The simplified Pourbaix diagram for iron in water and air

The steps to be followed to produce a Pourbaix diagram of the system are as follows. 

The dependence of the cell voltage (which is a measure of the oxidising potential as well as the free energy of the reaction) as a function of the concentrations of the species present is expressed via the Nemst equation as  

Note that as long as the Fe concentration is fairly close to the Fe + concentration this hne will always he close to 0.77 V. o Step 1(b)  

The concentration dependence of the position of this hne can be calculated as foUows. 

To plot this hne, use the equihbrium constant for the reaction, given by 

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