Fluoride selective electrode

The fluoride-selective electrode , and fluoride-sensitive electrode, fluoride ion electrode or shortly fluoride electrode called, is a measuring electrode whose potential on the concentration of fluoride ions (F ^- depends on the solution in which it is immersed). It serves as a sensor to determine the concentration of fluoride ions. It is an ion-selective electrode that, together with a separate or built-in reference electrode , must be immersed in the solution so that the voltage between the electrodes can be measured with a suitable measuring instrument - some devices also convert the voltage into a concentration. A fluoride electrode can be used in a fairly wide concentration range - typically from 1 · 10 ⁻⁶ to 0.1 molar, with adjustments in the range up to 1 · 10 ⁻⁷ mol / l or even 1 · 10 ⁻⁹ mol / l or up to the saturation limit. The determination method with the fluoride electrode is therefore the most important and most frequently used for fluoride. It is used, for example, for the direct determination of fluoride in drinking water , or for determination in liquids such as plasma , serum , saliva or milk .

construction

Schematic structure of a fluoride electrode and the measuring arrangement

The most important part of the fluoride-selective electrode is a membrane made of a solid fluoride ion conductor, mostly a single crystal made of lanthanum fluoride LaF ₃ , which has been doped with europium ions Eu ²⁺ .

The entire measurement setup can be passed through

Cu '| Ag, AgCl | KCl || solution to be measured | LaF ₃ | KF, KCl | AgCl, Ag | Cu

describe when a silver-silver chloride reference electrode is used, d. H.

• Cu '| Ag, AgCl | KCl is the silver-silver chloride electrode .
• KF, KCl | AgCl, Ag | Cu describes the solution (1 M KF, total KCl, total AgCl, alternatively e.g. also 0.1 M NaF and 0.1 M KCl) and the electrode within the fluoride electrode housing.

Measurement and concentration dependence

Strictly speaking, the electrode does not measure the concentration, but the activity of the fluoride ions. In order to obtain reliable measured values even for samples with fluctuating ionic strength , a relatively high ionic strength can be set before the measurement by adding a special buffer solution (TISAB, total ionic strength adjustment buffer). This also ensures that the pH value is not too high, as hydroxide ions can falsify the measurement. It also contains reagents that complex the trivalent ions of aluminum (Al ³⁺ ) and iron (Fe ³⁺ ) and thus prevent them from binding fluoride and thus simulating a lower fluoride concentration.

The voltage between the fluoride-selective electrode and a reference electrode is measured. It is given by according to the Nernst equation , taking into account the single negative charge of the fluoride ion

${\displaystyle {E=E^{\circ }-{\frac {RT}{F}}\ln {a_{\mathrm {F^{-}} }}=E^{\circ }-{\frac {RT}{F}}\ln {c_{\mathrm {F^{-}} }}-{\frac {RT}{F}}\ln {\gamma _{\mathrm {F^{-}} }}}}$.

With

${\displaystyle E}$	Electrode potential of the fluoride electrode or voltage measured against a specific reference electrode
${\displaystyle E}$°	Potential of the fluoride electrode or voltage against the same reference electrode with a fluoride activity of one ( ) ${\displaystyle a_{\mathrm {F^{-}} }=1}$
${\displaystyle R}$	Universal or molar gas constant , R = 8.31447 J mol −1 K −1 = 8.31447 CV mol −1 K −1
${\displaystyle T}$	absolute temperature (= temperature in Kelvin )
${\displaystyle F}$	Faraday's constant , F = 96485.34 C mol −1 = 96485.34 JV −1 mol −1
${\displaystyle a_{\mathrm {F^{-}} }}$	Activity of the fluoride anion, ${\displaystyle {a_{\mathrm {F^{-}} }=c_{\mathrm {F^{-}} }\gamma _{\mathrm {F^{-}} }}}$
${\displaystyle c_{\mathrm {F^{-}} }}$	Concentration of the fluoride anion

If the addition of TISAB ensures that the ionic strength remains approximately the same, the activity coefficients are the same for different samples and it is approximately the case

${\displaystyle {E=E^{\circ \prime }-{\frac {RT}{F}}\ln {c_{\mathrm {F^{-}} }}}}$

With

${\displaystyle {E^{\circ \prime }=E^{\circ }-{\frac {RT}{F}}\ln {\gamma _{\mathrm {F^{-}} }}}}$,

d. H. At constant ionic strength, constant pH and constant temperature, the measured voltage depends linearly on the logarithm of the fluoride ion concentration.

With the value of the electrode slope one obtains z. B. at 25 ° C

${\displaystyle {E=E^{\circ \prime }-59{,}2\,\mathrm {mV} \log {c_{\mathrm {F^{-}} }}}}$.

Historical

The fluoride electrode was in 1966 by a member of since 1996 to Thermo Fisher Scientific company belonging to Orion Research filed for a patent and then presented to the public. Also in 1966, details on the ionic conductivity of lanthanum fluoride were published, which is caused by very mobile fluoride ions. In the following year, the TISAB ionic strength adjustment method was developed. Fluoride-sensitive electrodes have also been used in the education of chemistry students since 1975 at the latest.

