Sodium fluoride

(endorsed 2005)

Sodium fluoride is used to artificially fluoridate water, to reduce the occurrence of dental caries. Use of sodium fluoride is more common in small fluoridation facilities.

General description

Sodium fluoride, NaF, is a white, odourless powder (or crystals), supplied in 25 kg bags. It is easily soluble in water, and the solubility varies little with temperature. It has a specific gravity of 2.78 at 20°C. The typical commercial grade of sodium fluoride is 97% purity, with about 44% fluorine. It has a bulk density of 1040–1440 kg/m³. The pH of a 1% solution is 6.5; that of a 4% solution is 7.6. Suitable materials for handling sodium fluoride include iron, steel, fibreglass-reinforced plastic and polyethylene.

Chemistry

Sodium fluoride is produced by neutralising hydrofluoric acid with either sodium carbonate or sodium hydroxide.

The dissolution of sodium fluoride in water forms fluoride ions ($\text{F}^{-}$) and sodium ions ($\text{Na}^{+}$) as follows:

$$\text{NaF}\leftrightarrow\text{Na}^{+}+\text{F}^{-}$$

Typical use in Australian drinking water treatment

Sodium fluoride is used to artificially fluoridate water, to reduce the occurrence of dental caries. In each State and Territory, except for South Australia, the fluoridation of drinking water is regulated by an Act of Parliament; New South Wales and Queensland also have regulations in force.

Sodium fluoride can be used in solution feed systems at a strength of 1–2%, or in a saturator system where water is passed through a bed of sodium fluoride crystals, thus producing a saturated solution. The water used for dissolving sodium fluoride should not have a hardness greater than 75 mg/L (as calcium carbonate, $\text{CaCO}_{3}$), because the presence of calcium and magnesium causes the formation of insoluble fluorides which may cause clogging problems.

When using sodium fluoride, it is good practice to add the chemical after drinking water has been treated, because fluoride ions may be adsorbed onto the surfaces of suspended matter in water.

The target levels of fluoride in fluoridated water in Australia vary between 0.7 and 1.0 mg/L. The lower concentrations apply in warmer climates, where more water is consumed.

For sodium fluoride of 97% strength (44% $\text{F}^{-}$), this range translates to a dose of sodium fluoride of 1.6–2.3 mg/L.

Contaminants

Sodium fluoride can contain traces of free acid or alkali, and also:

• arsenic

• lead

• silicate

• sulfate

Residual and by-product formation in drinking water

When employed in drinking water treatment, sodium fluoride should be used in such a way that any contaminant or by-product formed by the use of the chemical does not exceed guideline values in the Australian Drinking Water Guidelines.

Fluoride forms precipitates with many metals and other elements, but is notably insoluble with calcium; thus, scaling can occur when concentrated lime solution and concentrated fluoride solution come into contact. Locations for adding concentrated lime and fluoride solutions should be separated, to avoid this situation.

Status

Sodium fluoride was endorsed by the NHMRC for use as a drinking water treatment chemical in 1983. The revision undertaken in 2003 did not change the status of this chemical for the treatment of drinking water.

References

ANSI (American National Standards Institute)/AWWA (American Water and Wastewater Association) Standard no B701-99. AWWA CD-ROM (April 2003). Available at <www.awwa.org>

Clesceri LS, Greenberg AE and Eaton AD (eds) (1998). Standard Methods for the Examination of Water and Wastewater, 20th edition. American Public Health Association, Washington, DC.

Department of Health, South Africa (2003). Water fluoridation, A manual for water plant operators.

NSW Health (1957). Code of Practice for the fluoridation of public water supplies. NSW Fluoridation of Water Supplies Act 1957, NSW Government Gazette No. 135.