Graphical Representation of Redox Equilibria

Let us asume that the stabilization pond effluent is in equilibrium with the normal atmosphere that contains 21 percent oxygen. Knowing that the pH is 7,8, we find that 

For the oxygen reduction half-react ion, applying Eq. 7-8, we obtain 

Graphical presentation of redox equilibria, like the graphical treatment of acid-base, complexation, and precipitation equilibria is helpful in understanding complicated problems and in obtaining approximate solutions to equilibrium questions. For redox systems in natural waters the equilibrium condition is truly a boundary condition. In many cases, natural systems are not at equilibrium from a redox standpoint. The diagrams usually present an idea of what is possible, not necessarily of the existing or imminent situation. The graphical presentations of redox equilibria are seldom simple because redox reactions usually involve 

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. 

---

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. 

In this chapter we will examine oxidation-reduction stoichiometry, equilibria, and the graphical representation of simple and complex equilibria, and the rate of oxidation-reduction reactions. The applications of redox reactions to natural waters will be presented in the context of a discussion of iron chemistry the subject of corrosion will provide a vehicle for a discussion of the application of electrochemical processes a presentation of chlorine chemistry will include a discussion of the kinetics of redox reactions and the reactions of chlorine with organic matter finally, the application of redox reactions to various measurement methods will be discussed using electrochemical instruments as examples. 

The concept of electron activity is used in the description of oxidation-reduction equilibria and especially in the solution of problems that involve both redox and other equilibria such as acid-base, complexation, and so forth. In addition, this concept provides a useful basis for the graphical representation of complicated redox equilibria. The approach is extremely useful when working with redox equilibria in natural waters. 

---