Rotating disk electrode diffusion-convection layer

The thickness of the Nernst layer increases with the square root of time until natural - convection sets in, after which it remains constant. In the presence of forced convection (stirring, electrode rotation) (see also Prandtl boundary layer), the Nernst-layer thickness depends on the degree of convection that can be controlled e.g., by controlling the rotation speed of a -> rotating disk electrode. See also - diffusion layer. See also Fick s law. 

A Nemst stagnant-diffusion-layer model was used to accovmt for the diffusion impedance. This model is often used to account for mass transfer in convective systems, even though it is well known that this model caimot ac-coimt accurately for the convective diffusion associated with a rotating disk electrode. 

The most appropriate experimental arrangement for the quantitative determination of the stationary curves for the reaction is the rotating disk electrode technique, in which the convective transport is controlled mechanically and thus a constant diffusion layer for each species is achieved [101-106]. Modifications of this technique, to primarily collect the intermediates of the reactions, such as ring-disk electrode techniques [93,94] and a hanging meniscus rotating disk electrode... 

This expression, eq. (7.106) or (7.107), takes into account the effect of fluid motion on the diffusion layer 8, which is an adherent thin film on the electrode surface. Therefore, diffusion coupled with convection contributes to the total mass transfer process to or from the electrode surface and it is known as convective diffusion in which 6 is considered immobile at the electrode surface [66]. Recall that the limiting thickness of the diffusion layer is illustrated in Figure 4.5. Also, Levich equation describes the effect of rotation rate, concentration, kinetic viscosity on the current at a rotating-disk electrode. 

Fig. 4. Migration contribution to the limiting current in acidified CuS04 solutions, expressed as the ratio of limiting current (iL) to limiting diffusion current (i ) r = h,so4/(( h,so, + cCuS(>4). "Sulfate refers to complete dissociation of HS04 ions. "bisulfate" to undissociated HS04 ions. Forced convection" refers to steady-state laminar boundary layers, as at a rotating disk or flat plate free convection refers to laminar free convection at a vertical electrode penetration to unsteady-state diffusion in a stagnant solution. [F rom Selman (S8).]...

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