Fluoride Probe Manual
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Figure 1: Probe purchased from Daigger Scientific Inc.
- For short term storage (overnight/weekend storage):
- Fill probe cap with 50% TISAB solution and 50% distilled water.
- For long term storage:
- Rinse the electrode with DI water and store dry.
- Replace the cap to protect the sensing element.
- Always try to keep the probe in the box, preferably in a upright position so the solution does not leak.
- NOTE that no electrode should be left soaking in DI water for more than a few minutes at a time.
- A total ionic strength adjustment buffer (TISAB) must be used with each sample to adjust for the error caused by hydroxide ion interference and pH differences.
- Use a solution consisting of 50% TISAB and 50% sample containing fluoride to test samples with the fluoride probe.
- To conserve TISAB, use 5 mL TISAB with 5 mL of sample in a 20 mL sampling container.
- Compatible TISAB solutions:
- Included with Daigger Scientific fluoride probe: ISA AJ0FL1-500
- Spectrum Scientific: SPCM-T-210-4LTPL (Cheaper)
- Rinse the fluoride probe with DI water every time it is used to measure a sample in order to avoid contaminating samples.
- Dry probe with Kimwipes but NEVER touch the tip with fingers or rub it with Kimwipes. Carefully blot dry the tip with the Kimwipes.
- Connect the fluoride probe to the relay box using the BNC connector attached to the probe.
- Open ProCoDA on the computer.
- To add fluoride sensor: Go to Configuration > Sensor configuration and calibration > Icon with “Volts”
- Click on “Insert sensor.” Make sure the channel matches with the sensor input.
- To view voltage readings: Go to Graphs
- In order to take a reading, dip the probe tip into the 50% sample/50% TISAB solution. Wait until the voltage stabilizes (see image below).
- Make a copy of the Google Sheets spreadsheet.
- First, before every use, check the electrode slope to ensure probe is working properly.
- Make two standard solutions:
- Standard (E1): Mix 50 mL of DI water, 50 mL of TISAB solution, and 1 mL of 1000 ppm Fluoride stock. Take the voltage reading with the fluoride probe and record under the E1 column on the spreadsheet.
- Standard (E2): Mix 50 mL of DI water, 50 mL of TISAB solution, and 11 mL of 1000 ppm Fluoride stock. Take the voltage reading with the fluoride probe and record under the E2 column.
- Take the slope of these two readings (E2 - E1). If the probe is working properly the slope should be -56 ± 4mV.
- If the slope is not within range follow the procedures in the troubleshooting section.
The fluoride probe does not directly measure fluoride but instead takes a voltage reading which then is interpreted as a fluoride concentration. This is done by creating a standard curve of known fluoride concentrations and their associated voltages. The set of standards that the summer 2018 team used are as follows:
| Standard Concentration (mg/L) | Amount of 100 mg/L fluoride used (uL) | Amount of tap water used | Amount of TISAB used |
|---|---|---|---|
| 20 | 5000 | 20 mL (20,000 uL) | 25 mL |
| 15 | 3750 | 21.25 mL (21,250 uL) | 25 mL |
| 10 | 2500 | 22.5 mL (22,500 uL) | 25 mL |
| 5 | 1250 | 23.75 mL (23,750 uL) | 25 mL |
| 3 | 750 | 24.25 mL (24,250 uL) | 25 mL |
| 1 | 250 | 24.75 mL (24,750 uL) | 25 mL |
| 0.5 | 125 | 24.875 mL (24,875 uL) | 25 mL |
| 0.3 | 75 | 24.925 mL (24,925 uL) | 25 mL |
| 0.1 | 25 | 24.975 mL (24,975 uL) | 25 mL |
- Use the fluoride probe to record voltages for each of the above standards. Start from the lowest concentration (0.1 mg/L) and test the standards in order of increasing concentration.
- Graph the -log(Concentration) with the voltage to create the calibration curve.
- Instructions on how to create a calibration curve in Python are detailed here. Below is an example of a calibration curve.
- NOTE: Do NOT touch the probe tip. Use a Kimwipe to pat dry; do not rub the tip.
- Possible errors that may occur:
- The voltage reading does not stabilize after a considerable period of time
- The slope does not fall within range
- If any of these errors are encountered:
- Try rinsing the probe tip in DI water for 5 minutes.
- Try immersing the probe tip in a solution of 50% TISAB and 50% DI water.
- Use the shiny side of the provided polishing strips to polish the probe tip.
- Rinse with DI water, and blot dry with a Kimwipe.
- If voltage does not stabilize, try disconnecting probe (unscrew and screw it back) from the ProCoDa box and tightening all cables.