The Ion Exchange Membranes

Ion exchange membranes are the key components in electrodialysis. They consist of highly swollen gel-type polymer structures carrying fixed positive or negative charges. Polymer structures carrying negatively charged groups are referred to as cation exchange membranes, while those carrying positively charged groups are referred to as anion exchange membranes.

In a cation exchange membrane, the fixed negative charges are in electrical equilibrium with mobile cations in the interstices of the polymer. Figure 2 shows a cation exchange membrane with fixed anions and mobile cations; the latter are referred to as counter ions. The mobile anions, called co-ions, are more or less completely excluded from the polymer matrix because their electrical charge is identical to that of the fixed ions. Because of the exclusion of the co-ions, cation exchange membranes are preferentially permeable for cations. Anion exchange membranes which carry positive fixed charges and exclude cations are preferentially permeable to anions. The extent to which co-ions are excluded from an ion exchange membrane depends on the membrane as well as on the solution properties.

The most desirable properties for ion exchange membranes are:

• High permselectivity - the membrane should be permeable to counter-ions only

• Low electrical resistance - the membrane should have high counter ion permeability

• Good mechanical and form stability - the membrane should be mechanically strong and should have a low degree of swelling in diluate solutions

• High chemical stability - the membrane should be stable over the entire pH range and in the presence of oxidizing agents and organic solvents.

The properties of ion exchange membranes are determined by the base polymer and the type and concentration of the fixed charges. The base polymer determines the mechanical, chemical and thermal

Figure 2 Schematic drawing illustrating the structure of a cation exchange membrane.

stability of the membrane. The type and concentration of the fixed ions determine the permselectivity and the electrical resistance. The moieties often used as fixed charges are -SO3 and -COO- groups in cation exchange membranes and -R3N+ and -R2NH + groups in anion exchange membranes. The sulfonic acid group -SO33 is completely dissociated over the entire pH range, while the carboxylic acid group -COO3 is virtually undissociated in the pH range < 3. The quaternary ammonium group -R3N + again is completely dissociated over the entire pH range, while the tertiary ammonium group -R2NH+ is only weakly dissociated. Accordingly, ion exchange membranes are referred to as weakly or strongly acidic or basic in character depending on the charged groups they contain.

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