The thermal decomposition of NaHCO3

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.

Kinetic Analysis of Isothermal Decomposition Process of Sodium Bicarbonate Using the Weibull Probability Function—Estimation of Density Distribution Functions of the Apparent Activation Energies

“…[4]) with a good regression coefficient (0.99969 to 1.00000). The results are presented in Figure 3.…”

“…The apparent activation energy (E a ) can be calculated from the slope of the plot Àln (t in ) vs 1/T, which represents the special case of the ''model-free'' isoconversional method (Eq. [4]), for one (stationary) point on the conversion (a vs t) curve at a given operating temperature.…”

“…[4] is constant and E a can be determined from the slope of the straight line Àln t vs 1/T. The intercept ln [A/g(a)] is used to evaluate the pre-exponential factor when a certain form of g(a) is assumed.…”

“…[1][2][3][4] Keener et al [2] investigated the thermal decomposition of sodium bicarbonate over the temperature range of 380 K to 450 K and over the particle size range of 51 to 140 lm. They found that the shrinking core model, with chemical reaction as the rate controlling step, represents the good fit to the data in the investigated temperature range, with decomposition activation energy of 85.7 kJmol À1 .…”

“…It was found that the apparent activation energy (E a ) varies with the differences in the grain size of the investigated substance. [2][3][4] In the previous article of the sodium bicarbonate decomposition process under nitrogen atmosphere in nonisothermal conditions, the complete kinetic analysis of the investigated process was performed. [5] It was concluded that the decomposition process of NaHCO 3 in nitrogen atmosphere is a multistep mechanism that can be described by the two-parameter autocatalytic Sˇesta´k-Berggren (SB) reaction model.…”