Superposition of Concentration and Activation Polarization

We can also calculate the individual values of (with the equations reported in Section 5.2) and of by determining the concentration distribution in the diffusion layer, and from this, the distribution of solution conductivity. The resulting combined value of q coincides with the value determined from Eq. (6.32). 

The kinetic and polarization equations described in Sections 6.1 and 6.2 have been derived under the assumption that the component concentrations do not change during the reaction. Therefore, the current density appearing in these equations is the kinetic current density 4. Similarly, the current density appearing in the equations of Section 6.3 is the diffusion current density When the two types of polarization are effective simultaneously, the real current density i (Fig. 6.6, curve 3) will be smaller than current densities and ij (Fig. 6.6, curves 1 and 2) for a given value of polarization. 

The kinetic equations describing the joint effects of activation and concentration polarization are very complex and we shall consider only the the case of a simple first-order reaction of the type (6.2) proceeding in the presence in the solntion of an excess of a foreign electrolyte. To simplify the appearance of these equations (which even in this case are very cnmbersome), in this section we use a more compact notation that contains two new kinetic parameters  

A generalized rate constant that inclndes the potential dependence of the reaction rate  

In the case considered in this section of a joint action of concentration and activation polarization, in the polarization equation (6.10) we must take into account the concentration changes of the rectants near the electrode surface  

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