The heat of the thermal decomposition of monoclinic ammonium paratungstate, (NH4)10H2W12O42•4H2O, was measured in a HT-1000 microcalorimeter using three-step calorimetry and suitable thermochemical cycle. The standard enthalpy of the thermal decomposition reaction at 298.15 K and the standard enthalpy of formation at 298.15 K for monoclinic ammonium paratung-state were obtained to be (430.1±10.2) kJ•mol- 1 and -(13 423.7±14.8) kJ•mol-1, respectively.
The thermal decomposition of monoclinic ammonium paratungstate (APT) and amorphous ammonium metatungstate (AMT) has been studied over the temperature range 20 to 500℃ using thermogravimetry, evolved gas analysis, X-ray diffraction analysis, differential thermal analysis and scanning electron microscopy. A mechanism of decomposition which postulates the formation and subsequent decomposition of various lower ratio tungstates is proposed and substantiated by the experimental data.
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