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Kinetics and mechanism of silver dissolution in thiocarbamide solutions

The kinetics and mechanism of silver metal dissolution in thiocarbamide solutions in the presence of iron(III) sulfate (oxidizer) were studied. The dependences of the silver dissolution rate on the pH value, ratio betweenthe concentrations of iron(III) and thiocarbamide, and temperature were determined. The reaction rate constant of silver dissolution k i varies from 2.3 x 10 - 4 to 9.6 x 10 - 4 s - 1 as the temperature increases from 283 to 298 K. The equilibrium constants of silver dissolution for oxidation with formamidine disulfide and Fe(III) were found to be 4.75 x 106 and 1.95 x 10 3 , respectively. The reaction order with respect to carbamide was two or three, depending on its concentration. The activation energy of E a = 68.4 kJ/mol, which was calculated from the temperature dependence of the rate constant, suggests that the dissolution of silver in acidic carbamide solutions in the presence of iron(III) ions is a kinetically controlled process. In the regime of inhibition by an adsorption layer, the activation energy was found to be E a = 26.06 kJ/mol. Adsorption layers formed on the silver surface were studied with Auger spectroscopy and XRD analysis.