Species Analysis And Kinetics Study In The Electrochemical Oxidation Of Thiourea

In this thesis, various electrochemical methods, including cyclic voltammetry, chronoamperometry, chronopotentiometry and rotating disk electrode methods were combined with ex situ HPLC/UV-Vis/MS and in situ FTIR to investigate the electrochemical adsorption and oxidation of thiourea on a platinum electrode.In 0.1 M HCIO4 solution, the electrochemical oxidation of thiouea is an extremely complex process. Beside electron transfer, mass transport by the diffusion of reactant thiourea from bulk solution to the electrode surface and the products from the electrode surface region to the bulk solution, the adsorption of thiourea on the electrode surface, the transfer of oxygen and proton and other homogenous chemical reactions were also involved in the oxidation reactions.Two steps were involved in the electrochemical oxidation of thiorea. The first one occurs at potentials below 0.9 V. the oxidation of thiourea via a sluggish single electron transfer process producing thiourea radicals, and then formamidine disulfide is formed by fellowing homogeneous reactions. The formation of thiourea radical is the rate-determine step which controls the rate of the whole reaction. The standard heterogeneous electron transfer rate constant k0 ? 6*1 ? 10.5 cm.s" and the electron transfer coefficient ? ? O42. Beside formamidine disulfide, small amount of cyanamide were also found in the first oxidation step.The further oxidation of thiourea occurs at potentials above 0.9 V. In these potentials, the oxidation is much more complex and more species are involved. Potential step and chronopotentiometric measurements indicate that the number of electron transferred to be 3 eq ?mol" . Formamidine sulphinic acid is the major product, and cyanamide, thiourea and formamidine sulfonic acid were also detected in a small amount in the oxidation. These species can be further oxidized at potentials above 1.2 V, yielding sulfate, nitrogen gas and carbon dioxide.The oxidation products of thiourea, including formamidine disulfide, formamidine sulphinic acid and formamidine sulfonic can hydrolysis yielding thiourea, cyanamide, element sulfur, formadine sulphnic acid and a unidentified compound. Thiourea can be detected in the hydrolysis of formamidine sulphinic acid. The hydrolysis of formamidine sulfonic acid could produce urea and sulfide. As the rate constant of these reaction is quite small, their effects on the electrochemical oxidation of thiourea and its intermediate could be ignored.