Anodic Dissolution of Gold in Cyanide Solutions Containing Lead Ions: Effect of the Solution pH

The effect of the electrode potential on the gold dissolution rate in alkali–cyanide solutions with and without 10^–5 M of hydroxy compounds of lead is studied. With the compounds, the process rate passes through a maximum, whose potential E _m shifts in the negative direction and whose height drops with increasing pH. The pH dependence of E _m is linear, with the slope dE _m/dpH = –71 ± 5 mV, and correlates with that of the potential at which lead adatoms start to undergo desorption from the gold surface in alkali solutions. Without the compounds, the gold dissolution rate in alkali–cyanide solutions is independent of the solution pH at E < 0. Thus, the effect of the solution pH in this potential range is connected not with a direct participation of hydroxide ions in the anodic process but is of a secondary nature caused by the dependence of the region of adsorption of catalytically active lead adatoms on the hydroxide ion content in solution.