A study of the thermal decomposition of some metal nitrate hydrates by thermal analysis techniques

The thermal decomposition of fourteen metal salt hydrates was studied using seven thermal analysis techniques. From these investigations, the nature of the deaquation reactions was elucidated and other parameters such as the thermal stability temperatures, the existence of intermediate hydrates, and the composition of the decomposition products were determined. The thermal analysis techniques employed included thermogravimetry (TG), differential scanning calorimetry (DEC), electrical conductivity (EC), high pressure ESC, TG-magnetic susceptibility, and mass and infrared spectrometry. The compounds were studied to a maximum temperature of 700[degrees]C. In the high pressure DSC technique, a maximum pressure of 500 psi was used. The infrared and mass spectral methods did not involve a nitrogen gas atmosphere. All of the hydrated metal salt nitrates began to lose water of hydration in the 35-70[degrees]C temperature range. The transition metal nitrates, with the exception of nickel, decomposed directly to the corresponding metal oxide without the formation of intermediate compounds. Anhydrous metal nitrates began to lose oxides of nitrogen in the 265-370[degrees]C temperature range. The high pressure DSC curves exhibited splittings into multiple peaks as a result of the increased pressure. These splittings were interpreted as due to the evolution of a liquid water phase followed by its subsequent vaporization. The first endothermic effects in the thermal decomposition of all of the salts investigated was due to fusion in their water of crystallization. There was no change in sample mass at this stage. All of the compounds showed an increase in the minimum temperature at which each mass-loss occurred with increasing heating rate. The amount of gaseous products evolved in the mass spectral method was proportional to the temperature. Phase transitions and fusion of the metal salts were observed by the electrical conductivity method. From TG-magnetic susceptibility data, changes in the oxidation state of the central metal ion and also changes in the structure of the compound were observed. All of the experimental data corresponded to each other with respect to the dehydration and decomposition reaction temperatures.