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The preparation of non-permselective membrane for the divalent cation was attempted by coating it with thin vinylon film on one side of a commercial cation-exchange membrane. When the coating surface of the cationexchange membrane was set on the concentration-compartment side (cathode side), the permselectivity coefficient of the calcium ion to the sodium ion was equal to that of untreated cation-exchange membrane and no effect of the selective treatment was recognized. However, when the coating sufrace was directed to the desalting-compartment side (anode side), an effect of the selective treatment was observed; in this case, the permeability of the calcium ion was more depressed, corresponding with the formalation degree of the selective-treatment membrane; for example, the permselectivity of the calcium ion for the membrane which was formalated for 180 min at 30°C decreased about one half compared with the untreated membrane. The electric resistance of the cation-exchange membrane increased upon the coating with the vinylon film, and especially the resistance of the calcium ionic-form membrane indicated a remarkable increase.

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