AbstractWe elucidated the effect of the molecular weight of poly(N‐methyl‐4‐vinylpyridinium chloride) (PMVP) on its adsorbed or ion‐exchanged amount on the cation‐exchange membrane and the preferntial permselectivity of the membrane for sodium ion to calcium ion. The amount of the adsorbed or ion‐exchanged PMVP increases with time and then attains to a definite value at a definite concentration. Also, the ultimate value increases with increasing the concentration of PMVP. When the molecular weight of PMVP is 3090, the ratio of the pyridinium groups of PMVP ion‐exchanged with the sulfonic acid groups on the membrane surface to the total pyridinium groups of the cohered PMVP is in the range of 70–80. The relative transport number of calcium ions to sodium ions(PNaCa) decreases with increasing the amount of adsorbed or ion‐exchanged PMVP and the numbe rof non‐ion‐exchanged pyridinium groups of PMVP and the number of non‐ion‐exchanged pyridinium groups of PMVP on the membrane surface. The smaller the molecular weight of PMVP, the more the adsorbed or ion‐exchanged amount and the more preferable the per
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