Electrochemistry of selected quinones at immiscible n-octyl-2-pyrrolidone/aqueous interface using a three-phase electrode system

Highlights: • We measured redox behaviour of 1-aminoanthraquinone (AQ) and 2,3-dichloro-1,4-naphthoquinone (NQ) at a three-phase-electrode. • Reduction of AQ and NQ is accompanied with transfer of cations from the aqueous to the organic phase. • The reduction potential varies with ionic potential of the transferred ion, not with hydrophobicity. • This is due to ion-pair formation between the quinone and the transferred cations. -- Abstract: Both endogenous and synthetic (e.g. drugs) quinones are essential functional moieties in various biological systems and their activity is mainly governed by their electrochemical properties. In recent years they are also more readily applied in novel energy storage devices, such as batteries and solar cells. In this work, the redox behaviour of 1-aminoanthraquinone (AQ), and 2,3-dichloro-1,4-naphthoquinone (NQ) is studied at a three-phase junction formed by n-octyl-2-pyrrolidone and aqueous electrolyte solution on a glassy carbon working electrode. We show that the two quinones behave quite differently when undergoing reduction at the three-phase junction. AQ underwent a 1-step, 2-electron reduction resulting in transfer of cations from the aqueous phase, while reduction of NQ occurred in two steps, the first of which was accompanied by a transfer of the NQ^{• −} radical to the interface with the reduction potential dependent on the anion present in the aqueous phase due to salting out effects. After that, the quinone was able to undergo a second reduction process forming a dianion which similarly to AQ resulted in a transfer of cations from the aqueous phase. Importantly, the cation transfer potential is determined by ion-pair formation with the quinone, rather than the solvation energy of cation in the pure organic solvent.