Effect of pH on Electrode Potential

A particularly important complex interaction with metal ions is the formation of hydroxo complexes in aqueous solutions. In an aqueous solution, OH ions are always present and their concentration is related to the pH value. Hence, the presence of hydroxo complexes is pH dependent and, depending on stability constants, there are pH regions in which free metal ions are practically nonexistent (cf. Pourbaix diagrams). 

One should note, however, that for electrode processes, including those determining corrosion, the relevant equilibria are those of species present at the electrode surface and, hence, their pH limits as well. Drazic has shown recently that those limits can be significantly shifted on the pH scale if energies of adsorption of those species are taken into account in the calculation of stability constants (fig. 56).  

Likely Mechanisms of Metai Ion Discharge and Their Kinetic Consequences 

These events are (a) transport of the reacting species from the bulk of solution to the position at which charge transfer takes place (b) the electrode process itself, i.e., the act of reaction or a reaction sequence leading to the change of the valency state of the metal, as indicated by the stoichiometry of the electrochemical reaction and (c) a sequence of events in the transformation 

This structure of the electrode process determines the electrical properties of the interface, i.e., the electrode impedance, the responses to electrical perturbations, and the current densities vs. potential relationship, which are normally measured in contemporary experimental electrochemistry. 

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A good representation of the effect of pH on electrode potential is due to Pourbaix. A Pourbaix diagram, such as the one shown in Figure 5, defines... 

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