ELECTROCHEMICAL REDUCTION OF LUCIGENIN ON MERCURY IN AQUEOUS-MEDIA

A more comprehensive study than those previously reported in the liter ature was carried out on the electrochemical reduction of 10,10'-dimet hyl-9,9'-baicridinium ion (lucigenin) in aqueous solutions. Lucigenin (L(2+)) exhibits one, two, or three irreversible voltammetric reductio n peaks, depending on its concentration and the scan rate. The first p eak is due to the reduction of the first monolayer of molecules adsorb ed on the electrode. This process involves two simultaneous one-electr on transfers that yield a molecule (P), also immobilized at the electr ode, which undergoes a conformational change to the form P'. In additi on, the reagent molecules that reach the electrode by diffusion and th e product (P) comproportionate to yield the intermediate radical R(.+) . New reactant molecules can displace the product because the monolaye r is reversibly adsorbed. The transfer coefficient of the process and the activation energy of the conformational change were determined. Th e second peak is due to the reduction of a second monolayer of adsorbe d lucigenin molecules. This takes place via the first monolayer of pro duct molecules adsorbed on the electrode. Finally, the third peak aris es from reduction of lucigenin molecules adsorbed on the previous mono layers. The overvoltage required to reduce lucigenin increases with in creasing number of monolayers formed. The formation of new insoluble m onolayers blocks the faradaic process altogether in the second scan at more positive potentials than those for the second or third peak.