Potentiometry and the Detection of Fluoride

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pH meter

pH meters use a ion selective glass electrode:
Thin glass membrane connects sample with a reference solution
Glass membrane typically made of silica

Potential difference builds across membrane

Potential difference depends on ions concentrations on either side of glass membrane

pH meter selective
for H+ ion

Conduction is achieved by ion exchange between singly charged cations
on the glass lattice with H+ from solution:


Then between internal solution and glass


Ion Selective Electrodes
Highly selective for a specific ion
Selective binding to analyte by ion exchange, crystallization, or complexation 

Types of Ion Selective Electrodes
Crstyalline Membrane Electrodes
Ex: LaF3 for F-
Non crystalline Membrane Electrodes
Ex: Silicate glass for Na+ and H+

Lanthanum Fluoride Electrode
Crystalline membrane electrode of LaF3, a natural conductor
Doping with EuF2 which has one less F- anion
Vacancies in crystal structure allow for ion “hopping” and thus a conduction of charge
Interference by hydroxide ion at pH > 8


Ion Hopping in a Doped Lanthium Fluoride Electrode
F- anions move through vacancies. 

Selectivity
Electrodes can also be sensitive to ions of the same charge
  which can cause interference
Need to evaluate sensitivity of your technique to other interferents
Activity, not concentration, is directly measured
Total ionic strength of buffer kept constant to minimize ionic effects

Potentiometry
Measure potential difference develops from fluoride ions present
Use potentiometer: high impedance voltmeter that draw virtually no electrical current
Ion concentration can be determined from the measured potential using the Nernst Equation:



Can be used to determine fluoride concentration in drinking water

Fluoridation
Since 1945, communities across the United States have been adding fluoride (F-) to public water supplies.
Fluoride fights tooth decay
Optimal fluoride level recommended for the prevention of tooth decay: 0.7 - 1.2 parts per million

One of three compounds typically added:


Fluorosilicic acid: liquid by product of phosphate fertilizer manufacture; expensive distribution (shipping)





Sodium Fluoride: crystalline form easy to handle, but expensive; reference standard
Sodium Fluorosilicate: powder, fine crystal; easy to ship

Fluoridation does not change taste, smell, or appearance of drinking water.


Experimental Results
(Skoog and Leary 1997)

Calibration curve constructed using public water samples.
Higher concentrations of fluoride are associated with more negative potential
Linear until very low concentrations

References

Frant, MS and Ross, James W. "Use of a Total Ionic Strength Adjustment Buffer for ELectrode Determination of Fluoride in Water Supplies."  Analytical Chemistry (1968).  40(7):1169-71

Skoog, Douglas A., and Leary, James. J.  Principles of Instrumental Analysis.  Fort Worth: Saunder College Publishing, 1997.  Print.

http://www.cdc.gov/fluoridation/