XPS analysis of hydroxide ion surface reactions on reactions on CeF3 and LaF3 fluoride ion‐selective electrodes

Abstract Lanthanum fluoride and cerium fluoride single crystals, used as the sensing membranes of the fluoride ion‐selective electrode (F‐ISE), were investigated for their hydroxide interference and surface reactions with OH− in high pH conditions. While these membranes show fast response and excellent Nernstian behavior over a wide concertration range in buffered F− solutions, they deviate from the theoretical slope at high pH. CeF3, in particular, exhibits a much larger deviation from the Nernstian slope and a substantially slower response to a F− activity change compared to LaF3. This larger deviation is due to more extensive and faster formation of hydroxo‐complexes, with the release of a greater amount of the fluoride ion into the hydrated gel layer. The slower response of the CeF3 membrane after contact with OH− is due to the formation of CeIV oxide on the surface, which acts as a blocking layer to the exchange of F−.