In situ underpotential deposition study of lead on silver using the electrochemical quartz crystal microbalance. Direct evidence for lead(II) adsorption before spontaneous charge transfer

In situ studies using the electrochemical quartz crystal microbalance (EQCM) have provided unique means for demonstrating the existence of adsorbed anionic lead species at silver and also for tracking anion expulsion during the underpotential deposition (UPD) discharge process. UPD studies of lead(II) at silver were carried out over the pH range 1 to 13. In perchloric acid solutions and acetate buffers, where only cationic and uncharged lead(II) species exist in solution, the UPD process is accompanied by an increase in the mass of the electrode. However, in borate buffers and sodium hydroxide solutions, where anionic lead(II) species predominate, the UPD process is accompanied by a decrease in electrode mass. This mass decrease is caused by the reduction of adsorbed, negatively charged lead(II) species to UPD Pb(0) and concomitant expulsion of the negatively charged ligand, borate or hydroxide. The potential of zero charge of silver is very negative so that the electrostatic adsorption of anionic lead(II) species is strongly favored, while cationic and uncharged lead(II) species, such as lead acetate, do not adsorb.