The key components in electrodialysis and related processes are the ionexchange membranes. There are three different types of ion-exchange membranes: (1) cation-exchange membranes, which contain negatively charged groups fixed to the polymer matrix, (2) anion-exchange membranes, which contain positively charged groups fixed to the polymer matrix, and (3) bipolar membranes, which are composed of an anion- and a cation-exchange layer laminated together. The fixed charges of an ionexchange membrane are in electrical equilibrium with mobile ions in the interstices of the polymer matrix as indicated in Fig. 1 which shows schematically the structure of a cation-exchange membrane with negative charges fixed to the polymer matrix. The most desired properties of ionexchange membranes are high permselectivity, low electrical resistance, good mechanical and form stability, and high chemical and thermal stability. In addition to these properties, bipolar membranes should have high catalytic water dissociation rates.
The properties of ion-exchange membranes are determined by two parameters, that is, the basic material they are made from and the type and concentration of the fixed ionic moiety. The basic material determines to a large extent the mechanical, chemical, and thermal stability of the membrane. The type and the concentration of the fixed ionic charges determine the permselectivity and the electrical resistance ofthe membrane, but they also have a significant effect on the mechanical properties of the membrane and their swelling in solution.
The following moieties are used as fixed charges in cation-exchange membranes:
In anion-exchange membranes, fixed charges may be the following moieties:
The sulfonic acid group is completely dissociated over nearly the entire pH range, while the carboxylic acid group is virtually undissociated in the pH range <3. The quaternary ammonium group again is completely dissociated over the entire pH range, while the secondary ammonium group is only weakly dissociated. Accordingly, ion-exchange membranes are referred to as being weakly or strongly acidic or basic in character.
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