potentiometry and the detection of fluoride
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Potentiometry and the Detection of Fluoride . 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 - PowerPoint PPT PresentationTRANSCRIPT
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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+ ions
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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-
Oxygen Silica
Cation(Na+ or H+)
SiO4 Glass Membrane
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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+
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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
LaF3
LaF2+ + F-
VACANCY
F- La3+ Eu2+
Ion Hopping in a Doped Lanthium Fluoride ElectrodeF- anions move through vacancies.
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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
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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
E is the potential difference E0 is the standard cell potential Q = [ion outside]/[ion inside]
R, F are constantsT is temperature n is # of e-
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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
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Fluoridation
• 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
One of three compounds typically added:
Fluoridation does not change taste, smell, or appearance of drinking water.
Na+ -F
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Experimental Results • Calibration curve
constructed using public water samples.
• Higher concentrations of fluoride are associated with more negative potential
• Linear until very low concentrations
Fluo
ride
Ion
(ppm
)
Millivolts
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Experimental Results
• INSERT HUMAN DATA