Sodium trimetaphosphate

Sodium trimetaphosphate (also STMP), with formula Na3P3O9, is one of the metaphosphates of sodium. It has the formula Na3P3O9 but the hexahydrate Na3P3O9*(H2O)6 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.

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:
 * 3 NaH2PO4 -> Na3P3O9  +  3 H2O

The trimetaphosphate dissolves in water and is precipitated by the addition of sodium chloride (common ion effect), 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.


 * 3 NaH3PO4 + 3 NaCl -> Na3P3O9 + 3 H2O + 3 HCl

Hydrolysis of the ring leads to the acyclic sodium triphosphate:
 * Na3P3O9 +  H2O  →  H2Na3P3O10

The analogous reaction of the metatriphosphate anion involves ring-opening by amine nucleophiles.