XPS ANALYSIS OF HYDROXIDE ION SURFACE-REACTIONS ON CEF3 AND LAF3 FLUORIDE ION-SELECTIVE ELECTRODES

Lanthanum fluoride and cerium fluoride single crystals, used as the se nsing membranes of the fluoride ion-selective electrode (F-ISE), were investigated for their hydroxide interference and surface reactions wi th OH- in high pH conditions. While these membranes show fast response and excellent Nernstian behavior over a wide concentration range in b uffered F- solutions, they deviate from the theoretical slope at high pH. CeF3, in particular, exhibits a much larger deviation from the Ner nstian slope and a substantially slower response to a F- activity chan ge compared to LaF3. This larger deviation is due to more extensive an d faster formation of hydroxo-complexes, with the release of a greater amount of the fluoride ion into the hydrated gel layer. The slower re sponse of the CeF3 membrane after contact with OH- is due to the forma tion of Ce-IV oxide on the surface, which acts as a blocking layer to the exchange of F-.