⚗️ Fluoride in water
Potentiometric determination of fluoride ion (F⁻) concentration in water using an ion-selective electrode (ISE). Reference method for drinking water.
Overview
Fluorides (F⁻) occur in natural waters at concentrations usually not exceeding 1.5 mg/L. They are an important drinking water quality parameter—at low concentrations (0.7–1.0 mg/L) they have a preventive effect on tooth enamel, while at concentrations above 1.5 mg/L they can cause dental and skeletal fluorosis.
The PN-EN ISO 10359-1 standard describes an electrochemical method for determining dissolved fluorides in drinking water and lightly polluted waters using a fluoride ion-selective electrode (ISE). The method is suitable for concentrations from 0.2 mg/L to 2000 mg/L F⁻.
The permissible concentration of fluorides in drinking water in Poland is 1.5 mg/L (Ministry of Health Regulation). Fluoride determination is required in routine monitoring of waters intended for consumption, groundwater and surface waters, and industrial wastewater (e.g., from aluminum, glass, and ceramic industries).
Method principle
The method is based on measuring the potential of a fluoride ion-selective electrode (ISE) immersed in a water sample. The electrode membrane is made of lanthanum fluoride monocrystal (LaF₃), in which only F⁻ ions are mobile. The electrode potential is proportional to the logarithm of the fluoride ion activity in solution (according to the Nernst equation). Before measurement, TISAB buffer (Total Ionic Strength Adjustment Buffer) is added to samples and standards, which stabilizes the ionic strength of the solution, maintains pH in the range 5–5.5, and decomplexes fluorides bound with aluminum and iron.
Applications
- Drinking water quality control (Journal of Laws 2017 item 2294—limit 1.5 mg/L)
- Groundwater and surface water monitoring
- Industrial wastewater control (aluminum, glass, chemical industries)
- Mineral and therapeutic water testing
- Water fluoridation process monitoring
- Water control in semiconductor industry (HF etching)
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 0.2–2000 mg/L F⁻ |
| Detection limit | 0.02 mg/L |
| Accuracy | ±2% in measurement range |
| Sample pH (after TISAB) | 5.0–5.5 |
| Temperature | 20–25°C (samples and standards at same temperature ±1°C) |
| Stabilization time | 3–5 minutes per sample |
Standard
- Standard number
- ISO 10359-1:1992
- Title (PL)
- Jakość wody — Oznaczanie fluorków — Część 1: Metoda elektrochemiczna dla wody pitnej i wody o małym zanieczyszczeniu (brak odpowiednika PN-EN)
- Title (EN)
- Water quality — Determination of fluoride — Part 1: Electrochemical probe method for potable and lightly polluted water
Step-by-step procedure
1. Electrode preparation
Check ISE electrode condition—immerse in 1 mg/L F⁻ standard solution for 30 min for conditioning. Check electrolyte level in reference electrode.
2. Preparation of calibration standards
From 100 mg/L F⁻ stock solution, prepare a series of standards: 0.2; 0.5; 1.0; 2.0; 5.0; 10.0 mg/L in PE or PP volumetric flasks.
3. Adding TISAB
To each standard and sample (20 mL), add equal volume (20 mL) of TISAB II buffer. Mix.
4. Calibration—standard series
Immerse electrode in successive standards (from lowest concentration). Stir slowly on stirrer. Read potential (mV) after stabilization. Plot calibration curve E vs log[F⁻].
5. Calibration verification
Check curve slope—should be approximately -56 to -59 mV/decade at 25°C (theoretical value -59.16 mV/decade). Correlation coefficient r² > 0.999.
6. Sample measurement
To 20 mL sample, add 20 mL TISAB. Immerse electrode, stir slowly. Read potential after stabilization (±0.3 mV for 1 min). Read concentration from calibration curve.
7. Quality control
Every 10 samples, measure control sample (CRM). Deviation max ±10% from certified value. At end of series, check repeatability of standard.
8. Electrode storage
After work completion, immerse ISE electrode in 1 mg/L F⁻ standard solution or in deionized water. Never store dry.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Ion meter / pH meter with ISE input | Thermo Orion Star A214, Mettler Toledo SevenCompact S220, Hach HQ440d | 5,000–18,000 PLN |
| Fluoride ion-selective electrode (ISE) | Thermo Orion 9609BNWP, Hach Intellical ISEF121, Mettler Toledo perfectION F⁻ | 2,000–5,500 PLN |
| Reference electrode (if ISE not combined) | Thermo Orion 900200, Ag/AgCl | 800–2,000 PLN |
| Magnetic stirrer | IKA C-MAG MS 4, Heidolph MR Hei-Standard | 1,500–4,000 PLN |
| Polyethylene / polypropylene vessels (50–100 mL) | PE beakers, sample cups (not glass!) | 5–20 PLN/piece |
| Automatic pipettes | Eppendorf Research Plus, Brand Transferpette S | 800–2,500 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| TISAB II buffer | 68-04-2 | Total Ionic Strength Adjustment Buffer—acetic acid, sodium citrate, NaCl, NaOH; pH ~5.0–5.5. Available ready-made (Thermo, Hach) or prepared in laboratory. Add 1:1 to sample. |
| Fluoride standard 100 mg/L F⁻ | 7681-49-4 | Certified NaF standard solution for calibration. Prepare dilution series: 0.2; 0.5; 1.0; 2.0; 5.0 mg/L. |
| Sodium fluoride (NaF) p.a. | 7681-49-4 | For preparing standard solutions, dried at 105°C for 2 h. M = 41.99 g/mol. |
| Deionized water | — | Conductivity <0.1 mS/m, free from fluorides. For preparing standards and dilutions. |
Health and safety (OHS)
- Sodium fluoride (NaF)—toxic by ingestion and inhalation (H301, H311, H331). Use gloves, safety glasses, and lab coat.
- TISAB solution—contains acetic acid—irritating to skin and eyes. Use gloves.
- Work in well-ventilated area—avoid inhaling acetic acid vapors.
- Fluoride-containing waste—collect separately and dispose according to laboratory procedures.
- Safety glasses and lab coat required at all times.