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: 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 (US Pat. 03/0154888) and as a setting retarder for gypsum plaster.^[2]

Sodium trimetaphosphate^[1]

Names
Other names Sodium trimetaphosphate
Identifiers
CAS Number	7785-84-4 Y hexahydrate: 29856-33-5
3D model (JSmol)	Interactive image hexahydrate: Interactive image
ChEMBL	ChEMBL2107557
ChemSpider	22983 hexahydrate: 14394484
ECHA InfoCard	100.029.171
EC Number	232-088-3
KEGG	D02423
PubChem CID	24579 hexahydrate: 19755115
UNII	3IH6169RL0 Y hexahydrate: 9B14301IW7
CompTox Dashboard (EPA)	DTXSID7052789
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 Key: UGTZMIPZNRIWHX-UHFFFAOYSA-K hexahydrate: InChI=1S/3Na.H3O9P3.6H2O/c;;;1-10(2)7-11(3,4)9-12(5,6)8-10;;;;;;/h;;;(H,1,2)(H,3,4)(H,5,6);6*1H2/q3*+1;;;;;;;/p-3 Key: LHBMIJXEORAANY-UHFFFAOYSA-K
SMILES O=P(OP(O1)([O-])=O)([O-])OP1([O-])=O.[Na+].[Na+].[Na+] hexahydrate: O.O.O.O.O.O.[O-]P1(=O)OP(=O)(OP(=O)(O1)[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). N verify (what is YN ?) Infobox references

Although drawn with a particular resonance structure, the trianion has high symmetry.^[3]

Portion of the solid state structure of Na₃P₃O₉·(H₂O)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.^[4]

Synthesis and reactions

Trisodium trimetaphosphate is produced industrially by heating sodium dihydrogen phosphate to 550 °C, a method first developed in 1955:^[5]

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.^[6] STMP can also prepared by heating samples of sodium polyphosphate,^[2] or by a thermal reaction of orthophosphoric acid and sodium chloride at 600°C.^[7][8]

3 H₃PO₄ + 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.^[9]