Electrochemical oscillations and bistability in the redox processes of mercury ions, coupled with the self-induced convection of Hg surface

The negative differential resistance (NDR), a typical source of electrochemical instabilities, is usually caused by increasing adsorption of an inhibitor or desorption of the catalyst from the electrode surface. In this paper we describe, for the first time, oscillations and bistability which are associated with the NDR region caused by completely different mechanism – the potential-dependent onset and decay of convection associated with the electroreduction of Hg(II) and Hg(I) ions at the mercury electrodes of various geometries. Under potentiostatic conditions the current oscillations were observed in the presence of ohmic drops, while under galvanostatic conditions the large-amplitude oscillations of the electrode potential were observed only in the presence of excess of chlorides, producing a permanently existing layer of calomel in contact with the mercury surface. Oscillations were observed in various electrochemical reactors including the system resembling the “beating mercury heart” but operating on different basis. The outline mechanism of those oscillations was suggested. Additionally it was found that Hg2+ ions, undergoing fast surface synproportionation with Hg as the electrode material, are more efficient species than Hg22+ for the onset of convection and thus oscillations. These results enrich the set of electrochemical/oscillatory systems for the ones based on the alternate onset and decay of convection.