Ion transport, diffusion and hydrodynamics
If you are still with me, we have gone from electrolytes, and what happens when an electrode is dropped in an electrolyte, through the theory of electrodes and electrochemical cells, and now we look at what happens when the different species in the electrolyte begin to move. Why should they move at all? Because they are subjected to a force. This force can be electrical and cause charged particles (ions) to move, or can arise from the presence of a concentration gradient, in which case any species can move. The first is the subject of conduction and transport; the second is diffusion. They are linked and we shall bring the two concepts together in the chapter. In real life there is movement in electrolytes because of temperature gradients (convection) and if the solution is stirred, but I shall not attempt to model these phenomena here. I shall, however, look at the particular cases of when a cell is designed to give a known flow of electrolyte over the electrode. When the hydrodynamic flow is controlled, many of the problems associated with transport disappear and the current may be predicted in terms of easily determined properties of the solution and electrochemically active species.
Unable to display preview. Download preview PDF.