The Redox Potential of Dithionite and SO−2 from Equilibrium Reactions with Flavodoxins, Methyl Viologen and Hydrogen plus Hydrogenase

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Abstract
It has been shown that redox equilibria can be formed between dithionite ion (plus SO−2) and (bi)sulphite, and the low‐potential electron carriers flavodoxin and methyl viologen. The equilibria were established either by treating the oxidized electron carriers with dithionite, or by treating flavodoxin hydroquinone or methyl viologen semiquinone with (bi)sulphite. Similar redox equilibria were established between dithionite/(bi)sulphite and hydrogen using catalytic amounts of hydrogenase in the presence of a low‐potential electron carrier. The effects of pH and temperature on the equilibria were determined. The equilibria were analyzed to determine the redox potential of the dithionite/(bi)sulphite system. In accordance with the results of earlier kinetic studies, it was assumed that the reductant in dithionite solutions is the dissociation product SO−2. The calculated midpoint redox potential E' for the couple SO−2/HSO−3 at pH 7 and 25 °C was –0.66 V. The reductant is present largely as the dimer at concentrations of dithionite above about 10nM. Consequently, the midpoint potential, Em, of dithionite solutions becomes less negative as the concentration of dithionite is increased (ΔEm/Δlog S2O2−4= 29 mV). The theoretical potential of a solution of 1 M S2O2−4 and 2 M (bi)sulphite at pH 7 was calculated to be –0.386 V. This value is 59 mV more negative than that determined in 1911 by potentiometry, but considerably more positive than other values in the literature. The effects of pH on the equilibria showed that E' is controlled by the pK of (bi)sulphite at 6.9; the slope ΔE'/ΔpH was –59 mV below the pK and – 118 mV above the pK. The effects of temperature on the equilibria suggested that Em for dithionite changed by –1.6 mV/°C for a rise in temperature between 2 °C and 40 °C. If sodium dithionite is contaminated with small amounts of (bi)sulphite, its addition in large excess to a low potential electron carrier can cause oxidation of the carrier.