Ion transfer across electrodialysis membranes in the overlimiting current range: stationary voltage current characteristics and current noise power spectra under different conditions of free convection

Abstract

Laboratory-made cation-exchange membranes based on sulfonated polysulfone as well as commercially available membranes were investigated with respect to their concentration-polarization behavior in the overlimiting current range. Experiments carried out in a convection-free cell using reversible working electrodes have shown: (a) a significant deviation of the experimental limiting current from the theoretical value which decreased with increasing degree of sulfonation and (b) a second rise in current along with the onset of current-noise. The slope of the power spectra was ranging from −2.2 to −1.8 for a current >2 i lim. The reduced limiting currents were interpreted to be a consequence of partially impermeable membrane surfaces. After immobilization of the depleted diffusion layer by an agarose gel neither a second rise in current nor any excess noise could be observed. Power spectra recorded under basically different hydrodynamic conditions resembles those under suppressed free convection; the slopes were about −2. The experimental findings strongly suggest that the overlimiting behavior of the cation-exchange exchange membranes is caused by a convective flow different from a normal free convection. The theory of electroconvection based on an electriclaly inhomogeneous membrane-solution interface seems to be roughly compatible with the experimental results.