A fluoride selective electrode is a type of

ion selective electrode An ion-selective electrode (ISE), also known as a specific ion electrode (SIE), is a transducer (or sensor) that converts the activity (chemistry), activity of a specific ion dissolved in a Solution (chemistry), solution into an electrical potentia ...

sensitive to the concentration of the fluoride

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

. A common example is the

lanthanum fluoride Lanthanum trifluoride is a refractory ionic compound of lanthanum and fluorine. The LaF3 structure Bonding is ionic with lanthanum highly coordinated. The cation sits at the center of a trigonal prism. Nine fluorine atoms are close: three a ...

electrode.

Lanthanum fluoride electrode

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

(LaF₃), doped with

europium(II) fluoride Europium(II) fluoride is an inorganic compound with a chemical formula EuF2. It was first synthesized in 1937.Zhao Yongzhi, Ma Ying, Hou Shaochun, Zhang Wenjuan, Wang Jingjing, Ding Yanrong, Hao Yifan氟化亚铕研究现状(''lit''. Research Progr ...

(EuF₂) to create

lattice Lattice may refer to: Arts and design * Latticework, an ornamental criss-crossed framework, an arrangement of crossing laths or other thin strips of material * Lattice (music), an organized grid model of pitch ratios * Lattice (pastry), an orna ...

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 In battery technology, a concentration cell is a limited form of a galvanic cell that has two equivalent half-cells of the same composition differing only in concentrations. One can calculate the potential developed by such a cell using the Nernst ...

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: : Ag,AgCl, NaF, LaF3, fluoride analyte solution where: * Hg, 1/2Hg2Cl2 , KCl(sat) is an external reference electrode * NaF,NaCl , AgCl,Ag , 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 In electrochemistry, the Nernst equation is a chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction ( half-cell or full cell reaction) from the standard electrode potential, absolute tempe ...

: :''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

kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phys ...

s, * ''F'' is the Faraday constant (9.6485309×10⁴ C/mol). * ''a''_F⁻ is the activity of the fluoride ion.

References

*{{Cite journal , title = Determination of Fluoride in Toothpaste Using an Ion-Selective Electrode , last1 = Light , first1 = Truman S. , last2 = Cappuccino , first2 = Carleton C. , journal = Journal of Chemical Education , year = 1975 , volume = 52 , issue = 4 , pages = 247–50 , doi=10.1021/ed052p247, pmid = 1133123 , bibcode = 1975JChEd..52..247L Martin S. Frant and James W. Ross Jr. Electrode for Sensing Fluoride Ion Activity in Solution. Science 23 December 1966:Vol. 154. no. 3756, pp. 1553 - 1555 Electrodes Fluorine