Determination of Fluoride Ion Using an Ion Selective Electrode
Purpose:
The purpose of this experiment is to employ an ion selective electrode to determine fluoride concentration in toothpaste, and to examine the effect of changing the conditions on the measurements.
Introduction:
In recent years direct potentiometry has become important as an analytical technique largely because of the development of ion-selective electrodes (ISE).
ISEs are available to determine more than 20 different cations (e.g., Ag+, Na+, K+,
Ca2+, Cu2+) and anions (e.g., F−, Cl−, S2−, CN−) in aqueous solution. This type of electrode incorporates a special ion-sensitive membrane which may be glass, a crystalline inorganic material or an organic ion-exchanger. The membrane interacts specifically with the ion of choice.
In this experiment you will use a fluoride sensitive electrode to measure the fluoride-ion content of a solution. Fluoride is added to drinking water and toothpaste to inhibit dental caries; it is also present in effluents from many industrial processes, e.g., manufacture of fluoro- polymers. Flouride ISEs predominantly respond to free ionized F− in solution and can thus be used to measure this ion in the presence of other fluorine compounds, e.g., AlF63− or organofluorine compounds. In other words, the electrode responds to F− activity.
The electrode potential is related to the logarithm of the concentration of the measured ion by the Nernst equation.
Where R is the gas constant (8.314 V x coul / K), F is the faraday (96,487 coul), n is the magnitude of ion charge (1 for F−1). The factor 2.303 RT/F has a theoretical value of 59 mV at 25 ° The equation i s only valid for very dilute
C.
solutions or for solutions where the ionic strength is constant.
A graph of
electrode potential (mV) versus the logarithm of the concentration of fluoride
1
gives a straight line whose slope should be ~ -59.2 mV. This calibration relationship can then be employed to determine the concentration of fluoride in unknowns. The main common ion which interferes with this analysis is hydroxide ion. Consequently, it is necessary to use an acidic buffer solution to eliminate this interference. In this experiment, you will use this technique to determine fluoride in toothpaste.
Reagents and Equipments:
• NaF, dried at 100 ° for 1 hour.
C
• Total ionic strength adjustment buffer TISAB.
• Buffer (2).
• Unknown (tooth paste).
• Fluoride ISE and Ag/AgCl or SCE reference electrode.
• Multimeter or potentiometer capable of displaying mV potentials
PROCEDURE
Preparation of Standards
1. Dry the sodium fluoride in a 110 oC oven for a least 1 hour. After the drying period allow the salt to cool to room temperature in a desiccator.
2. Respectively label six of the 100.00 mL volumetric flasks with 1, 2, 3, 4, 5, 6,
Use a pipette to add 10.00 mL of TISAB to each flask.
3. Weigh 0.420 g of the cooled NaF to the nearest 0.001 g. Place the salt in
Flask 1. Add sufficient deionized water to dissolve the NaF and dilute the solution to the mark. Flask 1 now contains 0.1000 M F-1(aq)
4. Use a pipette to remove 10.00 mL of the solution from Flask 1 and transfer it into Flask 2. Dilute the solution in Flask 2 to the mark with deionized water. The solution in Flask 2 now contains 1.00 x 10-2 M F-1(aq). Similarly remove 10.00 mL of the solution in Flask 2 and transfer it into Flask 3. Dilute to the mark with deionized water, and the solution in Flask 3 now contains 1.00 x 10-3 M F-1(aq).
Continue the serial dilutions to yield fluoride standard solutions which contain
1.00 x 10-4 M in Flask 4, 1.00 x 10-5 M in Flask 5, and 1.00 x 10-6 M in Flask 6.
2
5. Label six of the 150-mL beakers with 1, 2, 3, 4, 5, 6. Pour the solution which is in each of the labeled flasks into the beaker with the corresponding label.
