Permselectivity cationic exchange membranes

Formulation. Consider two unity thick diffusion layers of a mixture of 1, 1-, and 1, z-valent3 electrolytes with a common anion, adjacent to a planar ideally permselective cation-exchange membrane. Direct the axis x normally to the membrane and let x = 0 coincide with the outer boundary of the diffusion layer. The diffusion layers will thus be located at 0 < x < 1 and 1 + Aelectro-diffusional transfer of ions across the membrane and the diffusion layers is... 

An ideal permselective cation-exchange membrane would transmit positively charged ions only, that is, for a transport number of a counterion in a cation-exchange membrane is T m = 1 and the permselectivity of the membrane is xFcm = 1. The permselectivity approaches zero when the transport number within the membrane is identical to that in the electrolyte solution, that is, for T = Tc is xFcm = 0. For the anion-exchange membrane the corresponding relation holds. 

M. Kogure, T. Sato, T. Tanaka, S. Yasujima, T. Suzuki and H. Ohya, Preparation of antimony acid cation exchange membranes by using metal alkoxide-gel method, Bull. Chem. Soc. Jpn., 1991, 1991, 1618 H. Ohya, R. Paterson, T. Nomura, S. McFadzean, T. Suzuki and M. Kogure, Properties of new inorganic membranes prepared by metal alkoxide methods. Part I A new permselective cation exchange membrane based on Si/Ta oxides, J. Membr. Sci., 1995, 105, 103-112. 

Fig. 6.1 Distribution of cations and anions in pores of a cation-exchanger membrane depends on pore radius which decreases in the sequence A- B— C. In case C the membrane becomes permselective. (According to K. Sollner)... 

In 1973, Dupont began to commercialize their first perfluorosulfonic add cation exchange membrane, Nafion. Since then until now, Nafion has been attracting much attention because of its superb chemical and thermal stability, high ionic conductivity, excellent permselectivity and good mechanical strength. Many approaches have been proposed to use this unique material as a modifier of electrochemical electrode surfaces. 

Another prototype of an ideally cation-permselective interface would be a cation-exchange membrane (C-membrane). Most practically employed C-membranes are extremely permselective, so that their polarization curves would be expected to coincide with those at electrodes (given the same... 

It is possible now to explain a number of important membrane properties in a qualitative way. The permselectivity i.e. the increased transference number of cations in a cation-exchange membrane respectively of anions in an anion-exchange membrane compared with the free solutions is due to the fact that the number of counterions is much higher than the number of co-ions. 

In 1939, Manegold and Kalauch assembled a three-compartment ED apparatus consisting of a permselective anion-exchange membrane and a cation-exchange one. It was, however, only in the early 1950s that the manufacture of selective membranes from ion exchangers allowed the multicompartment electrodialysers to be assembled (Shaposhnik and Kesore, 1997). 

Here, E, is the current utilization, / is the membrane permselectivity, T is the transport number, n is the number of cell pairs in the stack, Vw is the partial molar volume of water, and C is the concentration, a, c, s and w refer to anion, cation, solution and water, respectively, and the superscripts cm, am, c, and d refer to cation-exchange membrane, anion-exchange membrane, concentrate and diluate. 

The permselectivity of ion exchange membranes can be exploited as a preconcentration technique. Thus suppose two solutions (1 and 2) are separated by a cation exchange membrane which is totally impermeable to anions. Initially solution 1 contains a ions and solution 2 contains B4" ions. It can be shown (27, 28) that at equilibrium the following relationship holds... 

To restore electroneutrality, sodium ions are transported selectively in the electrochemical field gradient across the cation-exchange membrane from the anode to the cathode chamber. Ideally the membrane should be 100% cation permselective, therefore excluding any hydroxyl ion transport but in practice this is not the case and current efficiencies are always less than 100%. This current inefficiency is represented by the reaction of hydroxyl ions with chlorine. Patent applications for this method of chlor-alkali production appeared as early as 1949 (2). 

Because the ion exchange membrane shows ideal permselectivity at sufficiently low solution concentration, compared with the fixed ion concentration of the membrane, the difiusional flux is expressed as follows, for a cation exchange membrane... 

G.W. Bodamer and P.A. Cheltenham, Permselective films of anion-exchange resins, USP 2,681,319, 1954 Permselective films of cation-exchange resins, USP 2,681,320, 1954 B.L. Tye, Improvements to ion-exchange membranes, BP. 823,077, 1959 N. Tamura, Heterogeneous cation exchange membranes synthesized by the sulfonated emulsion copolymers of styrene/divinylbenzene and polypropyrene, Nihon Kagaku Zasshi, 1976, 1976, 654, etc. 

Z. Ogumi, Y. Uchimoto, M. Tsujikawa, K. Yasuda and Z. Takehara, Modification of ion-exchange membrane surface by plasma process. Part 2. Monovalent cation permselective membrane from perfluorosulfonate cation exchange membrane, J. Membr. Sci., 1990, 54, 163-174. 

Figure 4.9 Apparatus to measure permselectivity (transport number of X relative to chloride ions of an anion exchange membrane). C cation exchange membrane A anion exchange membrane to be measured 1,4 Ag—AgCl electrodes for current supply 2,3 Ag—AgCl wire probe electrodes to measure the voltage drop across the membrane.

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. 

Permselectivity of Specific Cations through the Cation Exchange Membrane in Electrodialysis... 

Y. Onoue, Y. Mizutani, R. Yamane and Y. Takasaki, Permselectivity of improved cation exchange membranes for NaCl-CaCl2 system, Denki Kagaku (J. Electrochem. Soc. Jpn.), 1961, 29, 544 T. Yawataya, H. Hani, Y. Oda and A. Nishihara, Thinly resin coated cation exchange resin membrane with permselectivity between uni- and di-valent cations, Dechema Monographien Band II, Verlag Chemie, Weinheim, 1962, p. 501. 

E. Katsube and T. Asawa, Electric potential drop across cation-exchange membranes with high permselectivity to univalent cations in electrodialytic concentration of sea water, Rep. Res. Lab., Asahi Glass, 1969, 19, 43. 

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