Viscosity electrode rotating rate limits

The rotating disc electrode (RDE) is the classical hydrodynamic electroanalytical technique used to limit the diffusion layer thickness. However, readers should also consider alternative controlled flow methods including the channel flow cell (38), the wall pipe and wall jet configurations (39). Forced convection has several advantages which include (1) the rapid establishment of a high rate of steady-state mass transport and (2) easily and reproducibly controlled convection over a wide range of mass transfer coefficients. There are also drawbacks (1) in many instances, the construction of electrodes and cells is not easy and (2) the theoretical treatment requires the determination of the solution flow velocity profiles (as functions of rotation rate, viscosities and densities) and of the electrochemical problem very few cases yield exact solutions. 

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. 

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