Coulostatic Deposition of Metals at Underpotential: II. The Effect of Heterogeneous Chemical Reactions on the Potential Relaxation Process

The coulostatically controlled underpotential deposition (UPD) processes Ag(I) + e = Ag on gold and platinum and Hg(II) + 2e = Hg on gold have been shown to lead to anodic potential excursions (1). Chemical reactions that precede and/or accompany coulostatic underpotential deposition can produce two other kinds of potential relaxation. For example, the potential of an oxidized gold electrode, on being open-circuited in Hg(I) solutions, shifts continuously to a more cathodic potential until equilibrium is reached. The Hg(I) reduces the gold oxide and then produces a UPD mercury layer on the gold surface through the disproportionation reaction [formula omitted] The potential shift ceases when the Nernst potential is reached. The potential of a reduced gold electrode, on being open-circuited in a Hg(I) solution, initially shifts anodically and then shifts cathodically until equilibrium is reached. The initial anodic shift is due to a conventional coulostatic deposition mechanism while the subsequent cathodic shift is due to the disproportionation mechanism given above.