Limiting current density and water splitting:

The electrical migration of cations and anions in electrodialysis procedure is much faster by the ion selective membranes than in solutions. as a result, cations and anions are not supplied from the dialysate solution side to the respective side at a sufficiently faster rate through that they are carried by the membrane. Same, once ions reach the other side of membranes they are not quickly carried within the bulk of the solution on the brine side. This results in depletion of ionic concentration on one side and builds up of ionic concentration on the other side of membranes. This is known as concentration polarization and this will become more severe along with rise in operating current density. At suitably high current density, the concentration of ions near the ion selective membrane on the dialysate side drops to zero and the current is carried through the hydrogen and the hydroxyl ions of water. This cause's additional ionization of water and within effect, the electrical driving force applied, to carry the ions, is utilised to split water alternatively. This phenomenon is detrimental to the process efficiency and is known as water splitting. This causes Joule heating also. The current density at that this phenomenon begins to occur is known as the limiting current density. Electrodialysis procedure is to be operated below the limiting current density.