2.6. Measurement of Ion-Exchange Capacity (IEC)

IEC is defined as the milliequivalent of cation-exchange groups per 1 g of dry membrane whose counter-ions are Na⁺ ions (meq/g-dry-Na form). To measure the IEC of a sample membrane, the membrane of 5 cm × 5 cm was immersed in 0.5 mol/dm³ KCl solution for 6 h to change the counter-ions with K⁺ ions. The membrane was then immersed in 50 cm³ of 0.3 mol/dm³ of NaNO₃ and stirred for 24 h to ensure that all of the K⁺ counter-ions were exchanged with the Na⁺ ions in the solution. The concentration of K⁺ ions C_K+ in the solution was measured using an ion chromatograph (Dionex ICS-1500). The membrane was dried under vacuum for 24 h, and its dry weight W_d was measured. The IEC of the membrane was obtained using the following equation: