DIFFUSION LAYER2

"While the concept of the diffusion layer is related to the concept of the hydrodynamic boundary layer, the two are not identical and neither is their thickness. The structure of the diffusion layer can be assumed to be relatively simple in the presence of a large excess of supporting electrolyte, which is usually the case in electroanalytical applications and in electrode kinetics research. Under these conditions, practically all the electrical current is carried by the ions of the supporting electrolyte, and the transport number of the reactant and the product is practically zero. When the current is initially turned on, the ions of the supporting electrolyte will migrate in the diffusion layer to/from the electrode (depending on their charge). However, since they do not take part in any electrode reaction, their concentration will increase/decrease at the electrode surface compared to that of their concentrations in the bulk solution. This will start the diffusion of these ions in a direction opposite to their migration. After steady-sate is reached, the diffusion will completely cancel the migration, and the net flux of these ions will be zero. They do not contribute to the mass transport in the diffusion layer; however, they are responsible for all the charge transport, that is, the resistance of the diffusion layer depends on the conductivity of the supporting electrolyte. On the other hand, the reactant and the product will diffuse to/from the electrode surface, and they will carry all the mass. The situation remains the same even if either the reactant or the product is an electrically neutral molecule. It is usually assumed that the concentration of all species changes linearly between the electrode surface and the edge of the diffusion layer. An example is the diffusion layer at the cathode surface during electroplating of copper from a solution containing a small amount of copper chloride and a large concentration of sulfuric acid. All the current is carried by the ions of the sulfuric acid (hydrogen cations and sulfate anions) but the only possible electrode reaction is the reduction of the copper ions since the reduction of hydrogen ions cannot occur at the prevailing electrode potential (copper is lower in the electromotive series than hydrogen). "