A fluoride selective electrode is a type of
ion selective electrode sensitive to the concentration of the
fluoride
Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion of fluorine, with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts ty ...
ion. 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 at ...
electrode.
Lanthanum fluoride electrode
In the lanthanum fluoride electrode, the sensing element is a crystal of
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 at ...
(LaF
3), doped with
europium(II) fluoride (EuF
2) 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 (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 ...
s,
* ''F'' is the Faraday constant (9.6485309×10
4 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