What the Difference of the Ammoniu Metatungstate and Ammonium Paratungstate ?

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.

The thermal decomposition of ammoniu metatungstate(AMT)involved several steps in inert atmosphere: (i)That is release of crystal water between 25°C and 200°C resulte in dehydrated AMT. (ii) The formation of an amorphous phase between 200°C and 380 °C. (iii) It's from which hexagonal WO3 form between 380 °C and 500 °C. (iv) Then it's which transform the more stable WO3 between 500°C and 600 °C. A difference in air that the formed NH3 ignited with an exothermic heat effect, and nitrous oxides formed as combustion products. The thermal behavior of AMT was similar to ammonium paratungstate (APT), (NH4)10[H2W12O 42]•4H2O, the main difference is the lack of dry NH3 between 170°C and 240°C .

 

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