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Metaphosphates are condensation products of orthophosphoric acid (H3PO4) with the sum formula (M'PO3)n, where n=1, 2, 3, ..., (M': univalent cation). Metaphosphates are used in the manufacture of commercial products and occur naturally.


A metaphosphate is a salt or an ester of metaphosphoric acid, HPO3. Phosphoric acids can occur in rings (cyclic structures), chains (catena), branched, ultraphosphoric acids (cyclic and branched), or combinations, usually referred to as polyphosphoric acids. Each of the possible oligophosphates is linked together by HPO3 units. As an example trimetaphosphate can form a 6-membered ring. Tetrametaphosphates form a similar 8-membered ring.

Sodium metaphosphates

Trimetaphosphates were used in detergents, but they caused environmental problems; specfiically, extensive colonies of algae in lakes, resulting in eutrophication.[1] As trimetaphosphates are now banned in many parts of the world because of water pollution risks, they have been replaced in powder detergents by zeolites.[2]

Although sodium metaphosphate is the general term for any sodium polyphosphate salt with four or more phosphate units, the four-phosphate unit version of sodium metaphosphate has been used commercially as an oral care agent; i.e., a chelating agent in cosmetic formulations.[3] The four-phosphate unit version is cyclic.[3]

Sodium trimetaphosphate has served as a buffering agent and sodium hexametaphosphate as a corrosion inhibitor.[3] Both of these sodium metaphosphates are staight chains.[3] Because of the corrosive nature of each of these sodium metaphosphates, these ingredients can be used safely if each formulation is prepared to avoid skin irritation.[3]

Alkali and alkali-earth metaphosphates

The chain metaphosphates NaCa(PO3)3 and NaSr(PO3)3 are isostructural, crystallise in space group P(-1), and contain anionic metaphosphate chains of (PO3)n with ionic contacts to Na+ ions in distorted octahedral sites and Ca2+ (or Sr2+) in distorted dodecahedral sites.[4]

Transition metal metaphosphates

Thyroxine has a catalytic effect on the oxidation of As(III) by Mn(III) metaphosphate, wherein the reaction rate can be increased by the presence of orthophosphoric acid.[5]

Metaphosphate glasses

In Pb-Al-metaphosphate glasses, (1 - x)Pb(PO3)2.xAI(PO3)3 with 0 < or = x < or = 1, the glass transition temperature and density vary as a function of the Al concentration, and the bonding preferences determined for Al are consistent with the behavior observed in Na-AI and Ca-AI metaphosphates.[6]

Zinc oxide (0 up to 20 mol%) dopant incorporation into phosphate glasses, developed for biomedical applications, produces a significant increase in density; however, the thermal properties presented in glass transition, and melting temperatures are reduced.[7] The proportion of NaZn(PO3)3 increases with increasing zinc oxide content, NaCa(PO3)3 occurs as a second main phase, with CaP2O6 in minor amounts.[7] Regardless of the high hydrophilicity and surface reactivity of these zinc oxide containing glasses, they had lower biocompatibility, in particular 10-20 mol% ZnO, compared to both zinc free glasses.[7]


  1. Sodhi GS (2005). Fundamental concepts of environmental chemistry 2nd Edition. Alpha Sience Int'l Ltd. p. 537. ISBN 1842652818, 9781842652817 Check |isbn= value: invalid character (help).
  2. "Zeolites".
  3. 3.0 3.1 3.2 3.3 3.4 Lanigan RS (2001). "Final report on the safety assessment of Sodium Metaphosphate, Sodium Trimetaphosphate, and Sodium Hexametaphosphate". Int J Toxicol. 20 (Suppl 3): 75–89. PMID 11766135.
  4. Abrahams I, Hawkes GE, Ahmed A, Di Cristina T, Demetriou DZ, Ivanova GI (2008). "Structures of the chain metaphosphates NaM(PO3)3 (M = Ca or Sr)". Magn Reson Chem. 46 (4): 316–22. PMID 18306173. Unknown parameter |month= ignored (help)
  5. Pastor FT, Milovanović GA, Todorović M (2008). "Kinetic method for the determination of traces of thyroxine by its catalytic effect on the Mn(III) metaphosphate-As(III) reaction". Talanta. 74 (5): 1556–61. PMID 18371817. Unknown parameter |month= ignored (help)
  6. Tsuchida JE, Schneider J, Pizani PS, Oliveira SL (2008). "Lead and aluminum bonding in Pb-AI metaphosphate glasses". Inorg Chem. 47 (2): 690–8. PMID 18081273. Unknown parameter |month= ignored (help)
  7. 7.0 7.1 7.2 Abou Neel EA, O'Dell LA, Smith ME, Knowles JC (2008). "Processing, characterisation, and biocompatibility of zinc modified metaphosphate based glasses for biomedical applications". J Mater Sci Mater Med. 19 (4): 1669–79. PMID 18060479. Unknown parameter |month= ignored (help)

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