The mechanism of the electrochemical oxidation of thiourea

Cyclic voltammetry and potentiostatic coulometry were used to study the electrochemical oxidation of thiourea in aqueous solutions of nitric acid, nitric acid—ammonium nitrate, and ammonium nitrate—ammonium hydroxide and in acetonitrile. These studies were carried out at glassy carbon and/or platinum working electrodes. In acetonitrile, the cyclic voltammograms show one oxidation peak at + 0.6 V and a reduction peak at —0.1 V. In aqueous solutions up to about pH 6, there is a second oxidation peak at 1.3 V which is irreversible and its height is sensitive to acidity. These experiments have confirmed that in acidic and neutral solutions the oxidation of thiourea proceeds via a slow 1-e transfer reaction producing a radical (NH₂)₂—C—S]⁺. Further direct oxidation of this radical takes place only at higher potentials (ca. 1.2 V) and involves hydration and protontransfer equilibria. Otherwise, C,C'-dithiodiformamidinium ion is formed by a fast dimerization reaction. Coulometric and chronopotentiometric measurements have shown that in strong acid the second oxidation step involves one electron, while at lower acidities the further oxidation involving three (and possibly five) electrons proceeds in two (or three) steps of very similar potential.