Metaphosphate

Editor-In-Chief: Henry A. Hoff

=Overview=

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.

=Introduction=

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. 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.

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. The four-phosphate unit version is cyclic.

Sodium trimetaphosphate has served as a buffering agent and sodium hexametaphosphate as a corrosion inhibitor. Both of these sodium metaphosphates are staight chains. 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.

=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.

=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.

=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.

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. 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. 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.

=References=