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
3D model (JSmol)	Interactive image
ECHA InfoCard	100.029.171
EC Number	232-088-3
PubChem CID	24579
UNII	3IH6169RL0 Y
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
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). 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]

References

1. Lide, David R. (1998). Handbook of Chemistry and Physics (87 ed.). Boca Raton, Florida: CRC Press. pp. 4–86. ISBN 0-8493-0594-2.
2. Klaus Schrödter; Gerhard Bettermann; Thomas Staffel; Friedrich Wahl; Thomas Klein; Thomas Hofmann (2008). "Phosphoric Acid and Phosphates". Ullmann's Encyclopedia of Industrial Chemistry. Ullmann’s Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_465.pub3. ISBN 978-3527306732. S2CID 94458523.
3. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 530. ISBN 978-0-08-037941-8.
4. Tordjman, I.; Durif, A.; Guitel, J. C. (1976). "Structure Cristalline du Trimétaphosphate de Sodium Hexahydraté: Na3P3O9*6H2O". Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry. 32 (6): 1871–1874. Bibcode:1976AcCrB..32.1871T. doi:10.1107/S0567740876006560.
5. Thilo, Erich; Grunze, Herbert (December 1955). "Zur Chemie der kondensierten Phosphate und Arsenate. XIII. Der Entwässerungsverlauf der Dihydrogenmonophosphate des Li, Na, K, und NH4". Zeitschrift für anorganische und allgemeine Chemie. 281 (5–6): 262–283. doi:10.1002/zaac.19552810504.
6. Bell, R. N. (1950). "Sodium Metaphosphates". Inorganic Syntheses. Vol. 3. pp. 103–106. doi:10.1002/9780470132340.ch26. ISBN 9780470132340.
7. Minh, Doan Pham; Ramaroson, Jocelyn; Nzihou, Ange; Sharrock, Patrick; Depelsenaire, Guy (1 January 2012). "A New Route for the Synthesis of Alkali Polyphosphate from Economical Starting Materials: Preparation and Characterization of Sodium Cyclotriphosphate" (PDF). Phosphorus, Sulfur, and Silicon and the Related Elements. 187 (1): 112–120. doi:10.1080/10426507.2011.590950. S2CID 98164275.
8. Pham Minh, Doan; Ramaroson, Jocelyn; Nzihou, Ange; Sharrock, Patrick (14 March 2012). "One-Step Synthesis of Sodium Trimetaphosphate (Na 3 P 3 O 9 ) from Sodium Chloride and Orthophosphoric Acid" (PDF). Industrial & Engineering Chemistry Research. 51 (10): 3851–3854. doi:10.1021/ie201085b. S2CID 98235073.
9. Bezold, Dominik; Dürr, Tobias; Singh, Jyoti; Jessen, Henning J. (2020). "Cyclotriphosphate: A Brief History, Recent Developments, and Perspectives in Synthesis". Chemistry – A European Journal. 26 (11): 2298–2308. doi:10.1002/chem.201904433. PMC 7065162. PMID 31637774.