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

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 > 2i(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 membranes is caused by a convective flow different from a normal free convection. The theory of electroconvection based on an electrically inhomogeneous membrane-solution interface seems to be roughly compatible with the experimental results.