Effect of concentration of lithium ions on the voltammetric responses of nitro-substituted aromatic sulfides in dimethylformamide on glassy carbon electrodes

Lithium salts are used as supporting electrolytes and ion-pair forming reagents during voltammetric investigations in aprotic solvents. In the present work, these ions are found to have a significant influence on the voltammetric responses of aromatic sulfides in dimethylformamide given their concentration and the cathodic potential limits applied. At very low concentrations (< 4 mM) and cathodic potential limits (− 1.4 V), lithium ions form ion-pairs with the anion radicals generated by the reduction of nitro groups on the electrode surface. This is the conventional lithium ion-pair formation effect reported in the literature. With increasing lithium ion concentration and increasing cathodic limit, the ion-pairs tend to form an insoluble salt film on the electrode surface. This leads to inhibition of further electron transfer. The inhibiting effect does not seem to correlate with the size of the reactant organic molecule involved. At cathodic potential limits exceeding − 2 V, lithium ions appear to undergo direct reduction and subsequent reaction with trace levels of water in the solvent to produce a passive LiOH layer. This inhibits all further electron transfer.