E-pH Diagram of Water

The following example illustrates how the stability or predominance diagram of water can be constructed from its basic thermodynamic information. Equation (4.33) describes the equilibrium between hydrogen ions and hydrogen gas in an aqueous environment  

Adding sufficient OH to both sides of reaction in Eq. (4.33) results in Eq. (4.34) in neutral or alkaline solutions  

At higher pH than neutral, Eq. (4.34) is a more appropriate representation. However, since the concentrations of H and OH ions are related by the dissociation constant of water expressed in Eq. (4.35), Eq. (4.33) and (4.34) basically represent the same reaction for which the thermodynamic behavior can be expressed by Nernst equation. 

Equation (4.33) and its alkaline or basic form, Eq. (4.34), delineate the stability of water in a reducing environment and are represented in a graphical form by the sloping line (a) on the Pourbaix diagram in Fig. 4.10. Below the equilibrium reaction shown as line (a) in this figure, the decomposition of H O into hydrogen is favored while water is thermodynamically stable above the same line (a). As potential becomes more positive or noble, water can be decomposed into its other constituent, oxygen, as illustrated in Eqs. (4.38) and (4.39) for respectively the acidic form and neutral or basic form of the same process. 

And again these equations are equivalent and only reflect the pH condition of the environment. The corresponding Nernst expression 

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