Comment on using cyclic voltammetry to determine a standard potential of a reversible redox couple involving oxidation or reduction of a gas

There is a common practice to equate the midpoint potential from cyclic voltammetry (CV) for a reversible redox couple that involves oxidation or reduction of a gas to the formal potential, which is equal to the standard potential in the limit of infinite dilution (when all activity coefficients are one).We show that in order to use CV to approximate a standard potential of a redox couple that includes a gas, a correction term equal to (RT/F)ln(1/kH cp) must be applied, where kH cp is the Henry's law constant in the units of M/bar. The correction term arises from the difference in the standard state used to define a standard potential (1 bar for a gas in the gas phase) and the condition of equality of standard concentrations for electroactive species in solution from CV (1 M for a soluble component of a gas dissolved in a liquid). We illustrate the significance of this correction for the O2 electrode that exceeds 120 mV for the most common electrolyte solvents. Ignoring this requirement leads to systematic errors in the reported standard electrode potentials measured by CV