Sodium trimetaphosphate

Sodium trimetaphosphate, with formula Na₃P₃O₉, is one of the metaphosphates of sodium. It has the formula but the hexahydrate is also well known. It is the sodium salt of trimetaphosphoric acid. It is a colourless solid that finds specialised applications in food and construction industries: it is used as a phosphorylating
agent for ascorbic acid to stabilize vitamin C mixtures against thermal decomposition; in the construction industry, sodium trimetaphosphate is used to prevent the shrinkage of
gypsum plaster boards and as a setting retarder for gypsum plaster.
Although drawn with a particular resonance structure, the trianion has high symmetry.

Synthesis and reactions

Trisodium trimetaphosphate is produced industrially by heating sodium dihydrogen phosphate to 550 °C, a method first developed in 1955:
The trimetaphosphate dissolves in water and is precipitated by the addition of sodium chloride, affording the hexahydrate. STMP can also prepared by heating samples of sodium polyphosphate, or by a thermal reaction of orthophosphoric acid and sodium chloride at 600°C.
Hydrolysis of the ring leads to the acyclic sodium triphosphate:
The analogous reaction of the metatriphosphate anion involves ring-opening by amine nucleophiles.