To accurately determine the concentration of fluoride ions in a drinking water sample using an ion-selective electrode ISE , follow these steps:1. Gather necessary equipment and reagents: - Fluoride ion-selective electrode F-ISE - Reference electrode usually Ag/AgCl - pH meter or ion meter with mV readout - Magnetic stirrer and stir bar - Standard fluoride solutions e.g., 0.1, 1, 10, and 100 ppm - TISAB Total Ionic Strength Adjustment Buffer solution - Deionized water - Graduated cylinders and pipettes - Beakers or Erlenmeyer flasks2. Calibration: a. Prepare a series of standard fluoride solutions by diluting the stock fluoride solution with deionized water. For example, prepare 100 mL of 0.1, 1, 10, and 100 ppm fluoride solutions. b. Add an equal volume of TISAB solution to each standard fluoride solution to maintain a constant ionic strength and adjust the pH to around 5-5.5. This ensures that the fluoride ions are free and not complexed with other ions. c. Connect the F-ISE and reference electrode to the pH or ion meter. d. Place the electrodes in the first standard fluoride solution e.g., 0.1 ppm and allow the reading to stabilize. Record the mV reading. e. Repeat the process for each standard fluoride solution, rinsing the electrodes with deionized water between each measurement. f. Plot the mV readings against the logarithm of the fluoride concentration log C to create a calibration curve.3. Sample preparation: a. Collect a representative drinking water sample in a clean container. b. Measure a known volume of the water sample e.g., 50 mL and transfer it to a beaker or Erlenmeyer flask. c. Add an equal volume of TISAB solution to the water sample to maintain a constant ionic strength and adjust the pH to around 5-5.5.4. Measurement: a. Place the electrodes in the prepared water sample and allow the reading to stabilize. Record the mV reading. b. Rinse the electrodes with deionized water and store them according to the manufacturer's instructions.5. Data analysis: a. Use the calibration curve to determine the fluoride concentration in the water sample. Locate the mV reading of the sample on the y-axis, and find the corresponding log C value on the x-axis. b. Convert the log C value to the actual concentration C by taking the antilog 10^log C . c. If necessary, adjust the concentration value for any dilution factors introduced during sample preparation.By following these steps, you can accurately determine the concentration of fluoride ions in a drinking water sample using an ion-selective electrode.