Oxidation-Reduction Potentials

One way to quantify whether a substance is a strong oxidizing agent or a strong reducing agent is to use the oxidation-reduction potential or redox potential. Strong reducing agents can be said to have a high electron-transfer potential. Strong oxidizing agents have low electron-transfer potential. Oxidizing and reducing agents occur as couples, with a strong reducing agent coupled with a weak oxidizing agent and vice versa. Since these processes involve the transfer of electrons, the measurement of the resulting charge separation can be quantified in voltage measured between the couple and a standard hydrogen half-cell with 1 molar hydrogen and 1 atmosphere of hydrogen pressure on it. The redox potential of the hydrogen is zero at pH=0, but for tabulations a pH=7 is used for the hydrogen and under those conditions its redox potential is -0.421 volts.

As a useful reference, the redox potentials of couples that commonly occur in biochemistry can give insight into their roles in biological energy processes. A negative number for this potential indicates a reducing agent whereas a strong oxidizer will have a positive redox potential. Since the obtaining of useful energy in a biological process generally involves oxidation of some foodstuff, it follows that a large negative redox potential like -0.32 V for NAD+ suggests a useful role in producing a reduced product, like NADH. A reduced coenzyme NADH can be oxidized to provide energy for biological processes.

Standard Redox Potentials E'0
Half reaction
Half reaction
Succinate + CO2 + 2H+ +2e-α-ketoglutarate + H2O
Acetate + 2H+ +2e-acetaldehyde
2H+ +2e-H2
α-ketoglutarate + CO2 + 2H+ +2e-isocitrate
Cystine + 2H+ +2e-2 cysteine
NADP+ + 2H+ +2e-NADPH + H+
NAD+ + 2H+ +2e-NADH + H+
Acetaldehyde + 2H+ +2e-ethanol
Pyruvate + 2H+ +2e-lactate
Oxaloacetate + 2H+ +2e-malate
FAD + 2H+ +2e-FADH2
Fumarate + 2H+ +2e-succinate
Ubiquinone + 2H+ +2e-ubiquinol
2 cytochrome b(ox) +2e-2 cytochrome b(red)
2 cytochrome c(ox) +2e-2 cytochrome c(red)
2 cytochrome a3(ox) +2e-2 cytochrome a3(red)
1/2 O2 + 2H+ +2e-H2O

This data is from Karp, Table 5.1 for standard conditions


Chemical concepts

Reduction concepts

Ch 5
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