Ammonium paratungstate

Ammonium paratungstate

Identifiers
CAS Number	11120-25-5
ChemSpider	21241467
ECHA InfoCard	100.031.228
UNII	GCP1SFG106
InChI InChI=1S/10H3N.41O.12W/h10*1H3;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;/q;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;5*-2;;;;;;;;;;;;/p+10 Key: LZTNPCZGBGTJFN-UHFFFAOYSA-X InChI=1/10H3N.41O.12W/h10*1H3;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;/q;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;5*-2;;;;;;;;;;;;/p+10/r10H3N.12O3W.5O/c;;;;;;;;;;12*1-4(2)3;;;;;/h10*1H3;;;;;;;;;;;;;;;;;/q;;;;;;;;;;;;;;;;;;;;;;5*-2/p+10 Key: LZTNPCZGBGTJFN-GEIIXCJIAC
Properties
Chemical formula	(NH4)10(H2W12O42)·4H2O
Molar mass	3132.2 g/mol
Appearance	White crystalline powder
Boiling point	Decomposes at 600 °C
Hazards
EU classification (DSD)	not listed
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Ammonium paratungstate (or APT) is a white crystalline salt of ammonium and tungsten, with the chemical formula (NH₄)₁₀(H₂W₁₂O₄₂)·4H₂O.^[1]

Ammonium paratungstate is produced by separating tungsten from its ore. Once the ammonium paratungstate is prepared, it is heated to its decomposition temperature, 600 °C. Left over is WO₃, tungsten(VI) oxide. From there, the oxide is heated in an atmosphere of hydrogen, reducing the tungsten to elemental powder, leaving behind water vapor. From there, the tungsten powder can be fused into any number of things, from wire to bars to other shapes.^[2]

Structure

In more recent literature (post World War II period), the anion in (NH₄)₁₀(W₁₂O₄₁)·5H₂O has been shown to be [H₂W₁₂O₄₂]¹⁰⁻, containing two hydrogen atoms, keeping two hydrogen atoms inside the cage. The tungsten-oxygen cage, that is the heart of the anion requires 42 oxygen molecules. The correct formula notation for ammonium paratungstate is therefore (NH₄)₁₀[H₂W₁₂O₄₂]·4H₂O. The [H₂W₁₂O₄₂]¹⁰⁻ ion is known as the paratungstate B ion, as opposed to the paratungstate A ion, that has the formula [W₇O₂₄]⁶⁻, similar to the paramolybdate ion. The existence of the paratungstate A ion, could not be confirmed by NMR spectroscopy, however.^[3]

Before about 1930, there has been some dispute about the exact composition of the salt, and both (NH₄)₁₀W₁₂O₄₁ and (NH₄)₆W₇O₂₄ 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."^[4]

Synthesis

When concentrating an ammoniacal solution of tungstic acid (i.e. hydrous WO₃), 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.

If a calcined WO₃ is used, refluxing the ammonia solution is advisable to speed up the dissolution of the oxide.

References

1. http://acronyms.thefreedictionary.com/Ammonium+paratungstate
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. Greenwood & Earnshaw, Chemistry of the Elements, 2nd ed. 1997, pp. 1012–1014
4. J. W. Mellor: Inorganic and Theoretical Chemistry, vol. XI, p. 812–813, Longmans Green & Co. 1931