Morpholinoethanesulfonic acid-based buffer system for improved detection limit and stability of the fluoride ion selective electrode

The zwitterionic morpholinoethanesulfonic acid (MES) is used as the basis for the development of a new total ionic strength adjustor buffer system for improving the analytical characteristics of fluoride sensor. Impedance analysis, atomic force microscopy (AFM) and dissolution studies of the LaF₃ crystal, together with the potential stability and sensor sensitivity over time have aided in the elucidation of the processes that take place at the surface of the LaF₃ crystal, that seem to determine the sensor behaviour. Even though AFM analysis shows that both buffers used (MES or acetate) cause a similar increase in surface roughness, the data from the other studies suggest that in the first case there is a reversible ion exchange process at the interface, while in the second case this process is irreversible, leading to fast poisoning of the crystal surface. The use of 0.5 M MES and NaCl buffer adjusted to pH 5.50 allows for the continuous operation of the sensor under flow injection analysis conditions for at least 5 days with sensitivity of 60 mV per decade [F⁻], detection limit of 2.1×10⁻⁷ M [F⁻] and fast response time.