Pourbaix Eh-pH Diagrams

The ranges of Eh and pH over which a particular chemical species is thermodynamically expected to be dominant in a given aqueous system can be displayed graphically as stability fields in a Pourbaix diagram. These are constructed with the aid of the Nernst equation, together with the solubility products of any solid phases involved, for certain specified activities of the reactants. For example, the stability field of liquid water under standard conditions (partial pressures of H2 and O2 of 1 bar, at 25 °C) is delineated in Fig. 15.2 by 

For nonstandard partial pressures of the gases, these boundaries will be slightly displaced, but their slopes will remain the same. In Fig. 15.2, the stability field of water is only slightly narrowed by considering gas pressures a millionfold lower. 

The h-pH relations for the important iron-water system at 25 °C are summarized in Fig. 15.3 with some simplifications. First, it is assumed that no elements other than Fe, 0, and H are involved in a natural water system, the presence of CO2 would oblige us to include FeCOa (siderite), and sulfur compounds could lead to precipitation of iron sulfides in certain lh pH regimes. As it is, the only Fe-O-H solids we have considered are Fe metal, Fe(OH)2, and Fe(OH)3, whereas in practice magnetite (Fe304), hematite (Q-Fe203), goethite [a-FeO(OH)], and other Fe-O-H phases could be present. Indeed, our choice of solubility products for Fe(OH)2 and 

Fe(OH)3 is somewhat arbitrary, since these materials become less soluble as they age, that is, become better crystallized as time passes here, mid-range values are chosen. Furthermore, the hydrolytic sequence given in Fig. 13.6 for iron(III) has been ignored. A small modification is needed to take this into account, but for simplicity we may represent the hydrolysis of Fe +(aq) as leading directly to Fe(OH)3(s). Finally, certain of the boundary lines in Fig. 15.3 refer to a specific activity of aqueous iron ions. This is arbitrarily set at 10 mol (and, for convenience, activity is equated with concentration), but comparison with Fergusson for an iron ion activity of 10 shows that the qualitative features of the diagram are not very sensitive to this variable. 

When interpreting a Pourbaix diagram, one must not infer that a particular species cannot exist outside its stability field. The field boundaries represent contours at which that species and an alternative are equally important outside its stability field, that species is no longer dominant, and 

---

Figure 5.3 Pourbaix EH-pH diagram for sulfur-water system at 25°C and I atm. The shaded region represents experimental measurements for the test solutions 3.5wt.% NaCI + WOOppmw ABS, I wt.% NaCI + 1000 ppmw ABS, 3.5 wt. % NaCI + 500 ppmw ABS, deionized water + 1000 ppmw ABS, and I wt %NaCI + 500 ppmw ABS. (Reprinted from Lasebikan et a., 2011, with permission from Elsevier.)...

---