Potential of a Solution Containing Only One Redox Couple

2 Potential of a Solution Containing Only One Redox Couple 

Calculating the potential of a solution that contains only one redox couple is the simplest case. The potential is given by Nemst s law. For example, for a solution containing ferrous and ferric ions, the equilibrium potential is given by 

A particular case is that in which only one of the members of the couple Red/Ox exists alone in the solution. When Fe + is alone, a brutal application of Nernst s equation gives 

However, a platinum electrode dipping into such a solution exhibits a finite potential value. The dissolution of a pure ferrous salt may, in a first stage, lead to such a weak potential value of the solution that water itself is reduced in a second stage. Therefore, water plays the role of an oxidant, of course. As a result, traces of ferric ions are formed. Both forms of the couple are present and the solution exhibits a finite potential value predicted by Nernst s law, which is that found experimentally. Experience indicates that the solution potential, in this case, is not lower than 0.50 V (E° = 0.77 V). Inversely, when the solution contains only ferric ions, the potential does not tend toward +oo as expected. In this case, water plays the role of a reductor. Traces of Fe + are formed. The potential value the solution takes does not exceed about 1.05 V. 

These particular cases correspond to the first point of a redox titration. This point is, of course, not exploitable. 

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