The HI4110-51 fluoride module is for use with the HI4110 fluoride combination ion selective electrode (ISE). The module is a lanthanum fluoride (LaF₃) crystal membrane that contains openings within its lattice structure that allow fluoride ion exchange between the sample and internal solution. A charge imbalance develops between the sample and the internal cell in response to the fluoride ion activity. The potential change is proportional to the concentration of ions in solution.

The HI4110-51 fluoride combination module easily screws onto the HI4110 probe body, allowing for quick assemblage or replacement. Hanna recommends pre-soaking the fluoride sensor in a standard near expected concentrations before calibration. Conditioning in this way will activate the sensing crystal and help optimize the sensor response. Gently shaking the sensor down before measurement will place the internal solution next to the sensing membrane, ensuring reliable, accurate measurements.