Bulletin of the Chemical Society of Japan
Online ISSN : 1348-0634
Print ISSN : 0009-2673
ISSN-L : 0009-2673
Solid State Oscillatory Reaction. I. Thermal Decomposition Studies of [Co(NH3)6]Br3 and CoBr2·2H2O
Murlidhar D. PatilGopal N. NatuShantaram B. Kulkarni
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1992 Volume 65 Issue 4 Pages 1026-1032

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Abstract

Thermogravimetric analysis, TGA, of cobalt(II)bromide, [Co(NH3)6]Br3 –henceforth referred to by the shortname HCBR– was carried out using a wide diameter short one and a narrow diameter long sample cups. In the former type of cup the TGA trace was similar to that reported one. However, when the latter cup was used, the TGA showed oscillations in the % weight-loss. Cobalt(II)bromide, CoBr2, is formed as an intermediate product in the thermal decomposition of HCBR. The oscillatory nature of weight-loss curve was studied for both HCBR and CoBr2.
The oscillatory behavior in the present system has been confirmed on the basis of the following experiments:
(a) Dynamic TGA and differential thermal analysis, DTA, of HCBR and CoBr2·2H2O.
(b) Isothermal TGA by weight-loss method for HCBR and CoBr2.
(c) Thermal decomposition studies, in closed system, of (i) CoBr2 and of (ii) CoO in a mixture of O2 and Br2 gases, and of a mixture of (iii) Co3O4 and CoBr2 in dry nitrogen.
The oscillations in the composition of the solid residue were attributed to the formation of CoOBr as an intermediate. Final product of thermal decomposition of CoBr2 in air is Co3O4. Hence it is possible to imagine that the point of CoOBr formation makes branching off point. This sets the reaction to oscillate in the composition domain. Therefore, it was thought that Edelstein mechanism would be operative in this reaction. Alternately, the population growth model may also be used to explain this observation. Formation of CoOBr was thought to be the weight-loss step, i.e. decrease in the ‘population’ of CoBr2, while the formation of CoBr2 was the weight-gain step, i.e. increase in the ‘population’ of CoBr2 under the proposed reaction, CoBr2 + 1/2O2 CoOBr. The purpose of this article is, therefore, to present the interesting facts of this reaction before the readers, and not to ascribe any particular model for its explanation.

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© 1992 The Chemical Society of Japan
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