Migration And Limiting Currents:

The Limiting Current

The limiting current obtained with the dropping mercury electrode is caused through the extreme state of concentration polarization that results from the depletion of the concentration of the electro-reducible or electro-oxidizable substance at the electrode surface through the electrode reaction.  The oxidizable or reducible ions are supplied to the depleted region at the electrode surface through two forces:

1. A diffusive force that proportional to the concentration gradient at the electrode surface.

2.  An electrical force that proportional to the electrical potential difference between the surface and the solution.

So the limiting current is regarded as the sum of 'diffusion current' and a 'migration current'.