Ammonium paratungstate

Ammonium paratungstate

Identifiers
CAS Number	11120-25-5 Y
ChemSpider	21241467 Y
ECHA InfoCard	100.031.228
EC Number	234-364-9
PubChem CID	71306883
UNII	GCP1SFG106 N
CompTox Dashboard (EPA)	DTXSID20891417
InChI InChI=1S/10H3N.41O.12W/h10*1H3;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;/q;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;5*-2;;;;;;;;;;;;/p+10 Y Key: LZTNPCZGBGTJFN-UHFFFAOYSA-X Y 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 solid
Density	4.60 g/cm3
Boiling point	Decomposes at 600 °C
Hazards
GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H315, H319, H335
Precautionary statements	P261, P264, P271, P280, P302+P352, P304+P340, P305+P351+P338, P312, P321, P332+P313, P337+P313, P362, P403+P233, P405, P501
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Ammonium paratungstate (or APT) is a white crystalline salt with the chemical formula (NH₄)₁₀(H₂W₁₂O₄₂)·4H₂O. It is described as "the most important raw material for all other tungsten products."

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 WO₃ 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:

(NH₄)₁₀(H₂W₁₂O₄₂)·4H₂O → 12 WO₃ + 10 NH₃ + 6 H₂O

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

WO₃ + 3 H₂ → W + 3 H₂O

Structure

Structure of the [W₁₂O₄₂]^12- framework, emphasizing the coordination polyhedra. In APT, this anion binds two protons and is otherwise associated with ten NH₄⁺ counterions.

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 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.

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."

Other hydrates

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. The tetrahydrate is most significant in a commercial sense. Also known:

• decahydrate
• nonahydrate