Acta Chimica 50. (1966)

Kertész D.–Kőrösy F.–Székely J.: PH-dependent ion-permselective membranes

13 Acta Chimica Academiae Scientiarum Hungaricae Tomus 50, pp. 193—200, (1966) PH-DEPENDENT ION-PERMSELECTIVE MEMBRANES D. Kertész, F. de Kőrösy and J. Székely (Negev Institute for Arid Zone Research, Beersheva) Received May 9, 1966 Ion-permselective membranes are sheets of ion-exchange resins, contain­ing ionized or ionizable groups attached to a non-ionic polymer matrix. They also contain a system of pores or channels, multiply interconnected in space, through which ions are able to move in an aqueous medium. As long as the concentration of ions in the solution adjoining the membrane remains less than the concentration of fixed ions in the aqueous phase of the membrane, the overwhelming majority of mobile ions within it will be of opposite sign to its fixed charges. These counter-ions will populate the membrane and they may cross it either by simple diffusion or during electrolysis. Fundamentally, we are dealing with a system subject to the laws of the Donnán equilibrium between mobile and immobile ions. A part of the water within the pore system is built in around the fixed ions of the membrane, and the force keeping this water within decreases ap­preciably when these fixed ions become deionized. This happens, for instance, if they belong to the family of weakly acid or basic groups, and the pH of the solution is low enough or high enough, respectively, to counteract their elec­trolytic dissociation. The membrane deswells in this case to some extent, because the electrostatic field around the fixed ions has vanished and does not attract water molecules any more. At best, a much weaker dipole field remains. If the amount of water in the membrane were just on the limit of yielding a continuous aqueous phase even in the ionized, swollen state of the membrane, this deswelling by deionization could seriously change its pore size. Under favourable circumstances the contracting pore system may be­come too narrow either to allow the passage of larger ions which were able to pass it easily in the swollen state or, to allow the passage of ions at all. In the first case the membrane becomes selective for smaller as against somewhat larger ions, and in the second, its conductivity is going to decrease very strongly for all sorts of ions. Despic and Hills [1] found that the resistance of a membrane rose by one order of magnitude on deionization. Sollner and Gregor [2] describe a membrane with low capacity, which had twice the resistance in HC1 than in Acta Chimica Academiae Scientiarum Hungaricae 50, I960

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