Four measurement methods of membrane conductivity of an ion-exchange membrane, such as the constant-distance method, variable-distance method, multi-membrane method and a potential-step method, were compared by using smooth platinum (Pt) plates or platinized Pt plates as measuring electrodes. The conductivities of a Neosepta C66-5T membrane equilibrated with 0.05-5kmol·
-3 NaCl solution was measured at 298 K. An equivalent circuit model, composed of an overall resistance including electrolyte and membrane resistances, electrode-reaction resistances and electrode capacitance, was used for calculation of membrane resistance. Capacitance on a smooth platinum plate ranged between 15-60μF, but that on a platinized platinum ranged between 500-2500μF, which were a function of electrolyte concentration and electrode distance. The conductivities calculated from the resistances measured by different methods had similar tendencies, increasing with the increasing concentration of external NaCl solution, but showed scattered values. Since the measured resistance was sensitively affected by small capacitance, the conductivity estimated by the constant-distance method using smooth Pt plates showed the highest value. When using Pt-plated Pt plates, the calculation of conductivity was less affected by the capacitance. The membrane conductivity was calculated by subtraction of the solution resistance from the overall resistance including solution and electrolyte resistances. When the membrane resistivity is close to the solution electrolyte, the subtraction value becomes negligibly small, and therefore the calculation error becomes larger. A DC electrolysis method using a potential step is tested as an alternative method to a AC resistance method. Then a potential-step method using a Pt-plated Pt plates was found to be useful for measuring wide ranging conductivities of an ion-exchange membrane.
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