Formal Potential—Use It for Defined Nonstandard Solution Conditions

The E° values listed in Table 12.1 are for the case where all species are at an activity of 1 M. However, the potential of a half-reaction may depend on the conditions of the solution. For example, the E° value for Ce + q Ce is 1.61 V. However, we can change this potential by changing the acid used to acidify the solution. (See Table C.5 in Appendix C.) This change in potential happens because the anion of the acid complexes with the cerium, and the concentration of free cerium ion is thereby reduced. 

If we know the form of the complex, we could write a new half-reaction involving the acid anion and determine an E value for this reaction, keeping the acid and all other species at unit activity. However, the complexes are frequently of unknown composition. So we define the formal potential and designate this as E°. This is the standard potential of a redox couple with the oxidized and reduced forms at 1 M concentrations and with the solution conditions specified. For example, the formal potential of the Ce +/Ce + couple in 1 M HCl is 1.28 V. The Nemst equation is written as usual, using the formal potential in place of the standard potential. Table C.5 lists some formal potentials. 

The formal potential is used when not all species are known. 

Hydrogen or hydroxyl ions are involved in many redox half-reactions. We can change the potential of these redox couples by changing the pH of the solution. Consider the As(V)/As(III) couple  

In strongly acid solution, H3ASO4 wiU oxidize 1 to I2. But in neutral solution, the potential of the As(V)/As(III) couple (E° = 0.146 V) is less than that for 12/ , and the reaction goes in the reverse that is, I2 will oxidize H3ASO3. 

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FORMAL POTENTIAL—USE IT FOR DEFINED NONSTANDARD SOLUTION CONDITIONS... 

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