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 The Fluoride Ion-Selective Electrode: Procedure

The following is adapted from the EPA method for fluoride ion measurements.

  1. A stock solution of about 100 mg/L F- ion in buffer is provided. Record the actual concentration from the label.
  2. Prepare 50 ml each of a set of standard solutions by volumetric dilution of the stock solution with the 0.1 M acetate buffer.
  3. Calculate and record the actual concentrations of your standard solutions, but they should be approximately 10, 12, 14, 16, 18 and 20 mg/L F-.
  4. Exchange a clean 100-mL volumetric flask for one containing your “tap water” unknown.
  5. Dilute to volume with 0.1 M acetate buffer.
  6. The solutions are to be stirred during measurement. Center the beaker on the stir plate so that the stir bar spins slowly and smoothly in the solution and does not strike the electrodes.
  7. Starting with the least concentrated standard, place the electrodes in the solution, and, while stirring, allow the potential to stabilize for 1 to 2 minutes.
  8. Record the value.
  9. Rinse the electrodes with distilled water, wipe gently with a Kimwipe, and proceed to the next solution. Continue until all the standards and the unknown have been read.
  10. Repeat the measurement of standards and unknown until you have three readings for each solution. Be especially careful in rinsing the electrodes in going from a more concentrated to a less concentrated solution.