Redox as Eh in the Presence of Solid Phases

Consider a reaction where two solid phases are involved. Thus, in addition to electron transfer (redox process), one of the solid phases decomposes while the other one forms. For example, 

Reaction 5.25 is a half-cell reaction. In order for this half-cell reaction to move from left to right, there should be an electron sink. In soils, a most common electron sink is 02, although other soil minerals, as we shall see later in this chapter, can act as electron sinks (see also Chapter 6). The equilibrium expression (/ eq) for Reaction 5.25 is 

In this case, H+ activity cannot be set to 1 because the two solids, Fe(OH)3(s) and FeC03(s), cannot persist at such low pH. Unit activity, however, is assigned to all other 

For natural systems, a more accurate representation is HC03 = 10-3 M. From Equation 5.27, ° = - 24.82 kcal mol-V (l)(23.06) = - 1.08 V. By replacing the -AGInF terms of Equation 5.28 with Eh terms (see Equation 5.27), 

Using R, standard temperature (25°C), n = 1, and Faraday s constant gives 

---