X-Ray photoelectron spectroscopic (XPS) data of AgCI-coated, ion-selective electrodes exposed to solutions containing high concentrations of bromide and iodide ions revealed that the surface properties of the original electrode changed. The mixed phases, AgHr,Cl, --s and AgIsCI, --.F, seem to form initially on the electrode surface, with the existence of the AgICl phase much shorter than that of the AgBrCl phase. This indicates that the formation of the AgI surface layer is kinetically faster than the formation of the AgBr layer. In both cases XPS data showed the ultimate formation of AgBr and AgI layers on top of the original AgCI-coated, ion-selective electrode surface after being subjected to solutions containing either bromide or iodide ions. The conversion of the crystalline-coated, chloride-selective electrode surfaces to AgBr and AgI layers, however, transformed the chloride-selective electrodes into bromide-or iodideselective electrodes with electrode properties different from those of the original electrode.
The AgI-coated ion-selective electrode surface is more stable than either the AgCl or AgBr surfaces if exposed to solutions containing ions such as CI-, Br-, I-, Ag', and NH, for a week. A mixed AgCl,I,-, or AgBr,sI,_, phase is formed upon a week's treatment of h e AgI surface with either C1-or Br-, with s much smaller than 1. After a week's treatment with high concentrations of the ions, no new surface layer consisting of only AgC1 or AgBr is formed, as was observed in the case of AgCl and AgBr electrodes. Although AgI is much less soluble than either AgCl or AgBr, the absorption of chloride and bromide ions into the AgI surface nevertheless increases with time.
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