Characterization of an AgBr–Ag2S–As2S3–HgI2 ion-selective electrode membrane: a X-ray photoelectron and impedance spectroscopy approach

This paper presents the results of a detailed surface study of a chalcogenide (AgBr–Ag2S–As2S3–HgI2) mercury (Hg(II)) ionselective electrode (ISE) using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). EIS has shown that the Hg(II) ISE undergoes a photo-oxidation reaction in the presence of light. By contrast, XPS revealed that the outer surface layer of AgBr–Ag2S–As2S3–HgI2 becomes hydrated and depleted with respect to arsenic on exposure to air/water during membrane preparation/treatment. XPS failed to detect any arsenic compound which may be associated with oxygen, suggesting that the As–O species becomes removed from the surface. The adsorption of mercury (Hg2þ) and various ions (i.e., OH , NO3 and Cl ) have been studied, and a mechanism for their action is proposed. SEM and XPS have revealed that chloride ions undergo specific adsorption onto AgBr sites and the reaction is dictated by kinetic factors, whereas nitrate preferentially reacts with the chalcogenide arsenic component most likely via a redox process. By contrast, EIS revealed a decrease in charge transfer resistance after extended exposure of the AgBr–Ag2S– As2S3–HgI2 membrane in Hg2þ electrolyte, and XPS identified various changes in the surface species. It is proposed that the response mechanism of the chalcogenide Hg(II) ISE membrane involves a combined charge transfer and ion exchange reaction in Hg2þ electrolyte. # 2004 Elsevier B.V. All rights reserved.