Voltammetry of quinones in unbuffered aqueous solution: reassessing the roles of proton transfer and hydrogen bonding in the aqueous electrochemistry of quinones.

Cyclic voltammetry studies are reported for two representative quinones, benzoquinone and 2-anthraquinonesulfonate, in buffered and unbuffered aqueous solution at different pH's. While the redox reaction of quinones in buffered water is well described as an overall 2 e-, 2 H+ reduction to make the hydroquinone, a much better description of the overall reaction in unbuffered water is as a 2 e- reduction to make the strongly hydrogen-bonded quinone dianion, which will exist in water as an equilibrium mixture of protonation states. This description helps to unify quinone electrochemistry by bridging the apparent gap between the redox chemistry of quinones in water and that in aprotic organic solvents, where quinones undergo two sequential 1 e- reductions to form the quinone dianion.