Fluoride selective electrode


A fluoride selective electrode is a type of ion selective electrode sensitive to the concentration of the fluoride ion. A common example is the lanthanum fluoride electrode.

Lanthanum fluoride electrode

In the lanthanum fluoride electrode, the sensing element is a crystal of lanthanum fluoride, doped with europium fluoride to create lattice vacancies. Such a crystal is an ionic conductor by virtue of the mobility of fluoride ions which jump between lattice vacancies. An electrochemical cell may be constructed using such a crystal as a membrane separating two fluoride solutions. This cell acts as a concentration cell with transference where the fluoride transport number is 1. As transference of charge through the crystal is almost exclusively due to fluoride, the electrode is highly specific to fluoride. The only ion which significantly interferes is hydroxide. Generally such "alkaline error" can be avoided by buffering the sample to a pH below 7.

Cell diagram

The cell diagram of a typical experimental arrangement is:
where: