⚗️ Fluoride in water

Physicochemistry ISO 10359-1

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

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.

   ⏱ Time: 30 min

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.

   ⏱ Time: 15 min

3. Adding TISAB

   To each standard and sample (20 mL), add equal volume (20 mL) of TISAB II buffer. Mix.

   ⏱ Time: 2 min

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⁻].

   ⏱ Time: 3–5 min/standard • 🌡 Temperature: 20–25°C

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.

   🌡 Temperature: 25°C

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.

   ⏱ Time: 3–5 min • 🌡 Temperature: 20–25°C

7. Quality control

   Every 10 samples, measure control sample (CRM). Deviation max ±10% from certified value. At end of series, check repeatability of standard.

   ⏱ Time: 5 min

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

Reagents, media and consumables

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.

Frequently asked questions