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 – potential difference depends on ions

concentrations on either side of glass membrane

– pH meter selective for H+ ions

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

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

LaF3

LaF2+ + F-

VACANCY

F- La3+ Eu2+

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

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

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-

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

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

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

Experimental Results

• INSERT HUMAN DATA