Complex Electrochemical Reactions

At low values of polarization it will suffice to retain the first two terms of series expansions of the exponentials (i.e., to assume that 1 + AEIRT and y = 1 AEIRT). When substituting these values into Eq. (13.25) and taking into 

This equation is of the general form i = KAEI(M + NAE), where K, M, and N are constants. Derivative dildAE of this function has the value KMliM + NAE) and in the particular case where A = 0 the derivative has the value KIM. Thus, when allowing for Eq. (13.17) we find for polarization resistance p, which for AE = 0 is equal to dE/di  

It follows that from the slope of the linear section in the polarization curve close to the equilibrium potential, we can determine the exchange CD of the overall reaction. 

in the case of two-step reactions, different methods of determining the exchange CD generally yield different results (in contrast to the case of simple reactions discussed earlier) Extrapolation of the limiting anodic and cathodic sections of the semilogarithmic plots yields values and if, respectively, while the slope of the linear section in an ordinary plot of the polarization curve yields the value of ig. It is typical for multistep reactions that the exchange CD determined by these methods differ. 

The exchange CD determined by different methods will coincide only in the case of quasi-one-step reactions mentioned above. Thus, when the value of if is so much higher than if that the extreme anodic section cannot be measured and there is no break in the polarization curve, all three methods of determination lead to the same value of if. This implies that step 1 has no effect at all on the kinetics of the overall reaction and that its (high) exchange CD cannot be determined. The same conclusion holds in the opposite case of if s if. 

---

The degree to which an electrode will influence the reaction rates is different for different electrochemical reactions, hi complex electrochemical reactions having parallel pathways, such as a reaction involving organic substances, the electrode material might selectively influence the rates of certain individual steps and thus influence the selectivity of the reaction (i.e., the overall direction of the reaction and the relative yields of primary and secondary reaction products). 

The simplest type of complex electrochemical reactions consists of two steps, at least one of which must be a charge-transfer reaction. We now consider two consecutive electron-transfer reactions of the type ... 

Single Pulse Voltammetry Non-reversible and Complex Electrochemical Reactions... 

---