Ammonium paratungstate
Ammonium paratungstate is a white crystalline salt with the chemical formula. 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 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 equationFrom 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: