Sodium triphosphate (STP), also sodium tripolyphosphate (STPP), or tripolyphosphate (TPP),) is an inorganic compound with formula Na<sub>5</sub>P<sub>3</sub>O<sub>10</sub>. 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.
:2 Na<sub>2</sub>HPO<sub>4</sub> + NaH<sub>2</sub>PO<sub>4</sub> → Na<sub>5</sub>P<sub>3</sub>O<sub>10</sub> + 2 H<sub>2</sub>O
In this way, approximately 2 million tons are produced annually. Many related di-, tri-, and polyphosphates are known including the cyclic triphosphate (e.g. sodium trimetaphosphate). It binds strongly to metal cations as both a bidentate and tridentate chelating agent.
thumb|class=skin-invert-image|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 (water that contains high concentrations of Mg<sup>2+</sup> and Ca<sup>2+</sup>), detergents are deactivated. Being a highly charged chelating agent, TPP<sup>5−</sup> 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.
Other
Other uses (hundreds of thousands of tons/year) include ceramics (decrease the viscosity of glazes up to a certain limit), leather tanning (as masking agent and synthetic tanning agent - SYNTAN), anticaking agents, setting retarders, flame retardants, paper, anticorrosion pigments, textiles, rubber manufacture, fermentation, antifreeze." Toothpaste may contain sodium triphosphate.
Health effects
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.