Sodium triphosphate
Sodium triphosphate, also sodium tripolyphosphate, or tripolyphosphate is an inorganic compound with formula Na5P3O10. It is the sodium salt of the polyphosphate penta-anion, which is the conjugate base of triphosphoric acid. It is produced on a large scale as a component of many domestic and industrial products, especially detergents. Environmental problems associated with eutrophication are attributed to its widespread use.
Preparation and properties
Sodium tripolyphosphate is produced by heating a stoichiometric mixture of disodium phosphate, Na2HPO4, and monosodium phosphate, NaH2PO4, under carefully controlled conditions.
In this way, approximately 2 million tons are produced annually.
STPP is a colourless salt, which exists both in anhydrous form and as the hexahydrate. The anion can be described as the pentanionic chain 5−. Many related di-, tri-, and polyphosphates are known including the cyclic triphosphate. It binds strongly to metal cations as both a bidentate and tridentate chelating agent.
[Image:Triphosphate chelation.svg|thumb|180px|left|Chelation of a metal cation by triphosphate.]
Uses
Detergents
The majority of STPP is consumed as a component of commercial detergents. It serves as a "builder", industrial jargon for a water softener. In hard water, detergents are deactivated. Being a highly charged chelating agent, TPP5− binds to dications tightly and prevents them from interfering with the sulfonate detergent.
Food
STPP is a preservative for seafood, meats, poultry, and animal feeds. It is common in food production as E number E451. In foods, STPP is used as an emulsifier and to retain moisture. Many governments regulate the quantities allowed in foods, as it can substantially increase the sale weight of seafood in particular. The United States Food and Drug Administration lists STPP as Generally recognized as safe.
Other
Other uses include ceramics, leather tanning, anticaking agents, setting retarders, flame retardants, paper, anticorrosion pigments, textiles, rubber manufacture, fermentation, antifreeze." TPP is used as a polyanion crosslinker in polysaccharide based drug delivery. Toothpaste may contain sodium triphosphate.
High serum phosphate concentration has been identified as a predictor of cardiovascular events and mortality. Whilst phosphate is present in the body and food in organic forms, inorganic forms of phosphate such as sodium triphosphate are readily adsorbed and can result in elevated phosphate levels in serum. Salts of polyphosphate anions are moderately irritating to skin and mucous membranes because they are mildly alkaline.
Environmental effects
Because it is very water-soluble, STPP is not significantly removed by waste water treatment. STPP hydrolyses to phosphate, which is assimilated into the natural phosphorus cycle. Detergents containing phosphorus contribute to the eutrophication of many fresh waters.