An amphoteric Ion-permselective membrane

Abstract

A membrane made by reacting sulphochlorinated polyethylene with 1-amino-3-dimethylamino-propane containing water, was found to be amphoteric: about 5 6 of its capacity was due to dimethylamino groups while sulphonic acid groups accounted for the remaining 1 6 of its capacity. In acid the dimethylamino groups are ionised and the membrane behaves as a week anion-selective membrane. In alkali the aminogroups are deionised and the sulphonic acid groups are set free to make the membrane cation-active. These changes in ionisation with pH have the following consequences: (1)The membrane de-swells in length and in weight in its isoelectric region between pH 8–11. (2)The electric resistance increases by 3–4 orders of magnitude in this pH region as against both acid and strongly basic pH solutions. (3)In acid solutions the membrane is anion-selective, crosses the neutrality line at about pH 10 and becomes cation-selective in strongly basic solutions. (4)In the open states, that is in acid and strongly basic solutions, the membrane hardly distinguishes between the ion pairs Cl - and SO = 4 or Na + and K +. However, in the closed, high resistance, region Cl - is transferred 7–8 times more easily than SO = 4. The difference between transference of the two alkali ions remains slight. (5)A bipolar (sandwich) membrane made with adjoining layers of the above-mentioned membrane and a layer of polyethylene sulphonic acid was prepared. This sandwich rectifies the current in acid solutions but has a low, voltage-independent, resistance in strong alkali where it ceases to be bipolar and changes into a cation-selective membrane. It has a high, voltage-independent, resistance in its isoelectric region where the amphoteric side is uncharged and represents a high series-resistance beside the low resistance cation-selective layer.