Conductivity of ion exchange membranes

Important Quantities Connected with Electro Dialysis 3.5.1. Electrical Conductivity of Ion-Exchange Membranes The specific electrical conductivity of an ion selective membrane is given by ... 

The electric conductivity of ion-exchange membranes increases as temperature rises. This change may be characterized with an equation similar to the Arrhenius equation for viscosity. 

By the time the next overview of electrical properties of polymers was published (Blythe 1979), besides a detailed treatment of dielectric properties it included a chapter on conduction, both ionic and electronic. To take ionic conduction first, ion-exchange membranes as separation tools for electrolytes go back a long way historically, to the beginning of the twentieth century a polymeric membrane semipermeable to ions was first used in 1950 for the desalination of water (Jusa and McRae 1950). This kind of membrane is surveyed in detail by Strathmann (1994). Much more recently, highly developed polymeric membranes began to be used as electrolytes for experimental rechargeable batteries and, with particular success, for fuel cells. This important use is further discussed in Chapter 11. 

Pourcelly, G. (2002) Conductivity and selectivity of ion exchange membranes structure-correlations. Desalination, 147, 359. 

The unique capability of ion-exchange membranes to separate chemical species according to ionic charge makes it possible to conduct various electrochemical synthesis reactions otherwise difficult to perform. A number of such synthetic mechanisms are shown in Fig. 41. Although each may be applied individually, a recent trend has emerged toward assembling several electrochemical... 

Flemion is quite different from prior membranes in that it is based on specific perfluorinated copolymers with pendant carboxylic acid as a functional group. The introduction of carboxylic functions in the polymer has realized high permselectivity in cation transport with high conductivity, which is indispensable to electrochemical application of ion exchange membranes. 

During the last two decades, tremendous work has been conducted on properties of ion exchange membranes, their behavior under various environmental conditions, and their interaction with electrodes as well as in determining the limiting factors influencing operation and cell life. 

R. Arnold, Structure of ion-exchange membranes from acid absorption data, Aust. J. Chem., 1968, 21, 521-525 R. Arnold and D.F.A. Koch, Electrical conductivity of cation-exchange membranes in the hydrogen ion form, Aust. J. Chem., 1966, 19, 1299-1313 R. Wodzki, A. Nar bska and J. Ceynowa, Nonuniform distribution of the ionogenic groups in permselective membranes, Angew. Makromol. Chem., 1979, 78, 145-155. 

T. Sata, Properties of ion exchange membranes combined with conducting polymers anisotropically I. emf generation by redox reactions across composite membrane, Electrochim. Acta, 1992,37, 555. 

Ion exchange membranes have been used in various industrial fields, and have great potential for use in new fields due to their adaptable polymer membrane. As mentioned in the Introduction, membranes are characterized mainly by ion conductivity, hydrophilicity and the existence of carriers, which originate from the ion exchange groups of the membrane. Table 6.1 shows reported examples of applications of ion exchange membranes and the membrane species used in various fields. Various driving forces are usable for separation electrochemical potential, chemical potential, hydraulic pressure such as piezodialysis and pervaporation, temperature difference (thermo-osmosis), etc. Of these, the main applications of the membrane are to electrodialysis, diffusion dialysis, as a separator for electrolysis and a solid polymer electrolyte such as in fuel cells. 

Polymer electrolyte-aqueous electrolyte interfaces in the porous interior of proton conducting or ion exchange membranes for separation, desalination, and fuel-cell applications [287]. 

Fig. 6.2 Three-conductor model of ion-exchange membrane conductivity.

The set of equations above are used for calculation of ion-exchange membrane conductivity and selectivity versus external solution conductivity. 

The unique ability of ion-exchange membranes to conduct ions selectively is a foundation for technological processes based on the swapping ion mechanism. Double-exchange reaction by normal chemical methods is rather difficult, but it can be done easily with high efficiency in one stage in an electrodialyzer shown in Fig. 6.6. 

In undoped purified samples of MBBA, low-field conductance was attributed to thermal dissociation of trace impurities, but at fields greater than 1500 V/cm, electrode processes begin to interfere. Through the use of ion exchange membranes as an electrode coating, injection effects were supressed. Then one observes at low fields an... 

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