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Silver reduction in a thiosulphate + nitrate solution: study of a CE reaction scheme by means of polarization curves and ac impedance

1995; Elsevier BV; Volume: 397; Issue: 1-2 Linguagem: Inglês

10.1016/0022-0728(95)04124-6

1873-2569

S. Vandeputte, Els Verboom, Annick Hubin, J. Vereecken,

Electrodeposition and Electroless Coatings

Silver electrodeposition from an electrolyte consisting of 0.1 M NaNo3 + 0.025 M Na2S2O3 + 1 × 10−3 M AgNO3 (pH 12) was investigated by means of polarization curves and ac impedance. In order to work under quasi-stationary conditions, measurements were performed on a rotating disk electrode, the reactive surface of which was covered with a fine silver pre-plating. It is shown that for cathodic overvoltages higher than 30 mV the reduction mechanism followed a CE reaction scheme. The rate constants of both the preceding homogeneous decomplexation reaction of Ag(S2O3)3−2 to Ag(S2O3)− and the heterogeneous electron transfer reaction were determined. The low-frequency limit of electrohydrodynamic impedance measurements was used to check the calculated chemical reaction parameters. In earlier publications in which higher nitrate/thiosulphate ratios were used, the influence of this decomplexation reaction could not be detected. Also, it was found that the lower NO−3S2O2−3 ratio used here caused a substantial difference from the current-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.