Measurement of adsorption rates of anions on Au(111) electrodes by impedance spectroscopy

We have measured impedance spectra on Au(111) electrode in aqueous perchlorate solutions containing specifically adsorbing anions (SO42-, Cl-, Br- and I-) of about 10(-4) M concentrations at potentials where the dominant electrode process is the adsorption of the above anions. Accordingly. the impedance spectra have been analyzed in terms of the Frumkin-Melik-Gaykazyan adsorption impedance theory. With SO42-, Cl- we found the adsorption rates to be immeasurably high, with Br- and I- the apparent rate coefficients could be determined. For the halide anions this quantity increases in the order of Cl- > Br- > I-; and rapidly decreases with increasing electrode potential and coverage. We attribute this change to the decrease of the occupancy related hindrance factor overcompensates the increase of the rate coefficient. (C) 2002 Elsevier Science Ltd. All rights reserved.