1. Potentiometry and the Detection of Fluoride

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

3. Glass Membrane Electrode
Conduction is achieved by ion exchange between singly charged cations on the glass lattice with H+ from solution:
H+ + Na+Glass Na+ + H+Glass-
Then between internal solution and glass
H+Glass H+ + Glass-
SiO4 Glass Membrane
Skoog, Douglas A., and Leary, James. J. Principles of Instrumental Analysis. Fort Worth: Saunder College Publishing, Print.
Cation
(Na+ or H+)
Oxygen
Silica

4. 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+
Skoog, Douglas A., and Leary, James. J. Principles of Instrumental Analysis. Fort Worth: Saunder College Publishing, Print.

5. 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
Vacancy F-
La3+
Eu2+
LaF LaF2+ + F-
Skoog, Douglas A., and Leary, James. J. Principles of Instrumental Analysis. Fort Worth: Saunder College Publishing, Print.
Ion Hopping in a Doped Lanthium Fluoride Electrode
F- anions move through vacancies.

6. Selectivity
Electrodes can also be sensitive to ions of the same charge
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
Skoog, Douglas A., and Leary, James. J. Principles of Instrumental Analysis. Fort Worth: Saunder College Publishing, Print.

7. 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
E = E0-(RT/nF)lnQ
Skoog, Douglas A., and Leary, James. J. Principles of Instrumental Analysis. Fort Worth: Saunder College Publishing, Print.
E is the potential difference
E0 is the standard cell potential
Q = [ion outside]/[ion inside]
R, F are constants
T is temperature
n is # of e-

8. 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: parts per million

9. Fluoridation One of three compounds typically added:
Sodium Fluoride: crystalline form easy to handle, but expensive; reference standard
Fluorosilicic acid: liquid by product of phosphate fertilizer manufacture; expensive distribution (shipping)
Sodium Fluorosilicate: powder, fine crystal; easy to ship
Na+ -F
Fluoridation does not change taste, smell, or appearance of drinking water.

10. Experimental Results
Calibration curve constructed using public water samples.
Higher concentrations of fluoride are associated with more negative potential
Linear until very low concentrations
Fluoride Ion (ppm)
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):
Millivolts

11. Experimental Results
INSERT HUMAN DATA