Chronopotentiometrie Investigation of the Effects of Size, Charge and Nature of the Anion and Co-Ion on the Transport Phenomenon Using an Anion-Exchange, Membrane

The electrical responses of an interpolymer anion exchange membrane in aqueous solutions of NaCl (1 and 10 mM), NaNO₃, Na₂SO₄, CH₃COONa, KCl, CaCl₂ and NH₄Cl (each 10 mM) have been measured under steady state conditions by chronopotentiometric method to study the variation in the ion transport phenomenon as a function of size, charge and nature of the co-ions for a given cation or anion in solution. The membrane in these solutions exhibited an inflection clearly splitting its potential into two zones at the transition time (τ). The inflection was sharp in NaCl and NaNO₃, fairly well defined in CH₃COONa and diffused in Na₂SO₄. However, the inflection was much early in CH₃COONa and late in Na₂SO₄. as compared to that of NaCl which was followed by that in NaN03. The membrane behaviour with respect to chloride ion was found to vary with the change in co-ions Na⁺, K⁺, Ca²⁺ and NH⁴⁺. The potential drop across the membrane before and after the inflection was the highest in KCl and the least in NH₄Cl as compared to that found in the case of NaCl and CaCl₂ solutions respectively, which was nearly parallel. The inflection at t was found to be well defined in NaCl, KCl and NH₄Cl, but diffuse in CaCl₂. The measured data such as transition time (τ), membrane potentials E₀ at the instance and E_max at time (τ + 15) s after the application of the constant current, the potential difference (ΔE) between the two potential phases of the membrane at τ (a measure of the thickness of the diffusional layer in the membrane-solution interface on diluate side) and the inflection range (Δt) in membrane potential on the experimental time axis at an applied current 1.8 mA cm^-2, were compared to one another and discussed. Further, the values of (τ^1/2/Z_A) or (τ^1/2/Z_Cr_CA) (where Z_A and Z_C are ionic charge on the anion A and cation C, respectively, r_CA is cation to anion mole ratio), E₀ and (ΔE z_A) or (τ^1/2/Z_Cr_CA) were correlated with the ionic radii r_ or r+, respectively. The value of (τ^1/2/Z_A) decreased, while that of E₀ and (ΔE z_A) increased with the increase in ionic radius of the ion, A. Reverse trends were observed with the increase in ionic size of the cation (i.e., co-ion) in the solution. The effects of size and charge of the co-ions on the membrane phenomenon were elucidated and compared.