Nernst equation iron couple

This standard potential is for an OH concentration of 1 mol-L 1, which corresponds to pH = 14, a strongly basic solution. However, from the Nernst equation, we can calculate that, at pH = 7, this couple has E = —0.42 V. Any metal with a standard potential more negative than —0.42 V can therefore reduce water at pH = 7 that is, at this pH, any such metal can be oxidized by water. Because E° = — 0.44 V for Fe2+(aq) 4- 2 e Fe(s), iron has only a very slight tendency to be oxidized by water at pH = 7. For this reason, iron can be used for pipes in water supply systems and can be stored in oxygen-free water without rusting (Fig. 12.17). 

A redox couple that is wholly in solution can be analysed without recourse to a redox electrode - indeed, in the example given here, analysis with an iron rod would complicate the situation since the Fe " ", Fe " " system itself obeys the Nernst equation (equation (3.8)). 

Because electrode potentials are defined with reference to the H+/H2 electrode under standard conditions, E° values apply implicitly to (hypothetically ideal) acidic solutions in which the hydrogen ion concentration is 1 mol kg-1. Such E° values are therefore tabulated in Appendix D under the heading Acidic Solutions. Appendix D also lists electrode potentials for basic solutions, meaning solutions in which the hydroxide ion concentration is 1.0 mol kg-1. The conversion of E° values to those appropriate for basic solutions is effected with the Nernst equation (Eq. 15.15), in which the hydrogen ion concentration (if it appears) is set to 1.0 x 10-14 mol kg-1 and the identity and concentrations of other solute species are adjusted for pH 14. For example, for the Fc3+/2+ couple in a basic medium, the relevant forms of iron(III) and iron(II) are the solid hydroxides, and the concentrations of Fe3+ (aq) and Fe2+ (aq) to be inserted into the Nernst equation are those determined for pH 14 by the solubility products of Fe(OH)3(s) and Fe(OH)2(s), respectively. Examples of calculations of electrode potentials for nonstandard pH values are given in Sections 15.2 and 15.3. 

Applying the Nernst equation for the iron(III)/ iron(II) couple gives a value for the potential of the system directly. Using the couple cerium(IV)/ cerium(III) would give the same answer, but it would first be necessary to calculate a value for the concentration of cerium(IV), which in turn would require evaluation of the equilibrium constant for the reaction. 

Here, the iron(II) concentration is negligible with respect to the analytical concen-tration.s of the two cerium species. Substitution into the Nernst equation for the cerium couple gives... 

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