Amphoteric exchange membrane

Ion-exchange membranes are classified according their function as follows cation exchange membrane, anion exchange membrane, amphoteric ion exchange membrane, bipolar ion exchange membrane, and mosaic ion exchange membrane. 

Figure 3.15 Transport number of hydrogen ions relative to sodium ions (PNaH) and current efficiency of cations to the composition of an amphoteric ion exchange membrane. (O, ) quarternized membranes (N-methyl pyridinium and sulfonic acid groups) (A, A) tertiary amino groups membrane (pyridinium hydrochloride and sulfonic acid groups) (, A) PNa 11 (O, A) current efficiency (%). Electrodialysis was carried out at a current density of 20 mA cm 2 using a mixed solution of 0.25 N sodium chloride and 0.25 N hydrochloric acid for 60 min at 25.0 °C.

Recently, membranes prepared by a layer-by-layer method have been actively studied. For example, a cationic polyelectrolyte can be adsorbed on a fiat plate as a monolayer and then an anionic polyelectrolyte layer added to it. The same procedure can be repeated, thus forming an ultrathin, multi-layer, amphoteric ion exchange membrane.107 The multi-layers have been formed on a porous membrane in studies on separation membranes108 and, further, such membranes have been studied as new functional materials for sensors, devices for photoelectronics,109 etc. 

Attempts to concentrate acetic acid by thermo-osmosis have employed various anion exchange membranes acetic acid is concentrated by 1.5 times in concentrated steam on the hot side using diethylaminated polystyrene membrane groups.213 Sodium chloride solution is concentrated to the hot side using an amphoteric ion exchange membrane.214... 

Fig. 41. Variation of the electrical resistance of cationic exchange (curve 1), anionic exchange (curve 2) and amphoteric exchange (curve 3) membranes as a function of the external pH (Reproduced with permission from [183])...

Qiu JY, Zhang JZ, Chen JH, Peng J, Xu L, Zhai ML, et al. Amphoteric ion exchange membrane synthesized by radiation-induced graft copolymerization of styrene and dimethylaminoethyl methacrylate into PVDF film for vanadium redox flow battery applications. J Membr Sci 2009 334 9-15. 

Electrolysis of amino acid solutions is largely carried out to purify them from mineral salts and acids and to isolate the individual amino acids from their mixtures. The need for separate examination of electrolysis of amino acids with ion-exchange membranes arises from the fact that these compounds are amphoteric in nature [35], In acidic solution the amino acids exist in the form of — R — COOH cations, and when a direct electric current is passed through the solution they should migrate to the cathode. In alkaline solution they are present in the form of H2N — R — COO anions and migrate to the anode. If the pH of the solution corresponds to the isoelectric point of the given acid, it exists in the form of a bipolar ion and should not migrate to either electrode. 

With ion-exchange membranes it is possible to accomplish demineralization of organic compounds with high selectivity and to separate amphoteric compounds, such as amino acids. 

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