Sodium polytungstate

Sodium polytungstate (SPT) is a compound which has been successfully used in the manufacture of heavy liquids for a relatively short period.

Solid SPT are white crystals and aqueous SPT is a light yellow-green transparent solution. In powder form, SPT is a 12-fold aggregated isopolytungstate with a molar mass of 2986.12 g/Mol. The structure of polytungstate is modelled using octahedra in which the oxygen atoms are located at the corners and the tungsten atoms at the center of the octahedra. When represented as a spherical model, the oxygen atoms form a dense spherical shell while the tungsten atoms fill the open spaces in the octahedra. Because of this structure this substance may be considered a true metatungstate, represented structurally as Na6[H2W12O40]. In true metatungstates, both of the protons are located in the central empty space of the polyanion and are unable to penetrate the external shell of the spherical casing.

The correct formula is Na6[H2W12O40] or 3Na2WO4·9WO3·H2O.

Due to its very high solubility in water (max. density 3.1 g/cm3) SPT is widely used as a heavy liquid for gravity separation (sink swim analysis) and density gradient centrifugation. It has significant advantages when compared to the use of zinc chloride solution or the toxic halogenated carbons for sink swim analysis. Aqueous SPT is non-toxic, non-flammable, odorless, reusable and additionally it has a low viscosity.[1]

References

  1. ↑ Munsterman, Dirk; Kerstholt, Susan (1996). "Sodium polytungstate, a new non-toxic alternative to bromoform in heavy liquid separation". Review of Palaeobotany and Palynology 91: 417. doi:10.1016/0034-6667(95)00093-3.

Further reading

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