Metal ion transfer in a series of two elementary steps

For some metals the transfer of metallic ions involves a reaction intermediate of an adsorbed metallic ion complex which is coordinated with anionic ligands hence, the overall reaction occurs in a series of two elementaiy steps rather than one. Such a multistep transfer of ions can result, in the course of metallic ion transfer, from the reduction of the activation energy for ion transfer due to the formation of adsorbed intermediates. We examine a transfer reaction of divalent metallic ions via an adsorbed complex ion according to the steps in Eqn. 9-13  

The anodic and the cathodic currents i and i i for the formation of the adsorbed ion complex (Eqn. 9-13a) are given, respectively, in Eqns. 9-14 and 9-15  

In the rate equations, Eqn. 9-14 through Eqn. 9-18, we have assumed that Langmiiir s adsorption isotherm holds this assumption applies to the range of low adsorption coverages. 

If the anodic anion transfer (anionic adsorption, Eqn. 9-13a) to form an adsorbed metallic ion complex is the rate-determining step, the Tafel constant, a = 1 - p, win be obtained from Eqn. 9-14. If the anodic transfer of the adsorbed metallic ion complex (desorption of complexes, Eqn. 9-13b) is the rate-determining step, the Tafel constant, a = 2 - p, will be obtained from Eqns. 9-16 and 9-17. Similarly, if the cathodic anion transfer (anionic desorption, Eqn. 9-13a) is determining the rate, the Tafel constant in the cathodic reaction, a = 1 p, will be obtained from Eqns. 9-15 and 9-16 and if the cathodic transfer of a metallic ion complex (adsorption of complexes, Eqn. 9-13b) is determining the rate, the Tafel constant, a-sp, will be obtained from Eqn. 9-18. In this discussion we have assumed Pi = Ps P then, Eqns. 9-19 and 9-20 follow  

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