Electrode Reactions of Insoluble Salts

Most frequently, this electrode mechanism has been analyzed in connection with a mercury electrode hence the following reaction schemes are pertinent to this electrode. Note, that the electrosorption mechanism can serve as a theoretical basis for these processes as weU [135,139,140]. The simplest case of an accumula-tion/stripping equilibrium is given by the following equation  

Assuming that a mercurous salt is formed, reaction (2.205) can be rewritten as  

Reactions (2.205) and (2.206) are called second-order cathodic stripping reactions [134]. If the reacting ligand has a tendency to adsorb on the electrode surface, the following mechanisms are encountered [136,137]  

Reaction (2.208) is a first-order cathodic stripping reaction with adsorption of the ligand [136], whereas reaction (2.210) is of second order [137]. Considering a mercurous salt formation, reaction (2.210) is written in the following form  

For all reactions, the mass transport regime is controlled by the diffusion of the reacting ligand only, as the mercury electrode serves as an inexhaustible source for mercury ions. Hence, with respect to the mathematical modeling, reactions (2.205) and (2.206) are identical. This also holds true for reactions (2.210) and (2.211). Furthermore, it is assumed that the electrode surface is covered by a sub-monomolecular film without interactions between the deposited particles. For reactions (2.207) and (2.209) the ligand adsorption obeys a linear adsorption isotherm. Assuming semi-infinite diffusion at a planar electrode, the general mathematical model is defined as follows  

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