Application examples

The multitude of possible applications of fluoride electrodes include B. the fluoride determination in parts of plants or in sea water and the monitoring of the fluorine emissions of an aluminum smelter. In countries where drinking water is fluoridated , the electrode is used for online monitoring. The relatively reliable, fast and inexpensive analytical method is also suitable for use in student internships.

Individual evidence

1. ↑ Method for determining the concentration of fluoride (F ^- ) in Aqueous Solutions. NICO2000 Ltd, February 5, 2015, accessed March 12, 2015 . 
2. ^ Alberto Enrique Villa: Rapid Method for Determining Very Low Fluoride Concentrations Using an Ion-selective Electrode . In: Analyst . tape 113 , no. 8 , August 1988, p. 1299-1303 , doi : 10.1039 / AN9881301299 .
3. ↑ ^{a b c} A. D. Campbell: Determination of fluoride in various matrices . In: Pure and Applied Chemistry . tape 59 , no. 5 , p. 695-702, , doi : 10.1351 / pac198759050695 .
4. ↑ Carolyn A. Tylenda: Toxicological Profile for Fluorides, Hydrogen Fluoride, and Fluorine (Update) . Ed .: DIANE Publishing. 2011 ( cdc.gov [PDF]).
5. ↑ Jiao, Y., Duan, X.-H .: Recent research progress in the detection of fluoride ion . In: Chinese Journal of Pharmaceutical Biotechnology . tape 21 , no. 2 , 2014, p. 180–184 ( page no longer available , search in web archives: English summary with the indication "Ion selective electrode method is currently used most widely." ).
6. ↑ ^{a b} Patent US3431182 : Fluoride Sensitive Electrode and Method of Using Same. Applied on February 4, 1966 , published March 4, 1969 , inventor: Martin S. Frant. ‌
7. ↑ ^{a b} Martin S. Frant, James W. Ross Jr .: Electrode for Sensing Fluoride Ion Activity in Solution . In: Science . tape 154 , no. 3756 , December 23, 1966, pp. 1553–1555 , doi : 10.1126 / science.154.3756.1553 .
8. ↑ ^{a b} Martin S. Frant, James W. Ross Jr: Use of a total ionic strength adjustment buffer for electrode determination of fluoride in water supplies . In: Analytical Chemistry . tape 40 , no. 7 , June 1968, p. 1169-1171 , doi : 10.1021 / ac60263a005 .
9. ^ A. Sher, R. Solomon, K. Lee, MW Muller: Transport Properties of LaF ₃ . In: Physical Review . tape 144 , no. 2 , April 1966, p. 593-604 , doi : 10.1103 / PhysRev.144.593 .
10. ↑ ^{a b} Truman S. Light, Carleton C. Cappuccino: Determination of fluoride in toothpaste using an ion-selective electrode . In: Journal of Chemical Education . tape 52 , no. 4 , 1975, p. 247-250 , doi : 10.1021 / ed052p247 .
11. ↑ Alberto Enrique Villa: Rapid method for determining fluoride in vegetation using an ion-selective electrode . In: Analyst . tape 104 , no. 1239 , June 1979, p. 545-551 , doi : 10.1039 / AN9790400545 .
12. ^ Theodore B. Warner: Fluoride in Seawater: Measurement with Lanthanum Fluoride Electrode . In: Science . tape 165 , no. 3889 , 1969, p. 178-180 , doi : 10.1126 / science.165.3889.178 .
13. ↑ Jorge O. Ares: Fluoride Cycling near A Coastal Emission Source . In: Journal of the Air Pollution Control Association . tape 28 , no. 4 , 1978, p. 545-551 , doi : 10.1080 / 00022470.1978.10470608 .
14. ↑ Jianjun Zhao: Application of an ion-selective electrode to the potentiometric determination of fluoride. (PDF) (No longer available online.) In: Department of Chemistry and Biochemistry, Bern, practical course for physical chemistry I, thermodynamics, fluoride. University of Bern, 2006, archived from the original on April 2, 2015 ; accessed on March 11, 2015 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Carla Vogt: Practical Basics of Analysis - Experiment 6 - Ion-selective electrodes - Potentiometric fluoride determination. (PDF) In: Analytical Working Group - Institute for Inorganic Chemistry. Leibniz Universität Hannover, 2006, accessed on March 11, 2015 . Fluoride-specific electrode. (PDF) (No longer available online.) In: Institute for Analytical Chemistry, Analytical-Chemical Practical Course (270101 UE), Fluoride electrode: Part 1. University of Vienna, 2005, archived from the original on March 4, 2016 ; accessed on March 11, 2015 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Nicolas Bings, Thorsten Hoffmann: Practical Analytical Chemistry - Basic Practical Analytical Chemistry - AC II - Quantitative Chemical Analysis - Scriptum for the summer semester 2014. (PDF) Johannes Gutenberg University Mainz, August 14, 2014, accessed on March 11, 2015 . Hartmut Heinrichs, Albert Günter Herrmann: Practical course in analytical geochemistry . Springer-Verlag, Berlin Heidelberg 1990, ISBN 978-3-642-61286-2 , 14.14 F fluoride electrode, p.  
     
      
     
     488-492 , doi : 10.1007 / 978-3-642-61286-2_14 .