Potentiometry and the Detection of Fluoride

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

3. Glass Membrane Electrode SiO4 Glass Membrane • 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- 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+

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 LaF3 LaF2+ + F- Ion Hopping in a Doped Lanthium Fluoride Electrode F- anions move through vacancies. Eu2+ F- La3+

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

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 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: 0.7 - 1.2 parts per million

9. Fluoridation One of three compounds typically added: Na+-F • 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 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) Millivolts

11. Experimental Results • INSERT HUMAN DATA