Half Wave Potential (E_1/2)

In a CV containing a feature that has a forward and reverse wave, regardless of whether it is reversible or irreversible, E_1/2 defines the potential exactly in the middle of the two peaks. For example, if the wave that corresponds to the reduction of X to X^– has an E_p = –0.400 V and the wave that corresponds to the oxidation of X^– back to X has an E_p = +0.200 V, E_1/2 for the X/X^– redox couple is –0.100 V. It is important to report these values relative to the specific reference electrode in use, as they are meaningless without it. For a reversible, one electron wave in a CV measured at room temperature, the two E_p’s should be ~60 mV apart, meaning the E_1/2 should be ~30 mV positive of the E_pc and ~30 mV negative of the E_pa.

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An analyte’s standard potential (E⁰) is the E_1/2 under standard conditions, or 1 M concentration of analyte, 20 or 25 °C (depending on who you ask), and atmospheric pressure. For a given redox couple, the E_1/2 is equal to E⁰ if one assumes that the reduced and oxidized species have the same diffusion coefficients and move at a similar speed through solution. One may hear the E_1/2 described as the “standard potential,” the “standard reduction potential,” or the “redox potential,” but the term “redox potential” should be avoided.

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