INVESTIGATION OF THE MECHANISM OF SILVER DEPOSITION FROM THIOSULFATE SOLUTIONS BY MEANS OF AC-IMPEDANCE MEASUREMENTS AND SURFACE-ENHANCED RAMAN-SPECTROSCOPY

In order to optimize the reaction conditions for the process of silver deposition from thiosulphate solutions, the knowledge of the mechanis m of the reduction reaction is required. In the present paper, the rol e played by NaNO3 (added as supporting electrolyte) and Na2S2O3 (compl exing agent for Ag+) on the reduction process is studied by means of a c impedance and surface-enhanced Raman spectroscopy (SERS). In a solut ion made up of 0.1 M NaNO3, 0.025 M Na2S2O3 and 1 x 10(-3) M AgNO3 (pH = 12), with an equilibrium potential E(0) of -215 mV/sce, the reducti on follows a CE mechanism, but divergences are found at overvoltages s maller than 30 mV. SERS measurements on a pretreated silver electrode showed that at potentials between E(0) and about -250 mV/sce, the adso rption of S2O32- is stronger than at more cathodic potentials. The eff ect of the enhanced S2O32- adsorption on the electrochemical behaviour is hidden in solutions containing more NO3- by an overall acceleratio n of the reduction rate.