Some Examples of Potential-pH Diagrams

A very useful method of describing die stability of metals in aqueous solutions is the potential-pH diagrams introduced by Pourbaix. These equilibrium diagrams relate the reversible potentials of reactions of interest in corrosion studies to the pH and the concentration of different ionic species in solution. We shall use a number of examples to illustrate the principles involved, starting with the most basic diagram relating to water and some of the ionic and molecular species at equilibrium with it. 

To construct such diagrams, one has to identify the chemical and electrochemical reactions of interest and write the appropriate chemical equilibria and Nernst equations, respectively.  

In the following discussion all potentials are given against the standard hydrogen electrode (SHE), and the concentrations are written in square brackets, for clarity. The three most important equilibria for water are given below  

7) Unless otherwise stated, concentrations and partial pressures, instead of activities and fugacities, respectively, are used here and in all following equations, for simplicity. 

The region of stability of water at 25 °C is 1.229 V, independent of pH, since the reversible potentials for hydrogen and oxygen evolution change with pH in the same manner. This, incidentally, is the potential region in which the hydrogen/ oxygen fuel cell can operate. Thermodynamic considerations lead us to the 

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