Electrolyte diffusive flux determination through a highly selective cation exchange membrane

A new method for evaluating highly selective ion exchange membranes is devised, which is competitive with the counter-ion transport number determination method. It is based on the evaluation of a strong electrolyte leakage through a membrane. This study uses a CM2 polystyrene and divinylbenzene sulfonated cation exchange membrane, separating two solutions, one containing the electrolyte at a given concentration C-0 and the other permuted water. The electrolyte diffuses under its concentration gradient and induces an increase of the conductivity in the diluted compartment, allowing the determination of its flux. This new method is shown to be reliable and sufficiently accurate over a large concentration range. The bilogarithmic variation of the flux versus the concentration, for the KCl (10(-2) less than or equal to C, less than or equal to 4 mol L-1)/CM2/H2O system, shows the existence of three linear sections, each one corresponding to a diffusion state. The concentration range 10(-1)-2 mol L-1 seems to be the most interesting one for experimental and theoretical studies of the diffusion phenomenon of a strong electrolyte through an ion exchange membrane. In this domain the concentration is high enough that the proton contribution to the measured values of the conductivity can be neglected and sufficiently low that the osmosis flux, which would slow down the electrolyte as it diffuses, is not important.