Abstract
Thermal decomposition of ammonium paratungstate 5[(NH4)2O] 12WO3.4H2O has been studied by differential scanning calorimetry and thermogravimetry. The results suggest that the water is lost in two steps at the temperature region 318.7 - 441.3 K, while anhydrous ammonium paratungstate is converted in to WO3 in the next three steps in the temperature region 441.3 – 767 K. The phases have been characterized by IR spectroscopy, elemental analysis and X- ray diffractometry. Various kinetic equations have been applied to the isothermal DSC data to understand mechanism of the different decomposition steps. The isothermal data show that the dehydration process, observed in fist two steps, follows the Mampel’s unimolecular law of random nucleation, the third transition follows the Avrami - Erofeev equation, forth transition follows the diffusion controlled Ginstling – Brounshtein mechanism and the fifth transition obeys the linear law which is the rate determining step in this process.
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