Sodium fluorosilicate

(endorsed 2005)

Sodium fluorosilicate, $\text{Na}_{2}\text{Si}\text{F}_{6}$, is used to artificially fluoridate water, to reduce the occurrence of dental caries.

General description

Sodium fluorosilicate ($\text{Na}_{2}\text{Si}\text{F}_{6}$, also known as sodium silicofluoride, sodium hexafluorosilicate and disodium hexafluorosilicate) is a white or yellowish white, odourless, crystalline powder with a specific gravity of 2.7. Sodium fluorosilicate has very low solubility in water. The chemical is usually supplied at 98.5% purity (59.5% $\text{F}^{-}$) in 25 kg bags. It has a bulk density of 880–1150 kg/m³. Suitable handling material includes cast iron, rubber linings, steel and stainless steel, fibreglass-reinforced plastic, polyethylene and polyvinyl chloride.

Chemistry

Sodium fluorosilicate is produced by neutralising hydrofluorosilicic acid with sodium carbonate or sodium hydroxide, and then evaporating the solution.

The dissolution of sodium fluorosilicate in water forms the fluoride ion ($\text{F}^{-}$), as follows:

$$\text{Na}_{2}\text{Si}\text{F}_{6}\leftrightarrow\text{2Na}^{+}+\text{SiF}_{6}^{2-} \newline \text{SiF}_{6}^{2-}\leftrightarrow\text{SiF}_{4}+\text{2F}^{-} \newline \text{SiF}_{4}+\text{2H}_{2}\text{O}\leftrightarrow\text{SiO}_{2}+\text{4F}^{-}+\text{4H}^{+}$$

Typical use in Australian drinking water treatment

Sodium fluorosilicate is used to fluoridate drinking 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.

It is good practice to add sodium fluorosilicate after drinking water has been treated, because fluoride ions may be adsorbed onto the surfaces of suspended matter in water. In water that has been treated and disinfected, sodium fluorosilicate is usually added at a concentration of 0.2%. A good mixing system is required because sodium fluorosilicate has low solubility in water.

The targeted 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 fluorosilicate of 98.5% strength (59.5% F⁻), this range translates to a dose of sodium fluorosilicate of 1.2–1.7 mg/L.

Contaminants

Sodium fluorosilicate may contain traces of free acid and moisture, and also:

• arsenic

• cadmium

• iron

• phosphorus

Residual and by-product formation in drinking water

When employed in drinking water treatment, sodium fluorosilicate 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. Points for adding concentrated lime and fluoride solutions should be separated, to avoid this situation.

Status

Sodium fluorosilicate 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 B702-99. 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.