MECHANISM STUDIES ON LEAD ION-SELECTIVE CHALCOGENIDE GLASS SENSORS

The lead ion-sensing mechanism of PbS-AS2S3, AgAsS2 and PbS-Ag2S-As2S3 chalcogenide glass sensors has been studied. The following principal methods have been chosen: (i) a study of the sensitivity of Ag-free PbS-AS2S3 glasses to lead ions in solution; (ii) an investigation of induced exchange-site formation using XPS, AES and SIMS; (iii) Pb-212 tracer measurements to obtain information about lead ion exchange. It has been shown that Ag-free PbS-AS2S3 glasses are sensitive to Pb2+ ions in solution; their standard potentials are close to those of Ag-containing vitreous alloys. The formation of induced exchange sites governing the lead-ion sensitivity of Pb-free AgASS2 glass sensors has been confirmed by surface spectroscopies. Evidence for direct Pb2+ ion exchange at the membrane/solution interface for both PbS-Ag2S-AS2S3 and AgAsS2 glasses has been obtained by tracers. All these experiments indicate that the ion response of lead ion-selective chalcogenide glass sensors can be completely described within the framework of the modified surface-layer model.