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Introduction
Introduction
A conventional glass electrode used in the measurement of pH develops electrical potential in response to the activity of the hydrogen ion in a solution. A specific ion electrode is designed to develop a potential in response to the activity of the ion for which it is selective. In dilute solution, the activity of an ion approaches concentration, and thus such electrodes are useful for determining the concentration of ion under these conditions. This is particularly so when electrode response is compared with a calibration graph using the solution of known concentration. The specific ion electrode may also be used as an indicator electrode to detect the endpoint of a titration. A wide range of specific ion electrode is now available. These include electrodes specific for bromide, cadmium, chloride, cupric, cyanide, fluoride, iodide, lead, nitrate, and sodium ions.
For fluoride, the sensing element in the electrode is a specially treated crystal of lanthanum fluoride, but the electrode must be used in conjunction with a reference electrode such as a calomel electrode. The reference electrode may be separate, but the fluoride electrode is available as a combination electrode with the reference built into the electrode. The crystal of lanthanum fluoride is usually doped with europium to improve the conductivity. At each membrane-solution interface, the following equilibrium takes place:
LaF3 (s) ⇋ LaF2+(s) + F- (aq)
You can see that the formation of LaF2+ creates a charge at the surface. The equilibrium will be shifted to the left for the solution with a higher F- concentration, and the potential will become more negative relative to the other side of the membrane. It is this potential difference across the LaF3 crystal membrane that is measured and related to F- concentration. The relationship between ion activity and electrode potential is logarithmic:
where E is the reading generated from the fluoride ion selective electrode, Ea is a constant, R is the universal gas constant, T is the absolute temperature, F is the Faraday-constant, and aF- is the activity of the fluoride ion in the sample solution which is equivalent to the concentration of fluoride in water. When sensing an anion, the electrode potential becomes more negative with increasing concentration of fluoride due to the negative charge of fluoride.
Polyvalent cations such as Al3+ and Fe3+ and also hydrogen ions may complex fluoride ions, but most interferences are negligible above pH 5. Most interferences are eliminated by the addition of a buffer solution containing citrate, such as TISAB (Total Ionic Strength Adjustment Buffer). Note the composition of the TISAB (Total Ionic Strength Adjustment Buffer) solution used in the experimental section.
Figure 1: Basic structure of a fluoride ion selective electrode.
Methodology
Methodology
For the preparation of the TISAB solution, dissolve 58 g sodium chloride and 0.30 g sodium citrate in a mixture of 500 g cm-3 distilled water and 57 cm³ glacial acetic acid (pure). Cool in a water bath while adding 5 mol dm-3 sodium hydroxide until the pH is between 5.0 and 5.5 (use a pH meter). Cool and dilute to 1 liter with distilled water.
Preparation of calibration plot: Prepare a series of solutions containing between 0.0 and 2.0 mg dm-3 fluoride as in Table 1. Add the listed volumes of standard fluoride solutions containing 10.0 mg dm-3 fluoride to each of six 50 cm³ volumetric flask. Dilute each to the mark with distilled water and mix well.
Add 10 cm³ TISAB to 10 cm³ of each of the above solutions in the 25 cm³ beakers. Prepare also a solution containing no fluoride by adding 10 cm³ TISAB to 10 cm³ water. Rinse the electrode with distilled water before each measurement and dry with a soft tissue. Measure the potential developed by the specific ion electrode for each solution. Insert the electrode into the stirred solution and record the reading after a fixed period of about 3 minutes. For accurate work, the electrodes must be standardized several times a day, and all solutions should be at the same temperature. Refer to the following video for the measurement of Fluoride using the ion selective electrode.
Analysis of unknown water samples: Treat Unknown A, B, and C similarly with an equal volume of TISAB solution, measure the electrode potential under conditions identical to those used for the preparation of the calibration graph and obtain the fluoride concentrations from the calibration graph.
Table 1
Results and Raw Data
Results and Raw Data
Table 2: Result obtained from calibration standard.
Table 2: Result obtained from the analysis of unknown samples.
Task
Task
Prepare a technical report based on the given data in the above section. The result and discussion must include:
the calibration plot for fluoride.
Determine the concentration of fluoride in Unknown A, B, and C.
Answer the following Question.
Question:
Discuss how TISAB reduces the interference from other ionic species in measurement using an ion-selective electrode.
This experiment is utilizing an ion-selective electrode to determine the concentration of fluoride ions. This electrode gives respond specifically to fluoride ion and not other ions. Do you think pH electrode is an ion-selective electrode? Explain your answer.
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