Electrodeposition of Silver from Pyrophosphate Solution

Abstract

The characteristics of silver pyrophosphate solution have been examined from the conductivity and kinematic viscosity of solutions. The cathodic polarization curves excluding concentration polarization were determined by the use of a rotating disk electrode to investigate electrode reactions in silver electrodeposition. Each conductivity of solutions of potassium dicyanoargenatate( I ) K[Ag(CN)₂], potassium pyrophosphate K₄P₂O₇, potassium th iocyanate KSCN and the mixed solution appears to have an additive property (Fig.1). The kinetic viscosity was confirmed to be temperature dependent and the apparent activation energy E_c was 3.1 kcal/mol (Fig.2), and the overall electrode reaction in K[Ag(CN)₂]-K₄P₂O₇ solution can be written as follows:
[Ag(CN)₂]-+e→Ag+2CN-
The effect of ionic diffusion on the reaction was observed. The diffusion coefficient of this solution at the Ag⁺ concentration of 0.07 mol/l at 25°C was 1.39×10^-5cm/s and the Schmidt number was 718 (Fig.4). From the cathodic polarization curves, Tafel slope of 0.346 V was observed in the range of approximately -0.45∼-0.60 V (SCE) (Fig.7). In the charge transfer process in this range, the value of apparent coefficient α was 0.268, the apparent valence n was 0.638, the exchange current density i_o was 3.8 mA/cm2, and the apparent activation energy E_c was 7.7 kcal/mol (Fig.3 and Table 3). The activation overvoltage for hydrogen evolution from K₄P₂O₇ solution was higher than that from K[Ag(CN)₂]-K₄P₂O₇solution (Fig.9).