⚗️ Fluoride in water
In short
Determination of dissolved fluoride in drinking water, in lightly polluted waters and in some surface waters with a fluoride ion-selective electrode, after adding a buffer that adjusts the ionic strength and pH. The test is performed according to ISO 10359-1:1992; Range: from 0.2 mg/l to 2.0 g/l; after adding a known amount of fluoride, detection from 0.02 mg/l (1.1, 7.3). The procedure comprises 6 steps; it is used for: Water intended for human consumption — fluoride 1.5 mg/l (Journal of Laws 2026, item 748, Annex 1 Part B), Fresh, potable and lightly polluted water and some surface waters (1.1), Concentrations below 0.2 mg/l — after adding a known amount of fluoride (7.3).
At a glance
- Standard: ISO 10359-1:1992
- Category: Physicochemistry
- Procedure steps: 6
- Range: from 0.2 mg/l to 2.0 g/l; after adding a known amount of fluoride, detection from 0.02 mg/l (1.1, 7.3)
- pH of the measuring solution: 5.2 ± 0.2 (buffer adjusted to pH 5.2)
- Test portion: 25 ml of buffer + 25 ml of sample
Overview
STATUS (as of 29.10.2026). ISO 10359-1:1992 — first edition of 1.12.1992, current in the ISO catalogue, with no newer edition or draft. The standard has not been adopted as a Polish Standard — the catalogue of Polish Standards contains no standard numbered 10359-1; the Polish title is a working translation.
SCOPE (clause 1). Dissolved fluoride in fresh, potable and lightly polluted water and in some surface waters. The method is directly suitable for concentrations from 0.2 mg/l to 2.0 g/l; after the addition of a known amount of fluoride, concentrations as low as 0.02 mg/l can be detected (7.3). It is not suitable for waste waters and industrial effluents — that determination is covered by part 2 of the standard (ISO 10359-2: inorganically bound fluoride after digestion and distillation).
FLUORIDE IN WATER. Fluoride ions occur in almost all ground and surface waters; their concentration depends mainly on the hydrogeological conditions and is generally below 1 mg/l. Some industrial waste waters may contain higher concentrations. Calcium, magnesium, aluminium and iron(III) form sparingly soluble compounds or complexes of low dissociation with fluoride; stable boron-fluoride complexes also exist (introduction of the standard). With long-term consumption of water: below 0.5 mg/l — dental caries, 0.5–1.5 mg/l — concentrations optimal for oral health, 1.5–4.0 mg/l — dental fluorosis, 4.0–10.0 mg/l — dental and skeletal fluorosis, above 10 mg/l — crippling fluorosis.
INTERFERENCES (1.2). The electrode responds directly to hydroxide ions, and HF formed under acidic conditions lowers the result — therefore all test aliquots are buffered to pH 5–7. Calcium, magnesium, iron and aluminium bind fluoride in complexes or precipitates to which the electrode does not respond; the buffer therefore contains CDTA, which frees the bound fluoride. The BF₄⁻ anion is not decomplexed by the addition of the buffer.
LEGAL REQUIREMENTS. Water intended for human consumption: fluoride 1.5 mg/l (Regulation of the Minister of Health of 22 May 2026, Journal of Laws 2026, item 748, Annex 1 Part B item 18); measurement uncertainty up to 20 % of the parametric value, limit of quantification not greater than 30 % of that value (Annex 4 Part B). For fluoride in waste water, the regulation of 12 July 2019 (Journal of Laws 2019, item 1311, Annex 12 item 26) gives ion chromatography according to PN-EN ISO 10304-1 as the reference method, not this standard.
WHAT WE DO NOT GIVE HERE. The available text of the standard ends at the beginning of 7.4 (calibration): we do not give the volumes of standard solutions from Table 1, the calculation of the result or the precision data (further clauses and informative Annex A).
