AN ELECTROCHEMICAL QUARTZ-CRYSTAL MICROBALANCE STUDY OF SURFACE PROCESSES AT PT ELECTRODES MODIFIED WITH BI

Underpotential deposition (upd) and adsorption of Bi species have been studied at Pt electrodes in perchloric acid solutions using the elect rochemical quartz crystal microbalance (EQCM). Mass transients reveal extensive adsorption of cationic Bi species at potentials positive of the upd region (where the electrode surface is oxidized) in 0.1 M HClO 4. The bulk of this adsorption seems to derive from hydrolyzed Bi(III) species rather than Bi3+ since it is strongly dependent on pH, being much reduced when the acid concentration is increased to 1.0 M where h ydrolysis is largely absent. Increasing the pH leads to an increase in adsorption. The extent of adsorption of cations prior to development of the upd deposit in 0.1 M HClO4 leads to mass changes that are small er than expected when the potential is scanned into the upd region, si nce the deposit is formed largely from species already adsorbed at the electrode surface. The mass response also reveals an arrest on the po sitive-going scan that is suggested to be the result of a change in th e nature of Bi at the surface from Bi to BiOH2+ or Bi(OH)2+. When a Pt electrode with adsorbed Bi is examined in a 0.1 M HClO4 electrolyte f ree of Bi(III), a mass increase is seen to be associated with the tran sition between upd Bi and adsorbed Bi(III) and this is attributed to t he fact that the adsorbed Bi(III) is largely present as hydrolyzed spe cies.