A practical approach to measuring the ion-transport number of cation-exchange membranes: Effects of junction potential and analyte concentration

Ion-exchange membranes are crucial for realizing high-performance electrochemical devices. They exhibit properties that can be elucidated by considering accurate ion-transport numbers determined from membrane potential differences. Herein, the accurate sodium-ion transport numbers (tNa+m ) of various cation-exchange membranes were determined in sodium chloride solutions using two silver-silver chloride single-junction electrodes as references. The junction potential differences between these two electrodes were employed for the accurate measurement of membrane potentials in 0.01-1 M sodium chloride solutions and determined for five combinations of low-concentration/high-concentration sodium chloride solutions. They were sufficiently large to affect membrane potentials by 11-19% compared to the measured apparent potentials. It was found that the sodium chloride concentration should be at least 0.1 M, in which the optimal concentration difference was identified as 10 times between high- and low-concentration solutions. In addition, the stirring of the lowconcentration solution was essential for minimizing the diffusion boundary layer that distorted the apparent potentials. Consequently, it was proven that a 1/0.1 M concentration difference was most suitable for the determination of tm Na+ using the above setup, as the tm Na+ of a commercial cation-exchange membrane obtained under these conditions (0.995) was close to that reported previously (0.997).