The pe-pH Diagrams Incorporating Solids

The last step in complexity that we will explore in detail for the construction of diagrams to illustrate redox equilibria involves the addition of heterogeneous equilibria to redox and acid-base equilibrium diagrams. We will illustrate this system with a ps-pH diagram for iron species in aqueous solution containing no anions other than hydroxide, We will expand on this diagram later in this chapter during the discussion of iron chemistry. 

First we must identify which solids and solution components are important. For an unknown system we would achieve this by searching the literature for relevant reactions and equilibrium data. In this example the following components are of interest. 

The task of diagram construction is broken down into four stages. 

Boundaries are constructed between the solids of interest if more than one solid exists. 

Boundaries are constructed between solids and individual solution components. A concentration of dissolved component is selected and the stipulation is made that the solid/solution boundary is valid for the selected concentration of the indicated solution component. For our example we will use a Fe concentration of 10 M (5.6 /ug Fe/ liter) although in the later discussion of iron chemistry several sol id-solution boundaries are entered on a single diagram to represent different solution concentrations of Fe species at the boundary. 

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