No abstract

New EGA findings revealed that the small endothermal event preceding that of the main decomposition of commercial NaHCO 3 involves the simultaneous evolution of water and CO e. 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 NaHCO 3.

No abstract

The thermal decomposition of sodium dithionite in aqueous solution has been followed by means of e.p.r. signal intensity measurements. Two different reactions have been identified. The first one, catalyzed by bisulphite ion, proceeds according to the usual second-order autocatalytic reaction mechanism. The second one, much faster than the first one, appears after an induction period and ends as soon as a sufficient amount of decomposition product has been formed. Using previously reported values for the equilibrium constant for dithionite ion dissociation, values for the rate constant of the autocatalytic reaction are reported, and a mechanism for the decomposition is proposed.

Some particular methods of collecting and treating DSC data for the determination of kinetic parameters in thermal decompositions of solids are suggested. These are mainly concerned with an extrapolation method to avoid the effect of the sample mass; with an approximate method to obtain the activation energy and the exponent of the decayqaw for runs at constant temperature in which the total area under the thermal curve is not known with accuracy; and with a method to ascertain whether a particular decomposition takes place by a single mechanism or by a sequence of different mechanisms.The determination of kinetic parameters for thermal decompositions of solids by means of differential scanning calorimetry encounters a number of experimental difficulties. Among these are difficulties connected with the instrument, such as the effect of the sample mass, the effect of the rate of linear advance of temperature (scan speed, ss), the difference in thermal emissivity between the sample and the reference holders, etc.The aim of the present communication is to suggest experimental devices and particular treatments of data in order to meet and possibly overcome some of the above difficulties.The results reported have been obtained from a number of thermal decompositions of solids already investigated or under investigation.As experimental evidence had shown the dependence of kinetic parameters on the mass of the sample, in order to evaluate and eliminate such behaviour, we considered a solid whose decomposition kinetics had previously been studied. Sodium hydrogen carbonate was selected [1][2][3]. The results obtained (some of the runs have been omitted for clarity) are collected in Fig. 1 where the logarithms of the n = 1 rate constant of the equation d~/dt = k(1 -:0 n, deduced at constant scan speed by the methods already described [4], are plotted vs. T-1 in the Arrhenius diagram. The plots become progressively more curved the higher the mass of the sample; consequently, the activation energy which can be deduced assumes different values in different ranges of fractional decomposition (a), agreeing with lhe value reported in the literature only in a restricted central portion of the curve. The progressive curving suggested an extrapolation method based on plotting the fractional decomposition attained at a given temperature as a function of mass, d. Thermal Anal 5, 1973