Electrochemical detection of chloride by underpotentially deposited silver films on polycrystalline gold

This paper describes an electrochemical method for measuring dilute levels of chloride using an underpotentially deposited (UPD) Ag adlayer on polycrystalline Au substrates as a sensing agent. Specifically, chloride ions adsorb onto the Ag UPD adlayer and effect changes in the electrochemical deposition and stripping characteristics of the silver film. Cyclic voltammograms (CVs) of the native Au/Ag(UPD) electrode in 0.1 M H2SO4(aq) exhibit a primary stripping peak for the Ag UPD adlayer at 550 mV vs Ag(+/0), and chloride adsorption onto the Au/Ag(UPD) surface effects a peak shift to approximately 600 mV vs Ag(+/0), depending on the amount of adsorbed Cl-, as affected by the Cl- concentrations and contact times employed in the derivatization. The chloride-treated electrodes also exhibit a stripping peak at 275 mV that is not observed on the native substrate and increases in intensity with Cl- concentration and derivatization time. The integrated charge density for this latter stripping peak relative to that for the primary stripping peak at 550-610 mV provides a useful metric for quantifying adsorbed Cl- levels, and these values allow measurement of Cl- concentrations in dilute aqueous solutions. For Cl- concentrations between 0.5 and 100 microM, the kinetics of Cl- adsorption followed a transient Langmuir adsorption model and allowed measured surface coverages to be used for determining Cl- solution concentrations. Using contact times of 1 min for Cl- adsorption, the electrodes showed a linear response across Cl- concentrations of 0.5-20 microM.