Other Predominance Area Diagrams

For example, let us consider such a predominance area diagram for aqueous bromine species at 25° in which the diagram axes are pH and the negative log of the bromide concentration. 

We will use a similar approach to that used for constructing pe-pH diagrams. Lines representing equality of concentration between significant species are sought in the form of equations containing [Br ] and pH the two axes of the diagram. The species are 

To derive the relationship for HOBr and Bra it is necessary to combine equations (1) and (3). In logarithmic form we have 

Combining the relationship derived above for [HOBr] and [Brs ] with equation (2), we obtain 

This equation plots in the region where [HOBr] predominates over [OBr ] so that it does not represent a significant equality and is therefore not shown in Fig. 7-5. 

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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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