Mechanism of gold dissolving in alkaline thiourea solution

Reaction mechanism of gold dissolving in alkaline thiourea solution was studied by electrochemical methods, such as cyclic voltammetry, chronopotentiometry, AC impedance, linear sweep voltammetry. Apparent activation energy of anodic process of gold electrode dissolving in alkaline thiourea solution is 14.91 kJ/moh Rate determining step is the process of gold thiourea complex diffusing away from electrode surface to solution. The results of AC impedance and chronopotentiometry indicate that thiourea adsorbs on gold electrode surface before dissolving in solution. There does not exist proceeding chemical reactions. Formamidine disulfide, the decomposed product of thiourea, does not participate the process of gold dissolution and thiourea complex. Species with electro-activity produced in the process of electrode reaction adsorbs on the electrode surface. In alkaline thiourea solution, gold dissolving mechanism undergoes the following courses： adsorption of thiourea on electrode surface; charge transfer from gold atom to thiourea molecule; Au[SC（NH2）2]ads^＋ receiving a thiourea molecule and forming stable Au[SC（NH2）2]2^＋; and then Au[SC（NH2）2]2^＋ diffusing away from the electrode surface to solution, the last step is the rate-determining one.

Electrochemical kinetics of gold dissolving in alkaline thiourea solution

Kinetic parameters of the electrode reactions were measured by investigating steady-state current-potential behaviors. The results show that the apparent transfer coefficient of anodic process is 0.0582, diffusion coefficient of thiourea gold complex is 6.04×10~ -6 cm^2/s,anodic reaction order of thiourea is 2.0183, and anodic reaction order of OH^- is 0.0166. The theoretical kinetics equation of gold dissolving in alkaline thiourea solution is deduced, which indicates that anodic reaction order of thiourea is 2, and anodic reaction order of OH^- is 0. The theoretical values of the kinetic parameters are consistent with experimental values very well. The correctness of the mechanism is further demonstrated using apparent transfer coefficient according to the electrochemical dynamic equation of multi-electron reaction.

Electrochemical adsorption and passivation on gold surface in alkaline thiourea solutions

An electrochemical method was used to examine Au surface absorption and passivation under the most commonly employed pH conditions in an alkaline Au leaching system.The polarization at different pH values was obtained through the current step curve.At pH 10,no absorption layer was formed.When the pH was increased to 11,an absorption layer was formed through the Au electrode reaction.At pH 12,the entire system could not be stabilized,even after long durations because of the thiourea oxidation and decomposition.The samples were analyzed by scanning electron microscopy(SEM),energy-dispersive X-ray spectroscopy(EDS),and atomic force microscopy(AFM).AFM observations of the Au plating surface and SEM-EDS analyses for the Au-coated graphite surface indicated that a passivation membrane was formed on the Au surface after its use in the alkaline thiourea Au leaching system.Two-step leaching was used to confirm the conclusions drawn from the experimental results.The twostage experimental results further confirmed the existence of a passivation membrane.