Sodium hexametaphosphate (SHMP) is a hexamer of composition (NaPO3)6. Sodium hexametaphosphate of commerce is typically a mixture of polymeric metaphosphates, of which the hexamer is one, and is usually the compound referred to by this name. It is more correctly termed sodium polymetaphosphate.
1.1 Food additive
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SHMP is used as a sequestrant and has applications within a wide variety of industries, including as a food additive in which it is used under the E number E452i. Sodium carbonate is sometimes added to SHMP to raise the pH to 8.0–8.6, which produces a number of SHMP products used for water softening and detergents.
A significant use for sodium hexametaphosphate is as a deflocculant in the production of clay-based ceramic particles. It is also used as a dispersing agent to break down clay and other soil types for soil texture assessment.
It is used as an active ingredient in toothpastes as an anti-staining and tartar prevention ingredient.
As a food additive, SHMP is used as an emulsifier. Artificial maple syrup, canned milk, cheese powders and dips, imitation cheese, whipped topping, packaged egg whites, roast beef, fish fillets, fruit jelly, frozen desserts, salad dressing, herring, breakfast cereal, ice cream, beer, and bottled beverages, among other foods, can contain SHMP.
SHMP is prepared by heating monosodium orthophosphate to generate sodium acid pyrophosphate:
2 NaH2PO4 → Na2H2P2O7 + H2O
Subsequently, the pyrophosphate is heated to give the corresponding sodium hexametaphosphate:
3 Na2H2P2O7 → (NaPO3)6 + 3 H2O
followed by rapid cooling.
SHMP hydrolyzes in aqueous solution, particularly under acidic conditions, to sodium trimetaphosphate and sodium orthophosphate.
Hexametaphosphoric acid was named (but misidentified) in 1849 by the German chemist Theodor Fleitmann (1828–1904). By 1956, chromatographic analysis of hydrolysates of Graham's salt (sodium polyphosphate) indicated the presence of cyclic anions containing more than four phosphate groups; these findings were confirmed in 1961. In 1963, the German chemists Erich Thilo and Ulrich Schülke succeeded in preparing sodium hexametaphosphate by heating anhydrous sodium trimetaphosphate.[17