Kinetic Parameters and Mechanism of Gold Electrooxidation in Thiocarbamide Solutions

Abstract

Kinetics and mechanism of gold electrooxidation in sulfuric acid solutions of thiocarbamide ( TCA ) are studied by measuring voltammograms ( VA ) of gold and glassy carbon ( GC ) electrodes. In a potential ( E ) range from -0.2 to 1.2 V ( SCE ), VA of a gold electrode demonstrate three peaks. The first and the second peaks correspond to gold electrooxidation to [AuTCA ads ] + and [Au(TCA ads ) 3 ] 3+ ions at E of 0.152 and 0.554 V, while the third one corresponds to TCA electrooxidation. The electrooxidation of TCA on GC electrodes is characterized by two VA peaks at 0.983 and 1.437 V. The first peak corresponds to the formation of formamidine disulfide (FADS) ( k a1 = 6.40 × 10 -4 cm/s), while the second peak corresponds to TCA oxidation to sulfides and hydrosulfides ( k a2 = 7.78 × 10 -5 cm/s). The composition of adsorption layers formed at the Au electrode is analyzed by Auger spectroscopy. The introduction of sodium sulfide into TCA solutions eliminates the formation of adsorption layers and accelerates Au oxidation in such solutions. The rate constants of gold electrooxidation k a = 6.31 × 10 -3 cm/s and Au(I) electroreduction k c = 5.46 × 10 -4 cm/s in TCA solutions are estimated. Kinetic parameters (charge transfer coefficients, reactions orders in carbamide) are determined and the mechanisms of Au and TCA oxidation in sulfuric acid solutions are proposed.