Development of Electroconvection at the Undulate Surface of an Anion-Exchange Membrane in Sodium Chloride and Sodium Hydrogen Tartrate Solutions

Electroconvective (EC) vortex structures formed at the surface of an AMX anion-exchange membrane in 0.02 M NaCl and NaHT solutions have been compared. The AMX membrane surface is undulated and consists of alternating staggered hills and valleys. Electroconvective mixing zones have been visualized using the fluorescent agent rhodamine. In the case of NaCl, the vortex EC structures and concentration distributions have been calculated using a "basic" model involving the Nernst-Planck-Poisson-Navier-Stokes equations. According to the calculation, the zone of low concentration approximately coincides in its boundaries with a large vortex localized near the bottom of the valley. Visualization shows that the EC vortices are larger and the electrolyte concentration in them is higher in the case of NaCl compared with NaHT at the same current to limiting current ratio. The reason is more intense generation of H+ ions at the depleted solution/membrane boundaries, which is due to proton loss by a part of the tartrate anions when they enter the membrane. The generated H+ ions are additional charge carriers; their flux from the depleted membrane surface decreases (in absolute value) the space charge density at the membrane surface, and, hence, the intensity of electroconvection.