Chronopotentiometry and overlimiting ion transport through monopolar ion exchange membranes

In this paper chronopotentiometric measurements are described to study the overlimiting ion transport through a Neosepta CMX cation and AMX anion exch ange membrane. This technique is used to characterise the fluctuations in m embrane voltage drop observed in the overlimiting region of current-voltage curves and to investigate the structural inhomogeneity of the aforemention ed membranes. Above the limiting current the measurements show large voltag e drop fluctuations in time indicating hydrodynamic instabilities. The ampl itude of these fluctuations is increasing with increasing applied current d ensity. The fluctuations also occur when a set-up is used where there is no forced convection and the depleted diffusion layer is stabilised by gravit ation. Experimental transition times are found to be smaller than calculate d for an ideally permselective membrane and indicate a reduced permeable me mbrane area. The results can be related to the theory of electroconvection due to an inhomogeneous membrane structure. (C) 1999 Elsevier Science B.V. All rights reserved.