David Dollimore

The thermal decomposition of oxalates. A review

Abstract The thermal decomposition and stability of oxalates are reviewed from the viewpoint of their characteristic features rather than by a complete coverage of the existing literature. which is found to often be repetitive in nature.

A thermal analysis investigation of partially hydrolyzed starch

Porous starch granules are formed by the partial hydrolysis of starch using glucoamylases. These porous granules can be used to reveal structural information about the starch itself. Starches from four different sources were used as substrates in this study. These starches were partially hydrolyzed using glucoamylase. The material left behind after enzyme treatment was then treated to thermal analysis using TG and DSC to characterize the hydrolysis product. A structural analysis was made using SEM and powder X-ray diffraction to investigate whether the partial hydrolysis had any effect on the granule structure and the crystallinity of the different starches. In each case, except for potato starch, combustion in air took place at a lower temperature. On the other hand, the gelatinization temperature increased as the porosity increased.

The kinetic interpretation of the decomposition of calcium carbonate by use of relationships other than the Arrhenius equation

Abstract The kinetic parameters needed to describe the decomposition of calcium carbonate are obtained by a method which avoids the direct use of the Arrhenius parameters, the pre-exponential term A and the energy of activation E . It is shown that the approach is amenable to both an exact differential and integral analysis which carries certain advantages over existing methods. If needed, the method can be extended to allow the calculation of the Arrhenius parameters A and E , and in addition will show any dependence of E upon the fraction decomposed α, although it might be argued that the dependence of E is really upon the temperature.

Comparison of dolomite decomposition kinetics with related carbonates and the effect of procedural variables on its kinetic parameters

The three naturally occurring available carbonates in northwest Ohio are magnesite, calcite and dolomite. Dolomite is a double carbonate containing calcium and magnesium carbonate in equimolar concentrations. All three carbonates decompose via a single stage process in an atmosphere of nitrogen. The thermal behavior and the kinetics of decomposition were studied using the Arrhenius equation applied to solid-state reactions. It was found that calcite and dolomite supposedly decompose via a zero order mechanism while magnesite decomposes via a first order process. The energy of activation for the decomposition of magnesite, calcite and dolomite were 226.34, 192.50 and 175.05 kJ/mol, respectively. Similarly the ln A-values for magnesite, calcite and dolomite decomposition were 30.70, 20.73 and 18.76, respectively. Finally, the effect of procedural variables on the kinetic parameters of dolomite decomposition was investigated. The three procedural variables studied included flow rate, heating rate and sample size. The kinetic parameters and mechanism remain unaffected by a change in these variables. # 2002 Elsevier Science B.V. All rights reserved.

The thermal decomposition of oxalates: Part 26. A kinetic study of the thermal decomposition of manganese(II) oxalate dihydrate

Abstract The determination of the most probable mechanism function and calculation of the kinetic parameters of decomposition of manganese oxalate have been achieved by a new kinetic analysis procedure under nonisothermal conditions in both dry and wet N 2 . The isothermal kinetic analysis has also been performed in each atmosphere. Both the isothermal and nonisothermal analyses showed the most probable mechanism function is first order (Fl). The coincidence of results obtained by isothermal and nonisothermal analysis supports the idea that the proposed analysis procedure for nonisothermal conditions is a promising one.

Correlation between the shape of a TG/DTG curve and the form of the kinetic mechanism which is applying

Abstract A method of identifying the reaction mechanism in non-isothermal kinetics is presented based on the symmetry, width and height of the DTG curve. Using a computer program, which allows the calculation of a thermogravimetric curve from selected Arrhenius parameters, heating rate and reaction mechanism, it has been possible to study how these variables alter the shape of the DTG curves. It is shown that the value of α (fraction decomposed) at which the maximum rate is obtained is characteristic for any specific mechanism, and is altered only slightly by the magnitude of the Arrhenius parameters and the heating rate. As such, with complementary information based on the width of the DTG peaks, it is usually possible to identify the mathematical expression which describes the reaction.

A method of assessing solid state reactivity illustrated by thermal decomposition experiments on sodium bicarbonate

Abstract The thermal decomposition of sodium bicarbonate (NaHCO3) was studied under different atmospheres (dry nitrogen, air, and carbon dioxide), with various heating rates in order to characterize the substance. Various non-isothermal methods of kinetic analysis were employed in estimating the Arrhenius kinetic parameters, the activation energy and the frequency factor. All show that the most probable reaction mechanism under dry nitrogen and air is the first-order deceleratory mechanism, whereas under carbon dioxide it is the Avrami-Erofeev equation, with n = 1.5. Thermogravimetric and derivative thermogravimetric analysis (TGA and DTG) were employed for comparing the solid state reactivity of different samples of sodium bicarbonate. The reaction parameters, the extent of the reaction (α) and the reaction temperature were used in comparing the reactivities of various samples of sodium bicarbonate differing in particle size and surface area produced by grinding the substance in a ball mill. A method was utilized, termed here the αsample—αreference (αs—αr) method, by which the solid state reactivity of these samples could be compared with that of a reference. The terms αs, αr refer to the extent of reaction (here the extent of decomposition) at the same temperature for the sample (s) and reference (r).

The origin of the exothermic peak in the thermal decomposition of basic magnesium carbonate

Abstract The thermal analysis of basic magnesium carbonate has been investigated by thermogravimetry (TGA), derivative thermogravimetry (DTGA) and differential thermal analysis (DTA). The end product was magnesium oxide formed from the decomposition of magnesium carbonate. The composition of the samples studied varied, but the general formula that represented them was xMgCO3·yMg(OH)2·zH2O. In the present study, the thermal analysis of such magnesium carbonates in nitrogen, carbon dioxide and air was carried out at different heating rates and the origin of the exothermic peak was studied. Generally, the decomposition peaks were endothermic, but a persistent exothermic peak was noted, always accompanied by a very sharp drop in the TG curve, and the weight loss in this region may be significant. The exothermic DTA peak was found to be strongly influenced by the rate at which the samples were heated, sample size and atmospheric conditions.

Thermal stability of folic acid

Abstract This study attempts to identify the degradative process which folic acid undergoes under thermal stress. In order to facilitate the process, the various pieces of the chemical structure, namely, p -amino benzoic acid (PABA), pterin and glutamic acid as both its d - and l -isomers were investigated as separate entities. These structured pieces were then compared to the composite folic acid degradative thermogram in order to identify the peaks seen and provide direction for the interpolation of the degradative mechanism [Thermal stability of folic acid and associated excipients, M.Sc. thesis, 2001]. It was observed that none of the structural pieces could be superimposed as assumed earlier, and hence, an attempt was made to identify the decomposition products using various analytical techniques such as infrared (IR) spectroscopy, mass spectroscopy (MS) and X-ray diffraction (XRD) which suggested that the glutamic acid fragment is lost first as evidenced by acid loss and amide enhancement in the IR spectra. The vitamin was ultimately degraded to carbon fragments and that further identification was not necessary.

A generalized form of the Kissinger equation

Abstract An equation is deduced in this paper and an attempt is made to obtain the peak maxima information theoretically from the differential thermogravimetry curves. The calculations are critically assessed. The Kissinger method, which also deals with the peak maxima, is also discussed. A comparison of the equation due to Kissinger and that of the equation deduced in this paper shows that the Kissinger equation is a special case of this equation. The approximation made in the deduction of the Kissinger equation can be restated as “The extent of reaction at the peak maxima under different heating rates is the same”.