Sodium hexametaphosphate
(endorsed 2005)
Sodium hexametaphosphate can be used for control of corrosion, prevention of scale formation, and sequestration of unwanted precipitants.
General description
Sodium hexametaphosphate, (also known as SHMP, glassy phosphate or vitreous phosphate) is a white granular powder with a bulk density of 800–1500 kg/m³. It is highly soluble in water.
Sodium hexametaphosphate can be stored in rubber-lined containers, or in plastics, fibreglass-reinforced plastic, or stainless steel (type 316).
Chemistry
Sodium hexametaphosphate is produced by treating soda ash or caustic soda with phosphoric acid.
Polyphosphates keep metal ions in solution for a period of time, thus preventing deposition.
With time, sodium hexametaphosphate naturally reverts to orthophosphate, and thus loses its sequestering capability. This reversion can be accelerated by low pH, high temperature and the presence of oxides of certain materials (e.g. iron, calcium, copper and zinc). The reversion can occur in hot water systems or in reverse osmosis (RO) membranes, where it can cause fouling.
Typical use in Australian drinking water treatment
In water treatment plants, a thin layer of sodium hexametaphosphate formed on metal surfaces is used to control corrosion. The chemical is also used as a sequestering agent, to prevent unwanted precipitates or scales (e.g. iron, manganese, calcium or magnesium) from depositing.
Control of ferrous iron through sequestering is only effective up to concentrations of 3 mg/L ferrous iron. In water treatment, the amount of sodium hexametaphosphate should be controlled to ensure that concentrations do not exceed levels that would complex manganese or iron by more than 10%. Control of calcium carbonate (CaCO₃) scale rarely requires more than 1 mg/L of polyphosphate.
Contaminants
The purity of chemicals used in Australia for the treatment of drinking water varies, depending on the manufacturing process. The following chemical contaminants may be present in this product (JECFA):
arsenic
iron
fluoride
lead
Residual and by-product formation in drinking water
When employed in drinking water treatment, sodium hexametaphosphate 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.
Sodium and orthophosphate residues are present in finished water. Sodium hexametaphosphate naturally reverts to orthophosphate over time. Residual orthophosphate encourages biological growth.
The use of sodium hexametaphosphate in the water supply adds to the phosphorous load at the sewage treatment plant. Its use should therefore be considered in consultation with the manager of the sewage treatment plant.
Status
Sodium hexametaphosphate 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 B502-01. 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.
Gosselin RE, Smith RP and Hodge HC (1984). Clinical Toxicology of Commercial Products, 5th edition. Williams and Wilkins, Baltimore, II-121.
JECFA (Food and Agriculture Organization of the United Nations (FAO)/World Health Organization (WHO) Joint Expert Committee on Food Additives). Compendium of Food Additive Specifications. FAO Food and Nutrition Papers 52 (two volumes). Available at Lewis RJ (1993). Hawley’s Condensed Chemical Dictionary, 12th edition. Van Nostrand Reinhold, New York.
NRC (National Research Council) (1981). Drinking Water & Health, Volume 4. National Academy Press, Washington, DC.
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