Redox as Eh and the Standard Hydrogen Electrode SHE

A reaction demonstrating the conversion of an oxidized species to a reduced species without forming any precipitate is shown below  

Reaction 5.6 is known as a half-cell reaction to move from left to right, there should be an electron donor. This electron donor is the hydrogen half-cell reaction  

This reaction s redox level can be expressed in volts of Eh, which is obtained by 

Eh = ° + 0.059/n log(acti vity of oxidized species/activity of reduced species)  

This reaction shows that Eh is related to the standard potential ( ° - 0.77 V) and the proportionality of Fe3+ to Fe2+ in the solution phase. A number of J5° values representing various reactions in soils are given in Tables 5.2 and 5.3. Note that Equation 5.16 is analogous to the Henderson-Hasselbalch equation. A plot of Eh versus (Fe3+/Fe3+ + Fe2+) would produce a sigmoidal line with midpoint ° (Fig. 5.2) (recall that the Henderson-Hasselbalch equation gives the pKa at the titration midpoint see Chapter 1). To the left of ° (midpoint in the jc axis E° = 0.77 V) the reduced species (e.g., Fe2+) predominates, whereas to the right of ° (midpoint in the x axis), the oxidized species (e.g., Fe3+) predominates. 

---