The mass transport properties of selected membranes in potassium hydroxide solutions of various concentrations

• Main Author: Malpas, David George
• Published: Middlesex University 1992
• Subjects: 660
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.568381

Aspects of the mass transport processes occurring in the ion-exchange membranes, PUDO 193, a Cellophane membrane derived from regenerated cellulose and Permion 2291 40/30, a grafted co-polymer of polyethylene and methacrylic acid, were studied while the membranes were swollen in solutions of KOH of a wide range of concentrations. Measurement of the dimensional changes (length, width and thickness) of as received Cellophane films, after immersion in solutions of KOH, revealed a complex swelling behaviour with the majority of swelling resulting in changes in thickness. Unlike Cellophane, and as a consequence of their different structure, the swelling behaviour of Permion films were found to be less complex. The swelling behaviour was used to determine the porosity of the swollen membranes. The ion-exchange capacity of the mono-functional Permion was obtained by pH titration, a method unsuitable for use with Cellophane since it is not fully exchanged at low external electrolyte concentrations. The ion-exchange properties of Cellophane were determined using atomic absorption techniques and a crion tracer method which was deemed suitable since similar results were obtained for Permion by this method and by pH titration. Transport numbers of potassium ions and transference numbers of water through Cellophane and Permion as single membrane types were obtained using a meticulous technique, which minimised the unwanted effects of diffusion and osmosis. The method obtained transport numbers relevant to a specific concentration by holding solution composition constant and equal on either side of the membrane. The confidence gained in the technique, enhanced by careful cell testing and the application of correction factors, provided a sound basis for the measurement and subsequent understanding of transport behaviour in the less obvious situation, namely in assemblies of membranes. Measurements of the resistance of Cellophane and Permion ftlms equilibrated in solutions of KOH were made. Membrane resistance vs. membrane thickness plots were curved due to current refraction into the region where the membrane was clamped between the two halves of the conductivity cell. The data was linearised using a nonempirical correction factor and hence the accuracy of the measured membrane conductivity was improved. From the primary resistance data, electrolyte conductivity and the mobilities of the potassium and hydroxyl ions in free solution were calculated. The ratio of potassium to hydroxyl ion mobility in free solution formed a basis for a comparison with a similar mobility ratio in the membrane phase. This method allowed a comparison of mobilities in free solution and the membrane phase without the necessity of estimating a suitable tortuosity factor. Models of tortuosity were then introduced to assess the blocking characteristics of the two membranes. The transport properties of assemblies of Cellophane and Perm ion were investigated. Transport numbers entering and leaving the assembly were found to be different and depended on the orientation of the assembly. For the situation where transport numbers were greater leaving the assembly and at low current densities a steady state was reached and the depletion of electrolyte at the interface was replaced by diffusion, osmosis and hydraulic flow. At high current densities depletion of electrolyte at the interface was rapid causing the decomposition of the Cellophane. For the situation where transport numbers were greater entering the assembly, the assembly separated at the Permion/Cellophane interface due to the accumulation of electrolyte, the difference between measured and predicted transport numbers was accounted for by the effects of diffusion and osmosis which occurred mainly in the Cellophane.