Determination of the Mechanism by Reaction Orders

The order of the reaction with respect of species Sj is often used in the formal kinetics to elucidate the mechanism of the reaction. Similar approach is also employed in electrochemical kinetics. In so doing, it is necessary to keep in mind that the same component Sj takes part in two, anodic and cathodic, electrochemical processes. Therefore, two orders, Zgj and Zqj, respectively can be attributed to Sj-, besides, these quantities can be and often are different [1-3]. Some methods based on analysis of the reaction orders are considered below. 

According to Vetter [1, 3, 4], having determined the orders of electrochemical reactions of all substances Sy that are present in the solution independently of one another, it is possible to unambiguously determine the total formulas Sq, and Sg of the substances, which immediately take part in the charge transfer process. The following relationships are suitable for this purpose  

Several methods were proposed for determining the orders of reactions. One of them [3] deals with analysis of the dependence of a steady-state current density on concentration of the component Sj. The necessary condition that is imposed in this case requires keeping constant concentrations of the rest components, when the amount of Sj is varied. The analysis must be carried out at constant potential in the Tafel region. Changing concentration of one of the substances 5. and maintaining all others constant, relevant orders of the reaction can be determined from the changes of anodic or cathodic current densities at = const  

In another case, Vetter [2,3] proposed to analyze analogous dependencies of the exchange current density ig- This method requires additional information about the charge transfer coefficients and a, as well as stoichiometric coefficients Vy, involving in the net electrochemical process  

1) Stoichiometric coefficients ofthe reduced species are regarded tx be negative. 

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