Sodium tripolyphosphate
(endorsed 2005)
Sodium tripolyphosphate is used in drinking water treatment to control corrosion and soften water; it is also used as a sequestering and descaling agent, and to stabilise or disperse calcium and iron in the water distribution system
General description
Sodium tripolyphosphate, , is a white powder or granular solid, and is odourless. A 1% aqueous solution of sodium tripolyphosphate has a pH of 9.8; the pH of a concentrated solution (slurry) is about 10.5.
Appropriate handling materials for sodium tripolyphosphate include cast iron, steel, fibreglass-reinforced plastic, polyethylene and polyvinyl chloride; rubber-lined containers can also be used.
Chemistry
Sodium tripolyphosphate is manufactured by combining soda ash or caustic soda with phosphoric acid. The product is then heated to form crystalline solids.
Low concentrations of polyphosphate inhibit the precipitation of calcium salts, and therefore inhibit scale formation. If phosphate concentrations are increased, then calcium phosphate precipitates. A further increase in concentration results in the sequestration phenomenon, whereby calcium is sequestered, inhibiting scale formation. Sequestering is affected by pH, with a neutral to alkaline pH being more effective.
Sodium tripolyphosphate can be used as a corrosion inhibitor in combination with divalent cations such as calcium (). Positively charged colloidal complexes form, migrate to the cathode and create an amorphous polymeric film. This inhibition is most effective at a pH of 6.5–7.
Typical use in Australian drinking water treatment
The chemical is used in drinking water treatment to control corrosion and soften water; it is also used as a sequestering and descaling agent, and to stabilise or disperse calcium and iron in the water distribution system.
Polyphosphates can change the characteristics of corrosion, making it more uniform rather than a pitting type of corrosion. Polyphosphates have also been used to control oxidation of ferrous iron dissolved from pipes, and to reduce the formation of ‘red water’ (caused by contamination with hydrated iron oxide). When mixed with orthophosphate, polyphosphates may assist in the formation of an orthophosphate film, by complexing calcium or manganese in hard waters that might otherwise cause unwanted orthophosphate precipitates.
Typical doses for protection against scale, corrosion and prevention of ‘red water’ range from 0.5 to 20 mg/L, although doses of up to 50 mg/L may be used during mains cleaning.
For corrosion control in a cast-iron distribution system, an initial feed of 5–10 mg/L may be applied for several weeks, followed by a maintenance dosage of 1–2 mg/L; or a continuous dosage of 1–5 mg/L may be used.
For sequestration applications, a ratio of 3.4–5 parts sodium tripolyphosphate per water hardness (as CaCO₃) is recommended by manufacturers.
Control of post-precipitation in softened water typically requires a dosage of 0.5–2 mg/L.
Laboratory or pilot trials should be undertaken to determine the appropriate doses.
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
fluoride
iron
lead
phosphates
Residual and by-product formation in drinking water
When employed in drinking water treatment, sodium tripolyphosphate 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 orthophosphates are present in finished water and can cause problems. For example, phosphates increase biological activity in the distribution system, and polyphosphates both reduce the deposition of protective calcium-containing films and increase the solubility of metals, interfering with the formation of passivating films. Polyphosphates also soften asbestos-cement pipe by accelerating the depletion of calcium and inhibiting the formation of fibre-binding iron or manganese deposits. Similar effects can occur in cement-lined or concrete pipes.
The use of sodium tripolyphosphate 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 plant.
Status
Sodium tripolyphosphate was endorsed by the NHMRC for use as a drinking water treatment chemical in 2005.
References
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.
Lewis RJ (1993). Hawley’s Condensed Chemical Dictionary, 12th edition. Van Nostrand Reinhold, New York.
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