Chemistry:Sodium trimetaphosphate

Sodium trimetaphosphate

Names
Other names Sodium trimetaphosphate
Identifiers
CAS Number	7785-84-4 Y
3D model (JSmol)	Interactive image
EC Number	232-088-3
PubChem CID	24579
UNII	3IH6169RL0 Y
InChI InChI=1S/3Na.H3O9P3/c;;;1-10(2)7-11(3,4)9-12(5,6)8-10/h;;;(H,1,2)(H,3,4)(H,5,6)/q3*+1;/p-3
SMILES O=P(OP(O1)([O-])=O)([O-])OP1([O-])=O.[Na+].[Na+].[Na+]
Properties
Chemical formula	Na3P3O9
Molar mass	305.885 g/mol
Appearance	colorless or white crystals
Density	2.49 g/cm3 (anhydrous) 1.786 g/cm3 (hexahydrate)
Melting point	53 °C (127 °F; 326 K) (hexahydrate, decomposes to anyhdrous)
Solubility in water	22 g/100 mL
Solubility	insoluble in alcohol
Refractive index (nD)	1.433 (hexahydrate)
Structure
Crystal structure	triclinic (hexahydrate)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Sodium trimetaphosphate (also STMP), with formula Na₃P₃O₉, is one of the metaphosphates of sodium. It has the formula Na
3P
3O
9 but the hexahydrate Na
3P
3O
9 · (H
2O)
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.

Portion of the solid state structure of Na
3P
3O
9 · (H
2O)
6, as determined by X-ray crystallography. Highlighted is the P3O9 ring and some hydrogen bonding. Atoms are color coded as: purple = P, red = O, cyan = Na, white = H.

Synthesis and reactions

Trisodium trimetaphosphate is produced industrially by heating sodium dihydrogen phosphate to 550 °C, a method first developed in 1955:

3 NaH
2PO
4 → Na
3P
3O
9 + 3 H
2O

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 NaH
3PO
4 + 3 NaCl → Na
3P
3O
9 + 3 H
2O + 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.