B08 Fluoride Specific Ion Electrode

Description
A fluoride specific ion electrode with a lanthanum fluoride crystal is used to measure the fluoride in two water samples. Six standards are measured to construct a calibration curve. The lanthanum fluoride crystal acts as a membrane that dissolves fluoride internally when the sample is high in fluoride. A silver/silver chloride electrode is on the internal side of the fluoride membrane. As the fluoride ion concentration in the internal solution increases, chloride ions react with Ag to maintain electrical neutrality. The potential of this internal redox reaction is measured.

Equation
(sample) F^- + LaF₃ --> LaF₃ + F^- (internal)

AgCl <--> Ag⁺ + Cl^-

Ag --> Ag⁺ + e^-

Reference