Hydrofluorosilicic acid

(endorsed 2005)

Hydrofluorosilicic acid is used to artificially fluoridate water, to reduce the occurrence of dental caries. When dissolved in water, hydrofluorosilicic acid forms the fluoride ion–.

General description

Hydrofluorosilicic acid, $\text{H}_{2}\text{SiF}_{6}$ (also known as fluorosilicic acid, hexafluorosilicic acid), is a colourless to pale yellow liquid, poisonous and corrosive, with a pungent odour and irritating fumes. It can etch glass. It has a specific gravity of 1.18 at 20°C at 22% strength.

The acid is usually delivered by road tanker but can be supplied in drums. It is incompatible with glass and stoneware but can be stored in polythene drums, rubber-lined mild steel or polyvinyl chloride-lined plastic tanks.

Chemistry

Hydrofluorosilicic acid is a by-product of the preparation of chemical fertilisers from phosphate rock. The rock is ground up and treated with sulfuric acid, forming a gas by-product, which then reacts with water to produce a weak acid. This hydrofluorosilicic acid solution is subsequently concentrated to strengths of up to 30%. Manufacture of hydrofluorosilicic acid is limited, but because the acid is a by-product of the agricultural industry, it is generally readily available in Australia.

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

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

Typical use in Australian drinking water treatment

Hydrofluorosilicic acid 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.

In adding hydrofluorosilicic acid to drinking water, it is good practice to add the chemical after the 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, fluoridation is usually accomplished with a 20% hydrofluorosilicic acid stock solution. The acid solution, despite its pH of 1.2, has little effect on the pH of highly alkaline water, because relatively low amounts are used. However, the pH effect can be significant with water of low alkalinity.

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 an acid solution of 20% strength (15.8% $\text{F}^{-}$), this range translates to a dose of hydrofluorosilicic acid of 4.4–6.3 mg/L.

Contaminants

Chemical contaminants that may occur in hydrofluorosilicic acid solutions include inorganic and organic substances, and the following chemicals:

• arsenic

• lead

The concentrations of contaminants depend on the purity of the raw materials used in fertiliser production. Hydrofluorosilicic acid solutions also contain free hydrofluoric acid, which prevents the precipitation of solid silica when the acid is diluted in water.

Residual and by-product formation in drinking water

When employed in drinking water treatment, hydrofluorosilicic acid 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 these solutions should be separated, to avoid this situation.

Status

Hydrofluorosilicic acid 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 B703-00. AWWA CD-ROM (April 2003). Details 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.