Water sampling and analysis - Fluoride

NOTE: Article from the Geogenic Contamination Handbook

Fluoride concentrations in drinking water normally range from below 0.1 mg/L up to 10 mg/L, but can in some cases reach 20 mg/L or more. At fluoride concentrations of >20 mg F/L, the water is often saline and therefore not used for drinking or cooking. The WHO guideline value (and that of many national standards) is 1.5 mg/L, so quantification in this range is critical.

Fluoride measurement methods can be divided into colorimetric/photometric methods (semi-quantitative and quantitative) and potentiometric methods (using an ion-selective electrode, ISE). More sophisticated methods (e.g. ion chromatography, IC) do not deliver more accurate results than a carefully calibrated ISE and are not discussed in this manual (basic information on IC can be found in Fawell et al., (2006)).

Sampling and preservation

Table 4.3 Overview of commercially available semi-quantitative fluoride field test kits (this list does not include all available kits, and it is not intended as an endorsement of any of the companies or products listed)

For both field and laboratory testing, it is important that correct sampling procedures be followed. To ensure that the sample is representative, it should be freshly drawn from the aquifer. The groundwater should be pumped to ensure that at least one well volume of water is removed before collecting a sample. An alternative is to measure dissolved oxygen and/or pH can in the pumped water until the parameters have a constant value before taking a sample. The sampling bottle should be rinsed three times using the pumped water, making sure to keep the lid clean, before the sample is collected for analysis. Depending on the analytical method chosen, the fluoride measurement can be carried out directly on-site at the water source, or as soon as possible back in the laboratory.

If a sample is to be taken back to the laboratory, its volume should suffice for at least 5 fluoride analyses. The sample bottles should be filled to the top. The sample ID should be written on the bottle, or better still, on a label stuck on the bottle, with a waterproof pen BEFORE the sample is taken. A leaky sample bottle can render labels unreadable.

Plastic bottles are recommended, as glass bottles can easily break. Water samples should be transported and stored in a cool, dark and clean environment. If the samples are properly preserved, the fluoride measurements will still be reliable even if carried out several months after sampling (This is important when samples are collected for quality checking in a reference laboratory). However, it is better to analyse the samples as soon as possible, because some fluoride might precipitate in the presence of other ions.

Field test kits

Table 4.4 List of commercially available quantitative fluoride field test kits (this list does not include all available kits, and it is not intended as an endorsement of any of the companies or products listed))

A large number of fluoride field kits based on colorimetric techniques are commercially available (Tables 4.3, 4.4). The final colour of a test paper or a water sample is either compared visually with a colour scale (semi-quantitative) or more precisely against standard measurements using a photometer (quantitative). The colouring reagent SPADNS (1,8-dihydroxy-2-(4-sulfophenylazo)naphthalene-3,6-disulfonic acid) is commonly used as a reagent for fluoride determination.

The ease of operation of the photometers makes them attractive for organisations without well-trained laboratory staff. Some photometers are designed for field use. A drawback of all fluoride field test kits is the low upper detection limit; water samples often have to be diluted, increasing the risk of miscalculation.

Other semi-quantitative field test kits

Two field test kits are available in India; however, it might not be possible to ship these to other countries. The Orlab test kit and the test kit are developed by the National Chemical Laboratory in Pune and are distributed by the Chem-In Corporation.

Fluoride analysis in the laboratory

Ion-selective electrodes (ISE) are widely used for fluoride analyses. TISAB (total ionic strength adjustment buffer) is added to the diluted or undiluted water sample, which is stirred during the measurement. The electropotential is measured with an ion-selective electrode. This analytical method requires more laboratory experience than a photometric method.

The fluoride electrode has to be calibrated before use. The preparation of 3–8 standards with fluoride concentrations ranging between 0.05 mg/L and 2 mg/L is recommended. Dilution will be necessary if concentrations exceed 2 mg/L, so that sample concentrations lie within the calibrated range. Please note that high concentrations of dissolved aluminium in the sample can interfere with the ISE fluoride analysis. Suppliers of ISE provide manuals with information on calibration. A list of some suppliers and operation manuals is provided below:

• Hach
• Metrohm
• Mettler
• Thermo Orion

References

For references, please visit the page References - Geogenic Contamination Handbook.

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