Electric and electrokinetic transport properties of homogeneous weak ion exchange membranes

Abstract

The effect of H + ions on the membrane potential and electrical resistance of synthetic membranes has been investigated theoretically and experimentally for salt solutions of typical pH values. Employing the equations of Schlögl for the membrane potential Δϕ and the specific electrical resistance ϱ, membrane potentials and resistances have been calculated for binary and ternary electrolyte solutions. Moreover, the effect of a dissociation equilibrium of the fixed charges (COOH groups) in the membrane on the membrane potential and resistance have been considered. The ratios of the mobilities of the cations and the anions have been estimated at relatively large external salt concentrations from membrane potential curves measured with LiCl-HCl, HCl, CaCl 2-HCl, Na 2SO 4-H 2SO 4, and MgSO 4-H 2SO 4 using weak cation exchange membranes such as homogeneous cellulose acetate, cuprophane, and carboxy methyl cellulose, as well as strong cation exchange membranes such as ASAHI's CK-1 and a Nation membrane. From measurements of the electrical resistance of homogeneous cellulose acetate membranes at different salt concentrations but prescribed pH values for the systems LiClHCl, MgCl 2HCl, and MgSO 4H 2SO 4, individual ion diffusion coefficients have been estimated. Further, the concentration dependence of the ion diffusion coefficients in the membrane has been demonstrated. The counterion diffusion coefficients exhibit a strong change of their diffusion coefficients ranging from 10 −14 up to 10 −9 cm 2/sec in the concentration range 10 −4 mole/liter < c s < 10 −1 mole/liter, whereas the coion diffusion coefficients and the H +-ion diffusion coefficient vary slightly with the external electrolyte concentration and are of the order of 10 −8 to 10 −7 cm 2/sec and 8 × 10 −7 to about 3 × 10 −6 cm 2/sec, respectively.