Convection and diffusion hydrodynamic systems

Applying (5.39) to the Cottrell equation (5.19), we can deduce the expression for the diffusion layer thickness as 

The thickness increases with tl/2 hence the problems of natural convection, etc. at large t. The mass transfer coefficient is (compare with (5.40)) for c0 = 0 

The smaller 6 the larger the concentration gradient at the electrode surface, leading to higher currents. It is also useful in certain investigations that 6 is independent of t. Both these conditions can be satisfied by microelectrodes. They can also be obtained by imposition of forced convection at larger electrodes—hydrodynamic systems. 

In a fluid where there is both convection and diffusion—a hydrodynamic system—the flux is given by 

The forms of this equation in the three coordinate systems can be found in Table 5.3. 

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