Effect of the equilibria of multivalent metal sulfates on the transport through cation-exchange membranes at different current regimes

The transport of trivalent metals (Cr(III) and Fe(III)) through cation-exchange membranes is studied and compared with the transport of mono- and divalent ions by means of chronopotentiometry and current-voltage characteristics. The species present in solutions of sulfate salts of multivalent metals give rise to different transport properties depending on the applied current. The detection of various transition times in the chronopotentiograms and two limiting current densities in the current-voltage curves reveals a change in the species being transported in the diffusion boundary layers and inside the membrane phase. At overlimiting current regimes, two different mass transfer mechanisms can predominate depending on the electrolyte composition. For the most concentrated solutions, gravitational convection promotes a decrease in the diffusion boundary layer thickness and the resistance of the membrane system decreases, whereas electroconvection takes place for both concentrated and weak electrolytes when a certain threshold in the membrane voltage drop is surpassed. The initial salt concentration has a significant effect on the emergence of the overlimiting conductance, since the length of the plateau of the current-voltage curves diminishes as a consequence of the size of the cations involved and the equilibrium conditions in which they take part.