Ammonium Paratungstate Thermal Decomposition

The thermal decomposition of ammonium paratungstate tetrahydrate (APT·4H₂O), (NH₄)₁₀[H₂W₁₂O₄₂]·4H₂O, in air to tungsten trioxide, WO₃, was investigated under non-isothermal conditions using thermal analysis coupled on-line by a skimmer system to a quadrupole mass spectrometer (TA-MS) for evolved gas analysis (EGA), with X-ray powder diffraction (XRD), with Fourier transform infrared (FT-IR) and raman spectroscopy.

The decomposition is characterized by three endothermic and one exothermic step. The first endothermic step comprises the release of crystal water resulting in the formation of crystal-water-free ammonium paratungstate. During the second endothermic step ammonia is released leaving behind ammonium hydrogen paratungstate, (NH₄)₆H₄[H₂W₁₂O₄₂. The presence of protons in the “roasted” APT was supported by ex situ and in situ FT-IR spectroscopy (vibration bands at 1100 and 2160 cm⁻¹). Ammonium hydrogen paratungstate is the precursor for the metatungstate anion, [H₂W₁₂O₄₀]⁶⁻, which is formed during the digestion of “roasted” APT with hot water.


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