Ionic gradients at an electrode above the equilibrium limit current. 2. Transition to convection

When one member of a simple binary electrolyte is consumed at an electrode, an ion depleted layer will form at that electrode. Under constant current conditions this layer will have a net electrostatic charge opposite that of the electrode. This leads to an attractive electrostatic body force between the solution in the depletion layer and the electrode which increases with proximity to the electrode. There are three primary processes that can occur in this solution: electric field relaxation, convective motion, and ionic diffusion/migration. By ranking these processes according to the speed with which their effects propagate through the solution, we show that this depletion layer is highly unstable and will transition to convective motion on the micrometer scale, generally before any voltage artifact is generated. A Rayleigh number for electrically forced convection is calculated and shown to rise by 3 orders of magnitude at precisely the time convection must begin. In this type of system, electrically driven convection will begin at the depletion electrode and grow out into the bulk of the solution following a moving ion depletion gradient layer. This is demonstrated by DC conduction in an ethanol solution containing added HCl.