Biological Oxidation-Reduction Reactions:- Reduction Potentials Measure Affinity for Electrons
When two conjugate redox pairs are together in solution, electron transfer from the electron donor of one pair to the electron acceptor of the other may proceed spontaneously. The tendency for such a reaction de pends on the relative affinity of the electron acceptor of each redox pair for electrons. The standard reduction potential, E0,a measure (in volts) of this affinity, can be determined in an experiment such as that described in Figure 13–14. Electrochemists have chosen as a standard of reference the half-reaction
The electrode at which this half-reaction occurs (called a half-cell) is arbitrarily assigned a standard reduction potential of 0.00 V. When this hydrogen electrode is connected through an external circuit to another half-cell in which an oxidized species and its corresponding reduced species are present at standard concentrations (each solute at 1 M, each gas at 101.3 kPa), electrons tend to flow through the external circuit from the half-cell of lower standard reduction potential to the half-cell of higher standard reduction potential. By convention, the half-cell with the stronger tendency to acquire electrons is assigned a positive value of E0. The reduction potential of a half-cell depends not only on the chemical species present but also on their activities, approximated by their concentrations. About a century ago, Walther Nernst derived an equation that relates standard reduction potential (E0) to the reduction potential (E0) at any concentration of oxidized and reduced species in the cell:
where R and T have their usual meanings, n is the number of electrons transferred per molecule, and 
is the Faraday constant (Table 13–1). At 298 K (25 C), this expression reduces to
Many half-reactions of interest to biochemists in volve protons. As in the definition of ΔG0, biochemists define the standard state for oxidation-reduction reactions as pH 7 and express reduction potential as E0, the standard reduction potential at pH 7. The standard re duction potentials given in Table 13–7 and used throughout this book are values for E and are therefore valid only for systems at neutral pH. Each value represents the potential difference when the conjugate redox pair, at 1 M concentrations and pH 7, is connected with the standard (pH 0) hydrogen electrode. Notice in Table Reference cell of known emf: the hydrogen electrode in which H2 gas at 101.3 kPa is equilibrated at the electrode with 1 M H Test cell containing 1 M concentrations of the oxidized and reduced species of the redox pair to be examined 13–7 that when the conjugate pair 2H /H2 at pH 7 is connected with the standard hydrogen electrode (pH 0), electrons tend to flow from the pH 7 cell to the standard (pH 0) cell; the measured E0 for the 2H+/H2 pair is -0.414 V.
FIGURE 13–13 Oxidation states of carbon in the biosphere. The oxidation states are illustrated with some representative compounds. Focus on the red carbon atom and its bonding electrons. When this carbon is bonded to the less electronegative H atom, both bonding electrons (red) are assigned to the carbon. When carbon is bonded to another carbon, bonding electrons are shared equally, so one of the two electrons is assigned to the red carbon. When the red carbon is bonded to the more electronegative O atom, the bonding electrons are assigned to the oxygen. The number to the right of each compound is the number of electrons “owned” by the red carbon, a rough ex pression of the oxidation state of that carbon. When the red carbon undergoes oxidation (loses electrons), the number gets smaller. Thus the oxidation state increases from top to bottom of the list.
FIGURE 13–14 Measurement of the standard reduction potential (E0) of a redox pair. Electrons flow from the test electrode to the reference electrode, or vice versa. The ultimate reference half-cell is the hydrogen electrode, as shown here, at pH 0. The electromotive force (emf) of this electrode is designated 0.00 V. At pH 7 in the test cell, E for the hydrogen electrode is -0.414 V. The direction of electron flow depends on the relative electron “pressure” or potential of the two cells. A salt bridge containing a saturated KCl solution provides a path for counter-ion movement between the test cell and the reference cell. From the observed emf and the known emf of the reference cell, the experimenter can find the emf of the test cell containing the redox pair. The cell that gains electrons has, by convention, the more positive reduction potential.
