Origin and Interpretation of Pourbaix Diagrams

The equilibrium electrochemistry of an element in aqueous solution can be represented graphically using coordinates of equilibrium half-cell potential, E, and pH. These graphical representations, known 

A somewhat simplified Pourbaix diagram for the iron/water system is shown in Fig. 2.11. In this case, the possible solid phases are restricted to metallic iron, Fe304, and Fe203. A more detailed diagram and a diagram with Fe(OH)2 and Fe(OH)3 are shown subsequently. 

Interpretation of the Pourbaix diagram in Fig. 2.11 requires discussion of the experimental conditions under which, at least in principle, it would be determined. The coordinates are pH and electrode potential, and it is implied that each of these may be established experimentally. Their values will locate a point on the diagram, and from this point the equilibrium state of the system is determined. It is assumed that the pH may be established by appropriate additions of an acid or base. 

To establish any predetermined electrode potential, the experimental arrangement shown in Fig. 2.12 is used. The components and their functions include  

11 Simplified Pourbaix diagram for the iron/water system (iron/iron-oxides). Source Ref 9 

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