A fluoride selective electrode is a type of ion selective electrode

Ion selective electrode

An ion-selective electrode , also known as a specific ion electrode , is a transducer that converts the activity of a specific ion dissolved in a solution into an electrical potential, which can be measured by a voltmeter or pH meter. The voltage is theoretically dependent on the logarithm of the...

 sensitive to the concentration of the fluoride

Fluoride

Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...

 ion

Ion

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

. A common example is the lanthanum fluoride electrode.

Lanthanum fluoride electrode

In the lanthanum fluoride electrode, the sensing element is a crystal of lanthanum

Lanthanum

Lanthanum is a chemical element with the symbol La and atomic number 57.Lanthanum is a silvery white metallic element that belongs to group 3 of the periodic table and is the first element of the lanthanide series. It is found in some rare-earth minerals, usually in combination with cerium and...

 fluoride LaF₃, doped with europium

Europium

Europium is a chemical element with the symbol Eu and atomic number 63. It is named after the continent of Europe. It is a moderately hard silvery metal which readily oxidizes in air and water...

 fluoride EuF₂ to create lattice

Crystal structure

In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...

 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

Concentration cell

A concentration cell is a limited form of a galvanic cell that has two equivalent half-cells of the same material differing only in concentrations. One can calculate the potential developed by such a cell using the Nernst Equation. A concentration cell produces a voltage as it attempts to reach...

 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 (OH^-). 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:

Cu' | Ag,AgCl | KCl || solution | LaF₃ | KF,KCl | AgCl,Ag | Cu

where:

• Cu' | Ag, AgCl | KCl is an external reference electrode
• KF,KCl/AgCl,Ag/Cu is an internal reference inside the fluoride selective electrode

Reference electrode

Some commercially available reference electrodes have an internal junction which minimizes the liquid junction potential between the sample solution and the electrolyte in the reference electrode (KCl). The internal electrolyte is at fixed composition and the electrode response is given by the Nernst equation

Nernst equation

In electrochemistry, the Nernst equation is an equation that can be used to determine the equilibrium reduction potential of a half-cell in an electrochemical cell. It can also be used to determine the total voltage for a full electrochemical cell...

:

E = E⁰ − RT/F ln a_F⁻

where:

• E is the measured cell potential
• E⁰ is the standard cell potential
• R is the ideal gas constant
• T is the temperature in kelvins
• F is the Faraday constant (9.6485309×10⁴ C·mol⁻¹)
• a_F⁻ is the activity of the fluoride ion

History

The commercial development of ISE began when an engineer by the name of John Riseman thought he could develop a useful blood-gas analyzer. He teamed up with Dr. James Ross, an electrochemist from MIT. Together they formed Orion Research. Martin Frant, a chemist, joined them later. By the mid 1960s, the newly formed Orion Research Inc. was producing Calcium electrodes for use in blood gas analyzers (Frant, 1994). Since then numerous probes have been developed for the analysis of samples containing many different ions.