Method principle
When a fluoride ion-selective electrode comes into contact with a solution containing fluoride ions, a potential difference develops between it and the reference electrode, proportional to the logarithm of the fluoride ion activity (Nernst equation). The sensing element of the electrode is a doped lanthanum fluoride crystal. Temperature and ionic strength influence the potential difference, so they must be the same and constant during calibration and measurement. The fluoride activity also depends on pH — pH 5–7 is favourable. The total ionic strength adjustment buffer (TISAB) fixes the pH and the activity coefficient, so that the result refers to the fluoride concentration rather than to its activity. Fluoride electrodes operate between 0.2 mg/l and 2000 mg/l, where the potential is a linear function of the logarithm of fluoride activity (clause 3).
Applications
- Water intended for human consumption — fluoride 1.5 mg/l (Journal of Laws 2026, item 748, Annex 1 Part B)
- Fresh, potable and lightly polluted water and some surface waters (1.1)
- Concentrations below 0.2 mg/l — after adding a known amount of fluoride (7.3)
- Not for waste waters and industrial effluents — these are covered by ISO 10359-2 (1.1)
Key parameters
| Parameter | Value |
|---|---|
| Range | from 0.2 mg/l to 2.0 g/l; after adding a known amount of fluoride, detection from 0.02 mg/l (1.1, 7.3) |
| pH of the measuring solution | 5.2 ± 0.2 (buffer adjusted to pH 5.2) |
| Test portion | 25 ml of buffer + 25 ml of sample |
| Temperature | constant during calibration and measurement, e.g. (25 ± 0.5) °C; cell jacket supplied from a water bath at (25 ± 0.2) °C |
| Electrode | response of at least 55 mV per decade of fluoride concentration at 25 °C |
| Reading | when the potential changes by not more than 0.5 mV in 5 min; recorded at least 15 s after switching off the stirrer |
Standard
- Standard number
- ISO 10359-1:1992
- Title (PL)
- Jakość wody — Oznaczanie fluorków — Część 1: Metoda z sondą elektrochemiczną dla wody pitnej i wody lekko zanieczyszczonej
- Title (EN)
- Water quality — Determination of fluoride — Part 1: Electrochemical probe method for potable and lightly polluted water
Step-by-step procedure
-
Sampling
In polyethylene bottles thoroughly washed and rinsed with fluoride-free water. No preservative is normally necessary; analyse as soon as possible, preferably within 3 days (clause 6).
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Electrode preparation
The characteristics of the electrode change with time — the calibration curve is checked on the day of use. Before measurement, the electrode is immersed for 1 h in the cell containing reference solution 5 (Table 1); after rinsing with the first solution to be measured it is ready for use. After measuring high concentrations, it is reconditioned before measuring low ones (7.1, 7.2).
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Measuring solution
Filter the sample through a membrane filter (measurement without filtration is possible, but this is stated with the result). Pipette 25 ml of buffer followed by 25 ml of sample into a measuring cell; the pH must be 5.2 ± 0.2 — if necessary, adjust it with as little hydrochloric acid or NaOH solution as possible. If a precipitate forms, the analysis is carried out on a diluted sample and the dilution is taken into account in the calculation (7.2, notes 3–5).
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Measurement
Start a series with the lowest expected concentration and finish with the highest. When a constant temperature has been reached (e.g. 25 ± 0.5 °C), measure all solutions at this temperature: stirring bar in the cell on the magnetic stirrer, electrodes fixed in the solution, stirring rate about 180–200 min⁻¹. When the potential changes by not more than 0.5 mV in 5 min, switch off the stirrer and record the value after at least 15 s. Before the next measurement, rinse the stirring bar and electrodes with the next solution to be measured (7.2).
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Concentrations below 0.2 mg/l
To 25 ml of sample add 500 µl of standard solution I with a piston pipette and 25 ml of buffer with a volumetric pipette, then proceed as in step 4; subtract the amount of fluoride added when calculating the result (7.3).
