The Reactivity of Selenite toward Methyl Viologen in Mildly Acidic Aqueous Solutions

Abstract

The chemical and electrochemical reactivity of selenite, SeO32−, toward methyl viologen, MV2+, has been examined in mildly acidic (pH 4) aqueous electrolytes. Solutions containing these two species heated at 60°C for ca. 2 hours, were found to yield both 1-methyl-[4,4’-bipyridine]-1-ium chloride and elemental Se as the main reaction products, as evidenced from information derived from mass spectrometric and spectroscopic techniques. Cyclic voltammetric measurements recorded in acetate buffer (pH 4) with a glassy carbon, GC, electrode bearing a self-assembled monolayer, SAM, of N-ethyl-N’-octadecyl-4,4’-bipyridinium (EOB), which had been exposed to a 100 mM SeO32− solution in aqueous acetate buffer (pH 4) for over 200 min and then rinsed with ultrapure water, displayed a significantly smaller redox peak ascribed to the reduction of the viologen moiety in the EOB SAM. This observation is consistent with the loss of the redox active species within the layer and supports the view that SeO32− can cleave the N-C bond in EOB leading to the release of the viologen group into the solution. Cyclic voltammetric experiments involving a freshly prepared EOB SAM on GC in 1 mM SeO32− in the same buffer yielded a large reduction current with an onset potential virtually identical to that associated with the reduction of the EOB SAM in SeO32−-free solutions. On this basis, the extraordinary shift in the onset of SeO32− reduction induced by the presence of MV2+ in the same electrolyte reported by Koshikumo et al. [Electrochemistry (Tokyo, Jpn.), 81(5), 350 (2013)], may be explained by invoking formation of MV2+ aggregates on the GC surface in amounts too small to be detected by conventional voltammetric techniques.