Sodium hexametaphosphate (SHMP) is a salt of composition Na6[(PO3)6].[3] Sodium hexametaphosphate of commerce is typically a mixture of metaphosphates (empirical formula: NaPO3), of which the hexamer is one, and is usually the compound referred to by this name. Such a mixture is more correctly termed sodium polymetaphosphate. They are white solids that dissolve in water.
Uses
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.[4][5][6][7] It is also used as a dispersing agent to break down clay and other soil types for soil texture assessment.[8]
It is used as an active ingredient in toothpastes as an anti-staining and tartar prevention ingredient.[9]
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.[10][11][12]
Water softener salt
SHMP is used in Diamond Crystal brand Bright & Soft Salt Pellets for water softeners in a concentration of 0.03%. It is the only additive other than sodium chloride.
Sodium hexametaphosphate is the alkali salt of one of the series of polymetaphosphoric acids (acids formed by the polymerization of phosphate groups).[14] Hexametaphosphoric acid was first made in 1825 by the German chemist Johann Frederich Philipp Engelhart (1797-1853).[15] For his doctoral thesis, Engelhart intended to determine whether iron was responsible for the red color of blood. In order to purify his blood samples, Engelhart had found that he could coagulate the blood serum's albumin (dissolved proteins) by treating the blood with phosphoric acid. This contradicted the findings of the famous Swedish chemist Jöns Jacob Berzelius, who had stated that phosphoric acid did not coagulate water-soluble proteins such as egg white.[16] Berzelius and Engelhart collaborated with the intention of resolving the contradiction; they concluded that Engelhart had produced a new form of phosphoric acid simply by burning phosphorus in air and then dissolving the resulting substance in water.[17] However they did not determine the new acid's composition. That analysis was accomplished in 1833 by the Scottish chemist Thomas Graham, who named the sodium salt of the new acid "metaphosphate of soda".[18] Graham's findings were confirmed by the German chemists Justus von Liebig and Theodor Fleitmann.[19] In 1849 Fleitmann coined the name "hexametaphosphoric acid".[20][21]
By 1956, chromatographic analysis of hydrolysates of Graham's salt (sodium polyphosphate) indicated the presence of cyclic anions containing more than four phosphate groups;[22] these findings were confirmed in 1961.[23] In 1963, the German chemists Erich Thilo and Ulrich Schülke succeeded in preparing sodium hexametaphosphate by heating anhydrous sodium trimetaphosphate.[24]
Safety
Sodium phosphates are recognized to have low acute oral toxicity. SHMP concentrations not exceeding 10,000 mg/L or mg/kg are considered protective levels by the EFSA and USFDA. Extreme concentrations of this salt may cause acute side effects from excessive blood serum concentrations of sodium, such as: “irregular pulse, bradycardia, and hypocalcemia."[25]
^The Role Of Sodium Hexametaphosphate In The Dissolution Process Of Kaolinite And KaolinF. Andreola; E.Castellini; T.Manfredini; M.Romagnoli. Journal of the European Ceramic Society, Volume 24, Number 7, June 2004.
^Impact Of Dispersants On The Mechanical Strength Development Of Alumina-Spinel Self-Flowing Refractory Castables. Sasan Otroj, Mohammad Reza Nilforushan, Arash Daghighi, Reza Marzban. Ceramics – Silikáty 54 (3) 284–289 (2010)
^Effect Of Adding Sodium Hexametaphosphate Liquefier On Basic Properties Of Calcium Phosphate Cements. S. Hesaraki; A. Zamanian; F. Moztarzadeh. Journal of Biomedical Materials Research Part A. Vol. 88A, No. 2, 2009
^Study Of Clay’s Mineralogy Effect On Rheological Behavior Of Ceramic Suspensions Using An Experimental Design. Afef Jmal Ayadi; Julien Soro; Amel Kamoun; Samir Baklouti. International Journal of Recent Research and Applied Studies 14 (2). February 2013.
^ASTM D422 – 63(2007) Standard Test Method for Particle-Size Analysis of Soils.
Reprinted in French: Liebig, J. (1838). "Sur la constitution des acides organiques" [On the composition of organic acids]. Annales de Chimie. 2nd series (in French). 68: 5–93. See p. 39.
^Griffith, E.J.; Buxton, R.L. (1965). "The preparation and properties of the twelve-membered ring hexametaphosphate anion". Inorganic Chemistry. 4 (4): 549–551. doi:10.1021/ic50026a023. ; see p. 549.
^Van Wazer, J. R.; Kroupa, E. Karl (1956). "Existence of ring phosphates higher than tetrametaphosphate". Journal of the American Chemical Society. 78 (8): 1772. doi:10.1021/ja01589a086.