Ammonium paratungstate

Production

From tungsten ores

Tungsten ores, which are typically oxides, are digested in base to give solutions of tungstate together with many contaminating species. This crude extract is acidified and treated with sulfide to separate molybdenum trisulfide. Upon further acidification APT eventually crystallizes.

Laboratory methods

If a calcined is used, refluxing the ammonia solution is advisable to accelerate its dissolution.

Conversion to tungsten metal

Heating ammonium paratungstate to its decomposition temperature of 600 °C yields tungsten(VI) oxide, as described in this idealized equation :

From there, the trioxide is heated in an atmosphere of hydrogen, yielding elemental tungsten: :

Structure

The anion in has been shown to be , containing two hydrogen atoms, keeping two hydrogen atoms inside the cage. The correct formula notation for ammonium paratungstate is therefore . The ion is known as the paratungstate B ion, as opposed to the paratungstate A ion, which has the formula , similar to the paramolybdate ion. The existence of the paratungstate A ion, however, could not be confirmed by NMR spectroscopy.

Before about 1930, there has been some dispute about the exact composition of the salt, and both and were proposed. O.W. Gibbs remarked about this: :"The alkali tungstates are numerous and unusually complex. Salts of essentially different formulae approach so closely in percentage composition, that the differences lie very near the unavoidable errors of analyses. The analyses are hardly sufficiently close to decide the question upon purely analytical grounds."

Other hydrates

When concentrating an ammoniacal solution of tungstic acid (i.e. hydrous ), the product obtained is ammonium paratungstate. Below 50 °C, the hexahydrate is formed, whereas when the temperature of the solution is above 50 °C, the pentahydrate or heptahydrate is formed. The former crystallizes as triclinic plates or prisms, whereas the latter as pseudorhombic needles. The tetrahydrate is most significant in a commercial sense. The decahydrate and the nonahydrate4)10(H2W12O42) * 9(H2O): anionic "isomorphism"Acta Crystallographica, Section E: Structure Reports Online (2002) 58, (*) p93-p94--are also known.

References

References

1. "Tungsten, Tungsten Alloys, and Tungsten Compounds".
2. D. J. Jones, "Practical aspects of Sintering Tungsten and Molybdenum" (as referenced in ''Comprehensive Inorganic Chemistry'', J. C. Bailar Jr. et al., p. 744, vol. 3, 1st edition 1973)
3. (1972). "Die Kristallstruktur des Ammoniumparawolframat-tetrahydrats (NH₄)₁₀[H₂W₁₂O₄₂]·4H₂O". Zeitschrift für Kristallographie.
4. Greenwood & Earnshaw, ''Chemistry of the Elements'', 2nd ed. 1997, pp. 1012–1014
5. J. W. Mellor: ''Inorganic and Theoretical Chemistry'', vol. XI, p. 812–813, Longmans Green & Co. 1931
6. (1971). "Die Struktur des Ammoniumparawolframates (NH4)₁₀[H₂W₁₂O₄₂].10H₂O". Acta Crystallographica Section B.