Limiting-current measurement forced convection

The effective diffusivities determined from limiting-current measurements appear at first applicable only to the particular flow cell in which they were measured. However, it can be argued plausibly that, for example, rotating-disk effective diffusivities are also applicable to laminar forced-convection mass transfer in general, provided the same bulk electrolyte composition is used (H8). Furthermore, the effective diffusivities characteristic for laminar free convection at vertical or inclined electrodes are presumably not significantly different from the forced-convection diffusivities. 

Mass transport in amperometric systems in which the reagent stream is forced to flow along the surface of the electrode may be described in terms of convective diffusion. Effectively this means that at sufficiently high values of Pg, the Peclet number, the liquid above an electrode may be divided into two distinct zones. In one zone, far away from the electrode surface, convection is important, and the concentration profile is substantially flat. In the other zone, adjacent to the electroactive surface, there is a sharp concentration gradient here diffusion is the predominant mass transport process. The Peclet number is given by v l/D, where is the main stream fluid velocity, and / is the length of the electrode (measured in the direction of fluid flow). Under these conditions, the mass transport limited current z l for a reversible electrode couple (i.e. the concentration of the electroactive form is zero at the electrode surface) is given by... 

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