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  • Author or Editor: G. Guarini x
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Abstract  

A study of the thermal dehydration of α-NiSO4·6H2O has been performed by power compensation differential scanning calorimetry in flowing nitrogen. No significant differences in behaviour were observed using either uncrushed crystalline powders or single crystal slabs cleaved parallel to {001}. In good agreement with previous findings, the kinetic analysis of the thermal curves confirms the validity of an=2 Avrami-Erofeev equation (AE2) in isothermal experiments at low (338–343 K) temperatures or in the initial portions of variable temperature runs. The kinetic obedience is however of an ‘order of reaction’ type for the main portion of the variable temperature runs and, for isothermal experiments, in the upper part of the temperature range investigated. Values of activation energies and frequency factors are reported. Parallel studies by optical microscopy showed relevant changes of surface texture when partially (thermally or vacuum) dehydrated {001} cleaved surface were submitted to rehydration. This phenomenon (named orange peel formation) indicates that a dehydrated layer forms on the crystal surfaces preceding the appearance of product crystals (germination or nucleation). Microscopy also revealed that reaction goes on inside the crystal and that product formation takes place in the bulk phase, following lattice collapse in experiments at high heating rates. Combined with previous results, these new experimental findings allow us to formulate a mechanism for the present transformation, comprising three main rate processes: i)  the reaction (detachment of water molecules from their lattice positions in the reactant); ii)  the migration of the water molecules freed by the reaction through the initially formed, water-depleted layer enveloping the reactant crystal; iii)  the crystallization of such a layer to form the product.

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The thermal decomposition of NaHCO3

Renewed studies by DSC, SEM and FT-BR

Journal of Thermal Analysis and Calorimetry
Authors:
L. Dei
and
G. Guarini

Abstract  

New EGA findings revealed that the small endothermal event preceding that of the main decomposition of commercial NaHCO3 involves the simultaneous evolution of water and CO2. At very high sensitivity, EGA experiments evidenced that the above (limited) evolution of gases also took place from the recrystallized material for which thermal methods gave no indication of endotherms. Careful reexamination of previous DSC results indicated that for one kind of recrystallized material a very small endotherm had been neglected. Renewed experiments revealed that this endotherm can be enhanced if the samples are prepared by crushing and sieving in a wet atmosphere. Parallel FT-IR experiments on commercial and recrystallized materials demonstrated the presence of carbonate in samples that had previously been taken just beyond the first small endotherm; this confirmed the EGA results. SEM experiments showed that surface texture changes take place when samples are heated to temperatures just above that of the preliminary endotherm. On the basis of these new findings, the interpretation previously given to the small endotherm is revised and detailed knowledge is gained on the mechanism of decomposition of NaHCO3.

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The kinetics of the thermal dehydration of various kinds of BaCl2 · 2H2O and of BaCl2 · H2O are investigated using a differential scanning calorimeter. The loss of H2O proceeds in two steps: BaCl2 · 2H2O→BaCl2 · H2O→BaCl2 and is therefore revealed by two endothermic peaks.

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Abstract  

The thermal decomposition of four commercial powders and of differently stored single crystals of sodium hydrogen carbonate is studied by power compensation DSC and by optical and FT-IR microscopy. Independently of manufacturer, specified purity and price, the thermal curves of all the commercial powders show a more or less pronounced low temperature peak preceding the one due to the main decomposition. Such small peak is not observed when samples of laboratory recrystallized material are used. However the thermal behaviour of the latter preparation differs remarkably depending on storage conditions: the material kept in closed glass containers decomposes at temperatures higher than those of the material stored in a dessiccator in the presence of concentrated H2SO4. The observation by optical microscopy of the behaviour of the surfaces of single crystals coming from different storage conditions when the temperature is raised in a Kofler heater helps the interpretation of the data collected. The mechanism of the decomposition is discussed and the relevant kinetic parameters reported.

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Abstract

The literature reveals that the mechanisms of some solid-state dehydrations are more complicated than has been generally accepted. Reactions at a thin advancing reactant-product interface provide the geometric models on which the most widely employed rate equations are based. For some systems, this “thin interface” model is a simplification of observed behaviour. Elimination of water from crystallographic sites may occur to a significant extent within a much thicker zone of reactant towards which the active interface is progressing. Consequently the region of chemical change may not coincide with the region of structural transformation. Limited initial dehydration may occur across all crystal faces prior to the onset of a nucleation and growth process that is usually regarded as the dominant rate process in the dehydrations of many large crystals. Experimental observations for solid-state dehydrations are discussed and reaction mechanisms with different rate controlling processes are distinguished. Studies of dehydrations have contributed substantially to the theory of solid-state reactivity, and advances in understanding may have wider application to other solid-state reactants.

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Anthracene derivative dimers reveal a thermal monomerization at temperatures higher than the melting points of the corresponding monomers. Thermograms of these dimers are consisting of a broad endotherm followed by a small exotherm. A detailed investigation on 9-chloroanthracene dimer allowed to explain this peculiar behaviour.

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