Standard redox potentials in aqueous solution - Application to surface treatments in aqueous processes

Surface treatments (ST) are primarily carried out in an aqueous medium. All these processes, whether electrolytic or chemical, share electrochemistry as a common denominator and involve redox (oxidation-reduction) reactions. It is therefore clearly important to know the standard potentials of the redox couples involved.

The standard potential of an Ox/Red redox couple, abbreviated as $E_{\text{Ox}/\text{Red}}^0$ , is a thermodynamic quantity characteristic of the redox couple in question. Its value is expressed relative to a reference system (reference electrode) and reflects the oxidizing or reducing capacity of the Ox/Red couple. Thus, the higher the standard potential of the redox couple, the stronger the oxidizing power and the weaker the reducing power. Conversely, the lower the standard potential, the stronger the reducing power and the weaker the oxidizing power. A large number of these standard potentials have been compiled into a table that serves as a valuable and indispensable tool in chemistry for predicting the direction of reactions, even though, as we will see, it is more appropriate to use equilibrium potentials.

For simplicity, the metal reactant is often considered in its simplest cationic form, but in TS environments, it is primarily involved in anionic complexes. The standard potential of this new pair (anionic metal complex/metal) is then referred to as the apparent standard potential of the pair, and takes into account the formation constant of the metal complex in question.

Furthermore, it is necessary to predict the possibility and direction of the redox reaction. The equilibrium potential of a pair, $E_{\text{é}{\text{q}}_{\left(\text{Ox}/\text{Red}\right)}}$ , as given by Nernst's law, allows us to determine, among other things, the direction of the system's evolution. If the value of the applied potential is greater than $E_{\text{é}{\text{q}}_{\left(\text{Ox}/\text{Red}\right)}}$ , the system is subject to oxidation. Conversely, if it is less than $E_{\text{é}{\text{q}}_{\left(\text{Ox}/\text{Red}\right)}}$...