Fluoride
F⁻The ELIT fluoride electrode uses a mono-crystalline membrane for highly selective fluoride detection. The TISAB buffer is critical for this electrode, equalising ionic strength and releasing complexed fluoride.
The Selectivity Coefficient (SC) represents the approximate apparent increase in the measured concentration caused by 1 unit of the interferent. Effect (% error) = (interferent concentration × SC / analyte concentration) × 100.
| Interfering Ion | Selectivity Coefficient | Notes |
|---|---|---|
| Hydroxyl (OH⁻) | ~0.1 | Eliminated by keeping pH below 8. TISAB buffer also functions as a de-complexing agent to release fluoride bound in complex ions. |
Apparatus Required
- Ion-Selective Electrode: ELIT 8221
- Reference electrode: Single junction silver chloride (ELIT 001n)
- Dual electrode head (ELIT 201)
- ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter
- 150 ml polypropylene beakers, 100 ml volumetric flask, 1, 2, 5, 10 ml pipettes
Calibration
Calibrate with 1000, 100, 10, 1, 0.1 ppm F solutions. For samples with ionic strength > 0.01 M, pH outside 4–8, or fluoride in complex ions: mix all standards and samples with equal volumes of TISAB (1:1) before measurement.
Sample Preparation & Measurement
Low ionic strength, pH 4–8: no preparation — immerse in 50–100 ml sample. High ionic strength or pH outside range: mix 25 ml sample with 25 ml TISAB.
Results
Results are displayed as ppm and mol/l. If buffer solution has been added equally to standards and samples, figures will not need adjusting as all are affected by the same dilution factor. Allow 2–3 minutes stabilisation after electrode immersion. Wash and dry electrodes between samples to avoid cross-contamination.
Fluoride in Plant Material
Use the Standard Addition method. Sample preparation: wash plant material with deionised water, dry at 80°C for 24 hours, grind to pass a 60 mesh sieve. Weigh 2g into a 150 ml plastic beaker, add 100 ml of 0.1M perchloric acid, stir continuously for at least 20 mins. All calibration solutions must be mixed 50/50 with TISAB before measuring.
Fluoride in Toothpaste
Same procedure as plant material but disperse in pure water. Choose sample weight to give 10–100 ppm F in solution (e.g. 2g in 100 ml for ~2000 ppm toothpaste gives ~40 ppm). If solution exceeds 100 ppm F, use the Sample Addition method. Maximum solution concentration for this electrode is ~2000 ppm.
- Note the relatively narrow pH range (4 to 8) — this is the key operational constraint for the fluoride electrode.
- TISAB serves a dual purpose: equalising ionic strengths AND de-complexing fluoride from bound forms.
- For field measurements, electrodes can be immersed directly in natural water bodies if pH and ionic strength are suitable.