MASS TRANSPORT2

"The mass transport and the charge transport are typically ""decoupled"" in electrochemistry; that is, the reacting species and the charge carrying species are not necessarily identical. E.g., one would intuitively assume that during electroplating of copper from a solution of copper sulfate all the mass and charge required for the electrode reaction at the cathode would be carried by the copper cations in the solution. That is not the case at all. In the presence of a large excess of supporting electrolyte, all the current is carried by electromigration of the ions of the supporting electrolyte (both in the bulk solution and in the diffusion layer) while all the mass is carried by the copper ions by convection in the bulk solution and by diffusion in the diffusion layer. In the absence of any supporting electrolyte, all the mass is still carried by the copper ions, while the current is divided between the copper cations and the sulfate anions according to the ratio of their transport numbers (both in the bulk electrolyte and in the diffusion layer). The situation is maybe more self-explanatory in the case of an electrically neutral molecule reactant that cannot carry any current. "