Sodium hexametaphosphate SHMP

Food additive

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 drinks, among other foods, can contain SHMP.

Preparation

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.

Reactions

SHMP hydrolyzes in aqueous solution, particularly under acidic conditions, to sodium trimetaphosphate and sodium orthophosphate.

History

Hexametaphosphoric acid was named (but misidentified) in 1849 by the German chemist Theodor Fleitmann. 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.

Safety

Although sodium phosphates are generally of low toxicity (or even nutritious), sodium hexametaphosphate is very toxic. It causes “massive shock, irregular pulse, eventually bradycardia, and severe hypocalcemia