Amphoteric ion-permeable composite membrane

An amphoteric ion-permeable composite membrane having an NaCl and a sucrose rejection having the relation defined by the following equation:

• 2.times.(sucrose rejection in %)-(NaCl rejection in %).gtoreq.150
• and an NaCl rejection of at most 40% and a sucrose rejection of at least 80%, said member composed of a microporous substrate, and supported on the substrate, a thin amphoteric ion-exchange film formed of an active amino group-containing polymer which has been interfacially crosslinked at least at the surface portion of the thin film, said membrane being produced by forming a thin layer comprising an active amino group-containing polymer containing 1.0 to 23 milliequivalents/g, as an amino equivalent, of an active amino group selected from primary and secondary amino groups and 0 to 18 milliequivalents/g, as an amino equivalent, of a tertiary amino group and/or an ammonium salt group per molecule with the total amino content thereof being in the range of 2.0 to 23 milliequivalents/g on the microporous substrate and then contacting the thin layer on the substrate interfacially with a polyfunctional aromatic compound to interfacially crosslink the thin layer, said polyfunction aromatic compound containing at least two functional groups selected from carbonyl halide groups, sulfonyl halide groups, carboxylic acid anhydride groups, sulfonic acid anhydride groups and derivative groups of carboxylic acids and sulfonic acids having equivalent reactivity to the aforesaid halide and anhydride groups;
• the interfacially crosslinked polymer zone of the thin film containing at least one structural unit of the formula ##STR88## wherein Ar represents an aromatic ring, Y represents CO or SO.sub.2 provided that (m+n+1) group Y bonded to the group Ar are identical or different, M represents an atom or atomic grouping capable of being split off as a cation, and m and n each represent an integer of 0 or more provided that m+n=1-3, said polymer zone further containing a structural unit of formula (I) in which m is not zero when it contains a structural unit of formula (I) in which m is zero, and further containing a structural unit of formula (I) in which n is not zero when it contains a structural unit of formula (I) in which n is zero, and additionally containing at least 0.5 milliequivalents/g, based on the dry crosslinked polymer, of an ammonium salt group as an anion-exchange group and at least 0.05 milliequivalents/g of the dry crosslinked polymer of the group -YOM in formula (I) as a cation-exchange group, the equivalent ratio of the ammonium salt group to the group -YOM being in the range of from 20:1 to 1:2.