SILVER REDUCTION IN A THIOSULFATE PLUS NITRATE SOLUTION - STUDY OF A CE REACTION SCHEME BY MEANS OF POLARIZATION CURVES AND AC-IMPEDANCE

Silver electrodeposition from an electrolyte consisting of 0.1 M NaNO3 + 0.025 M Na2S2O3 + 1 X 10(-3) M AgNO3 (pH 12) was investigated by me ans of polarization curves and ac impedance. In order to work under qu asi-stationary conditions, measurements were performed on a rotating d isk electrode, the reactive surface of which was covered with a fine s ilver pre-plating. It is shown that for cathodic overvoltages higher t han 30 mV the reduction mechanism followed a CE reaction scheme. The r ate constants of both the preceding homogeneous decomplexation reactio n of Ag(S2O3)(2)(3-) to Ag(S2O3)(-) and the heterogeneous electron tra nsfer reaction were determined. The low-frequency limit of electrohydr odynamic impedance measurements was used to check the calculated chemi cal reaction parameters. In earlier publications in which higher nitra te/thiosulphate ratios were used, the influence of this decomplexation reaction could not be detected. Also, it was found that the lower NO3 -/S2O32- ratio used here caused a substantial difference from the curr ent-overpotential equation at cathodic overvoltages lower than 30 mV. In this potential region, the low-frequency limit of the amplitude of the ac impedance increased with the overvoltage.