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Calibration
The calibration function is established with five reference solutions in the corresponding concentration range; for the range 0.2–10 mg/l, pipette 25.0 ml of buffer into each of five measuring cells and add the volumes of working standard solutions given in Table 1 (7.4).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Millivolt meter | impedance not less than 10¹² Ω, resolution 0.1 mV or better (5.1) | — |
| Fluoride ion-selective electrode | stable readings; response of at least 55 mV per decade of fluoride concentration at 25 °C (5.2) | — |
| Reference electrode | calomel electrode filled with saturated KCl solution or silver/silver chloride electrode; single-junction, sleeve-type electrodes are preferable (5.3) | — |
| Measuring cells, 100 ml | made of polypropylene, with a thermostatted jacket (5.4) | — |
| Water bath | supplies the cell jacket with water at (25 ± 0.2) °C (5.5) | — |
| Magnetic stirrer | with a PTFE-coated stirring bar (5.6) | — |
| Membrane filtration device | filters with 0.45 µm pores (5.8) | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Sodium hydroxide, c(NaOH) = 5 mol/l | 1310-73-2 | dissolve cautiously (100 ± 0.5) g of NaOH in water, cool and dilute to 500 ml (4.1) |
| Total ionic strength adjustment buffer (TISAB) | — | in a 1 l beaker: 58 g of NaCl and 57 ml of glacial acetic acid (ρ = 1.05 g/ml) in 500 ml of water; when dissolved, add 150 ml of 5 mol/l NaOH solution and 4 g of CDTA (trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid), stir until dissolved, adjust to pH 5.2 with NaOH solution using a pH meter, transfer to a 1000 ml flask and make up to the mark; stable for about 6 months, do not use if a precipitate forms; commercially available (4.2) |
| Fluoride stock solution, 1000 mg/l | 7681-49-4 | dry sodium fluoride at 150 °C for 4 h and cool in a desiccator; dissolve (2.210 ± 0.001) g in water in a 1000 ml flask and make up to the mark; store in a screw-capped polyethylene container (4.3) |
| Working standard solutions | — | I — 10 mg/l (10 ml of stock solution to 1000 ml), II — 5 mg/l (5 ml of stock solution to 1000 ml), III — 1 mg/l (100 ml of solution I to 1000 ml), IV — 0.5 mg/l (100 ml of solution II to 1000 ml), V — 0.2 mg/l (20 ml of solution I to 1000 ml); stored in plastic bottles, usable for one month (4.3.1–4.3.5) |
| Water | — | distilled water or water of equivalent purity; reagents of recognized analytical grade (clause 4) |
Health and safety (OHS)
- Sodium hydroxide is dissolved in water cautiously and the solution is cooled before dilution to volume (4.1)
- Handle sodium hydroxide, glacial acetic acid and sodium fluoride in accordance with their safety data sheets
Frequently asked questions
Which standard describes this test?
The test is performed according to ISO 10359-1:1992 — “Water quality — Determination of fluoride — Part 1: Electrochemical probe method for potable and lightly polluted water”.
How does this method work?
When a fluoride ion-selective electrode comes into contact with a solution containing fluoride ions, a potential difference develops between it and the reference electrode, proportional to the logarithm of the fluoride ion activity (Nernst equation). The sensing element of the electrode is a doped lanthanum fluoride crystal.
What is the measuring range and accuracy?
Range: from 0.2 mg/l to 2.0 g/l; after adding a known amount of fluoride, detection from 0.02 mg/l (1.1, 7.3); pH of the measuring solution: 5.2 ± 0.2 (buffer adjusted to pH 5.2); Test portion: 25 ml of buffer + 25 ml of sample; Temperature: constant during calibration and measurement, e.g. (25 ± 0.5) °C; cell jacket supplied from a water bath at (25 ± 0.2) °C.
How long does the test take?
The procedure comprises 6 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Millivolt meter, Fluoride ion-selective electrode, Reference electrode, Measuring cells, 100 ml, Water bath, Magnetic stirrer, Membrane filtration device.
Where is this test used?
Water intended for human consumption — fluoride 1.5 mg/l (Journal of Laws 2026, item 748, Annex 1 Part B); Fresh, potable and lightly polluted water and some surface waters (1.1); Concentrations below 0.2 mg/l — after adding a known amount of fluoride (7.3); Not for waste waters and industrial effluents — these are covered by ISO 10359-2 (1.1).
What safety precautions apply?
Sodium hydroxide is dissolved in water cautiously and the solution is cooled before dilution to volume (4.1); Handle sodium hydroxide, glacial acetic acid and sodium fluoride in accordance with their safety data sheets.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number ISO 10359-1.