The DTA method has been used in studying the thermal dehydration and decomposition of NiSO4.nH20, accordingly of NiSO4~D20, (at n --7, 6, 4, 1) in a temperature interval of 20 to 900~ at a heating rate of 10 deg/min. The endocffccts observed show in all cases partial dehydration to monohydrate and evolution of the last molecule of hydrate water at a high temperature Tmaz --360~ for the hydrates and Tmax --360-335~ for the deutcratcs. At NiSO4.6H20 (6D20) and NiSO4.4D20 there occurs stepwise dehydration before the monohydrate as well. Decomposition of the anhydrous NiSO4 takes place at higher temperature which depends on whether it had been obtained from the respective deuterate ordinary hydrate. The one obtained from the deuterate undergoes decomposition at relatively lower temperature.Pertinent literature contains data about the thermal decomposition of NiSO4.7H20 and NiSO4.6H20, which are contradictory as regards the stages of the dehydration process. For instance, according to [1], hexahydrate and monohydrate are obtained as intermediates, while according to [2] monohydrate is obtained directly from the heptahydrate. E. Fruehard and A. Michel [3] assume that NiSO4.7H20 is dehydrated at several steps, the first to be obtained being NiSO4.6H20 which undergoes allotropic conversion and is then dehydrated to tetra-, tri-, di-, mono-and anhydrous NiSO4. The obtaining of tetra-and dihydrate is judged only by a change in the mass of the sample, without any isolation or identification of these phases. The same is with the stages of dehydration in [4] as well, the difference being that the authors assume the existence not of two but of four polymorphous modifications of NiSO4.6H20, without offering proof for them.
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