Fluoride Selective Electrode

A fluoride selective electrode is a specialized type of ion selective electrode that is sensitive to the concentration of fluoride ions. The most common type of fluoride selective electrode is the lanthanum fluoride electrode.

Lanthanum Fluoride Electrode

Sensing Element:
- Comprised of a crystal of lanthanum fluoride (LaF3) doped with europium(II) fluoride (EuF2).
- Doping creates lattice vacancies, enabling ionic conduction via the mobility of fluoride ions.
Functionality:
- Works as an ionic conductor due to the movement of fluoride ions between lattice vacancies.
- Operates as a concentration cell, where fluoride is the primary charge carrier (fluoride transport number = 1).
- High specificity for fluoride ions, with hydroxide ions (OH-) being the only significant interference, often termed "alkaline error". This can be mitigated by buffering the sample to a pH below 7.
Experimental Setup:
- The fluoride-selective electrode forms one half of a concentration cell, with a fluoride analyte solution.
- Contains an external reference electrode and an internal reference of NaF, NaCl.
- Minimizes the liquid junction potential with an internal junction in some commercial electrodes.

Reference Electrodes

External Reference:
- Often made of Hg|1/2Hg2Cl2 | KCl(sat).
Internal Reference:
- Uses NaF, NaCl inside the fluoride selective electrode.

Measurement and Response

The response of the electrode is described by the Nernst equation:
- E = _E_0 + (RT/F) ln _a_F.
- Where:
  - E is the measured cell potential
  - _E_0 is the standard cell potential
  - R is the ideal gas constant
  - T is the temperature in kelvins
  - F is the Faraday constant (9.6485309104 C/mol)
  - _a_F is the activity of the fluoride ion

Understanding Fluoride Selective Electrodes

Fluoride Selective Electrode

Lanthanum Fluoride Electrode

Reference Electrodes

Measurement and Response

Further Reading and References