The pe-pC Diagrams

Let us first examine an aqueous solution at 25°C in which the halfreaction between Fe + and Fe governs the redox conditions. The solution is maintained at pH 2 so that Fe and Fe are the major ferric and ferrous iron species, respectively, and the formation of hydrolysis products is negligible. There is a total analytical concentration of iron of Cj.Fe = 10- M. 

We proceed with the construction of this diagram using methods similar to those employed for preparing acid-base equilibrium diagrams. Although we use pe in our development of the diagram (Fig. 7-3), we can show EH versus pC too, since pe has a fixed relationship to E, for example, at 25 C, pe= 16.9E (Eq. 7-19). 

Like the pC-pH diagrams, the first equation we plot is the mass balance, which in logarithmic form is 

Now we will develop the -log concentration versus pe lines for Fe and Fe. In logarithmic form Eq, 7-22 becomes 

By a similar type of reasoning, a line for —log [Fe ] versus pe and —log [Fe versus pe can be developed in the region where pe pe°. These are plotted in Fig. 7-3 as lines 4 and 5, respectively. 

---

The pe-pC diagram conveys the idea that both Fe and Fe + have regions where they can be the predominant species in water at pH 2. However, if Fe + is to predominate and be in equilibrium with the Oa/HgO couple, the solution must essentially be stripped of dissolved oxygen. For example, at pe values of less than 13, Fe will predominate whep the oxygen partial pressure in equilibrium with the water couple is less than 10-22... 

---