The HOH E-pH Diagram

Since E-pH diagrams involve the pH which is a representation of the hydrogen ion concentration, the first of these equations is preferred. 

Once again, the equation showing H+ is preferred because one of the variables for the E-pH diagram is the pH. 

As is the pattern with other E-pH diagrams the area above the top dotted line shows the predominance of O2, the area below the lower dashed line shows the predominance of H2, and the area in between the dashed lines shows the predominance of HOH. In other words, at high potentials water decomposes into O2 and at low potentials water decomposes into H2. Values of E at H concentrations other than 1.00 M may be readily calculated by introducing the above equations and the appropriate E° values into the Nernst relation. 

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Figure 3.14 displays commonly used oxidants and reductants along with their potentials. Accompanying the redox couples is the HOH E-pH diagram since it represents the medium to which they are referred. 

These vertical cuts are called electron ladders. Rungs are placed on them at the proper E values, and proper species are indicated below and above the rungs. These are shown in Figure 3.3. The exact values of E for the HOH ladder have been calculated from the equations given above, and that for the Mg ladder from E = —2.36 + 0.030 log [Mg+ ], which is the equation that describes the line between Mg+ and Mg on the Mg E-pH diagram. 

Consider the electron ladder in Figure 3.13. This ladder was abstracted from the E-pH diagrams of HOH and Cr with all soluble species at 10°-° M and a... 

The sulfate species occupy considerable area on the E-pH diagram, including a large overlap with the HOH region, indicating that they are quite stable toward oxidation or reduction. 

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