Migration Current:

The migration current is because of the migration of reducible or oxidizable ions which is proportional to the electrical potential gradient at the electrode because ofattraction.

Unless this potential gradient is removed the limiting current cannot be proportional to the concentration of electroactive ions within the solution.  This migration current is commonly eliminated through adding 50 or 100 fold excess of an inert supporting electrolyte.

Generally used supporting electrolytes are potassium or sodium salts. The potassium ions cannot be discharged at cathode until the impressed voltage becomes large.  The large number of potassium ions therefore remains as a crowd around cathode and restrict the potential gradient to a region so very close to the electrode surface.  So there is no longer an electrostatic attraction operative to attract other reducible ions from the bulk of the solution.  In these circumstances the limiting current is solely controlled by the diffusion of the electroactive species through the concentration gradient adjacent to the electrode.