Redox Theory and Measurement

As a general rule, most reactions that involve electrons also involve protons. Oxidation usually releases protons or acidity. This is a basic cause of acid-mine drainage. Conversely, reduction usually consumes protons, and the pH rises. 

---

The potentials listed in Table 7.1 and plotted in Figure 7.1 are thermodynamic values and do not necessarily correspond to actual values measured in solutions, for reasons that will be discussed in this section. For example, aerobic soil solutions typically have measured values of about 400 to 500 mV, and O2 disappears from the soil at about 350 mV. These are much lower values than expected theoretically, since dissolved O2 should in principle maintain potentials closer to 1000 mV (see Figure 7.1). To understand discrepancies between theory and measurement, some knowledge of the principles involved in redox potential measurement is needed. 

The importance of solvent parameters such as DN and AN and the advantage of their use over physical-electrostatic parameters was further demonstrated by Mayer et al. [21], who studied correlations between the DN and AN of solvents and redox potentials and their chemical equilibrium and ion pair equilibria. According to the Born theory, redox potentials should depend linearly on the reciprocal of the solvent s dielectric constant. Plotting Em values of a redox such as Cd/Cd2+ versus 1/e of the solvents in which it is measured results in a very scattered picture. In contrast, it has been clearly shown by Mayer et al. [15] that redox potentials of metals (e.g., Zn/Zn2+, Cd/Cd2+, Eu/Eu2+) can be nicely... 

Kokholm, G. 1977. Redox Measurements Their Theory and Technique. ST40, Radiometer A/S Emdrupvej 72 DK-2400, Copenhagen NV, Denmark. 

THEORY OF REDOX CONDUCTION AND THE MEASUREMENT OF ELECTRON TRANSPORT RATES THROUGH ELECTROCHEMICALLY ACTIVE BIOFILMS... 

Martin RD, Unwin PR (1998) Theory and experiment for the substrate generation tip collection mode of the scanning electrochemical microscope application as an approach for measuring the diffusion coefficient ratio of a redox couple. Anal Chem 70 (2) 276-284. doi 10.102l/ac97068Ip... 

As the concentration of enzyme in the film is increased from 0 to 65 wt%, the diffusion constant falls by 2 orders of magnitude with a function that is close to exponential. Simplistic application of the Dahms-Ruff theory (which implicitly requires that a "mean-field" approximation holds) would predict a linear dependence on concentration. The latter approximation would require that physical diffusion of the Os sites be rapid compart to electron hopping, which is clearly not the case here. Theory based on rigorously fixed site redox molecules and extended electron transfer (ie. static percolation), would indeed predict an exponential decrease in electron hopping with concentration.(25) However, simulations by Blauch and Saveant for the case of tethered redox sites also leads to a behavior part-way between that predicted by "static percolation" and "mean-field" approximations, resulting in a functional form of close to that seen in the inset of Figure 6. (See Figure 3B of reference 15 where tjtp=0.1). It must be pointed out that the weak dependence of the film thickness, r, on enzyme content (Table I), leads to little electrochemically measurable decrease in site concentration as enzyme is increased. This of course makes a quantitative application of any of the models mentioned above rather difficult. 

---