion conductance was greater than the diffusional ion conductance by a factor of 10 to 100. The ion-solvent coupling coefficient was much less than the interionic one, although the electroosmotic conductance was ten-times greater than the membrane conductance. The reflection coefficient was close to unity. The electroosmotic properties were also discussed in terms of the conductance matrix.
A theory for the ion and solvent transports through membrane has been developed on the basis of nonequilibrium thermodynamics. The fluxes were represented as linear functions of the effective driving potentials. The theory is reduced to that of Katchalsky when the concentration ratio of two aqueous solutions is near unity and the solutions are dilute. The effect of solvent flow on the interionic correlations was discussed. It was pointed out that Despic and Hills’ theory of the electroosmotic effect on the membrane conductance is not valid with respect to both theoretical and experimental aspects. The comparison of the present theory with the previous one was also made. The six independent phenomenological coefficients (elements of the conductance matrix) were experimentally determined as a function of concentration with the concentration cell consisting of the amphoteric ion-exchange membrane and aqueous calcium chloride solutions. The membrane properties were discussed in terms of the conductance matrix. The diffusional ion conductance was much greater than the electrical ion conductance, as an indication of the failure of the Nernst-Einstein relation. The cation-anion coupling coefficient was close to unity whereas the ion-water coupling coefficients were less than 0.1. The solvent effects on the interionic conductance elements were found to be small in spite of the fact that the water permeability was extremely high. The distinction between the cationic and amphoteric ion-exchange membranes was explored in terms of the elements of the conductance matrix.
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