The pe-pH Predominance Area Diagrams

The construction of a pe-pH or E/ pH diagram is the next step in complexity in the graphical representation of redox equilibria. In such diagrams we establish areas of predominance in a pe-pH coordinate system for various species involved in redox, acid-base, precipitation, and complexation equilibria. 

We will illustrate the construction of a pe-pH diagram for the aqueous chlorine system. For the construction of such diagrams we must first state the ground rules for drawing boundaries between any two species. There are two basic rules  

Boundaries will only be drawn between the two major species under a given set of conditions. 

A boundary will be drawn between two species at the place where the concentration of the two species is equal. Thus, on one side of the boundary, one of the species predominates in concentration 

The first thing to consider is the stability domain of water. For the halfreaction, 

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In this section we will first present the development of a simple diagram (the so-called pe-pC or E -pC diagram) for a reaction that involves only electron transfer in a homogeneous system. Next we will consider the predominance area diagrams for presenting combined redox and acid-base equilibria. Finally, we will add a third dimension and consider systems with multiple phases using the pe-pH predominance area diagram. 

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