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Thermal decomposition of ammonium heptamolybdate was studied by means of differential scanning calorimetry and thermogravimetry. DSC showed that the material decomposes in the temperature region 463-608 K in static air atmosphere, in three steps. The products of decomposition were characterized by IR spectroscopy, elemental analysis and magnetic susceptibility measurements. All the phases are paramagnetic in nature. Kinetic analysis suggested that the first two steps are controlled by a diffusion mechanism, while the third step is controlled by nuclear-growth.On heat treatment, ammonium heptamolybdate is reported to decompose into molybdenum oxide in three steps in the temperature region room temperature to 633 K [1, 2]. The first step, corresponding to complete dehydration, is complete at 403 K, and is followed by decomposition. The decomposition results, which are based on thermogravimetric studies, seem to be semi-quantitative. There are no data on the kinetics of the transitions. The present study is aimed at the elucidation of the precise steps of decomposition of ammonium heptam01ybdate on heat treatment, determination of the kinetics of these steps and characterization of the resulting phases. The material was subjected to thermal analysis (DSC and TG-DTG) in static air atmosphere and in a continuous flow of nitrogen. |R spectra were recorded, magnetic susceptibility was measured and elemental analysis for N, H and Mo was performed to arrive at the sequence of transitions. With the program available in a TC 10 microprocessor, the kinetic parameters of the various steps involved were computed from the differential scanning calorimetric data under both dynamic and isothermal conditions. The results were analysed in light of the theory of solid-state reactions.

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