Ion transfer across electrodialysis membranes in the overlimiting current range: chronopotentiometric studies

Abstract A batch of highly permselective cation exchange membranes made of sulfonated polysulfone with varying exchange capacity was investigated by means of a chronopotentiometric method - the current-step transient. In 0.01 M CuSO 4 the experimental transition times were significantly smaller than predicted by the classical theory; yet with increasing ion exchange capacity of the membrane, the transition times approached the theoretical value. This was interpreted to be caused by a partially impermeable, i.e. conductively heterogeneous membrane surface. Similar experiments were carried out under conditions of free convection. At current densities well above the limiting value the steady-state polarization voltage of the system increased with increasing ion exchange capacity ( =decreasing heterogeneity) of the membrane. Chronopotentiograms taken at current densities close to the limiting value displayed a strong low-frequency voltage fluctuation, probably due to a hydrodynamic instability in the depleted diffusion layer. The experimental results were interpreted by applying the theory of electroconvection.