Kenneth S. Alexander

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.

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 new application for the Antoine equation in formulation development

Characterization of formulation components in pre-formulation and formulation studies will be made easier if a rapid method to evaluate the evaporation characteristics of an ingredient in the formulation is developed. This study aims at providing a simple and rapid thermogravimetric method for estimating the vapor pressure characteristics using the Antoine equation as the analytical tool. The heat treatment for the majority of benzoic acid derivatives follows zero-order rate processes that are in good correlation with their evaporation process. The optimum conditions for the rising temperature experiments were found when the heating rate was 10 degrees C/min in an atmosphere of dry nitrogen (100 ml/min). Methyl paraben was taken as the calibration compound since its Antoine constants are reported in the literature, and its selected thermodynamic parameters were evaluated using the Langmuir equation. The coefficient of vaporization (k macro) was determined to be 124,525+/-0.8, with units being reported in the S.I. system. The corresponding vapor-pressure plots were obtained for the remaining compounds and their Antoine constants calculated.

Estimating vapor pressure curves by thermogravimetry: a rapid and convenient method for characterization of pharmaceuticals.

The purpose of this study was to investigate a rapid method for the evaluation of vaporization characteristics for selected benzoic acid derivatives. The compounds studied in this context were the ortho-, meta- and para-derivatives of hydroxy and amino benzoic acids. Calculations for the order of reaction were first carried out for each of the compounds using methyl paraben as the calibration standard. Those compounds undergoing zero order, non-activated evaporation processes, were analyzed by the Antoine and Langmuir equations, conjointly. The coefficient of vaporization was obtained as 1.2 x 10(5)+/-0.8 Pakg (0.5)mol(0.5)s(-1)m(-2)K(-0.5). The vapor pressure values were used to determine the Antoine constants using the SPSS 10.0 software. This study attempts to outline a comprehensive thermogravimetric technique for vapor pressure characterization of single-component systems.

Calculation of vapor pressure curves for hydroxy benzoic acid derivatives using thermogravimetry

This study aims at providing a simple thermogravimetric method in estimating the vapor pressure characteristics using the Antoine equation as the analytical tool. The heat treatment for the majority of benzoic acid derivatives follows zero order rate processes that are in good correlation with their evaporation process. The optimum conditions for the rising temperature experiments were found when the heating rate was 10 8C/min in an atmosphere of dry nitrogen (100 ml/min). Methyl paraben was taken as the calibration compound since its Antoine constants are reported in literature and its selected thermodynamic parameters were evaluated using the Langmuir equation. The coefficient of vaporization k was determined to be 124525 � 0:8, with units being reported in the SI system. The corresponding vapor pressure plots were obtained for the remaining compounds that followed a zero order evaporation process and their Antoine constants were calculated using the Levenberg‐Marquardt least square curve fit method. # 2002 Elsevier Science B.V. All rights reserved.

The thermal analysis study of the drug captopril

Abstract Captopril is an antihypertensive drug currently being administered in tablet form. The thermal analysis study was carried out using a simultaneous TG–DTA unit. Both the isothermal and non-isothermal experiments are preformed to investigate the thermal degradation process of captopril in its natural state as a solid. The runs were performed in a flowing nitrogen atmosphere. Captopril melted at 106°C followed by decomposition. Based on the order of reaction, one method is used to identify the reaction mechanism in isothermal kinetics and two methods are used to identify the reaction mechanism in non-isothermal kinetics. These methods use the equations established by Avrami–Erofeev, Arrhenius and Freeman and Carroll. However, a kinetic analysis based on the “method of fit” using zero-order, first-order, and second-order equations showed that a first-order process gave a good fit for the Arrhenius plot at certain specific experimental conditions (i.e. very low sample mass). Overall, a second-order process followed by a first-order reaction for the main decomposition process of captopril showed an even better fit for the experiments. The possible reasons for this kinetic behavior are presented. There was up to 2% carbon remaining at 500°C. Thermal analysis was supplemented using Fourier Transform infrared spectroscopy (FTIR), X-ray diffraction and scanning electron microscopy (SEM) methods to identify the captopril with any degradation products which may have formed.

Isolation and identification of the intermediate and final products in the thermal decomposition of dolomite in an atmosphere of carbon dioxide

Extensive studies have been done on the behavior of dolomite when subjected to a rising temperature program. In this study a literature survey was prepared related to the thermal decomposition of dolomite and the probable mechanisms proposed by various workers for the above reaction. Dolomite undergoes a two-stage decomposition in one atmosphere of carbon dioxide; while the same decomposition proceeds via a single step at lower partial pressures of carbon dioxide. The intermediate and final products for the two-stage decomposition of dolomite were isolated and identified using thermogravimetry and X-ray powder diffraction. On each of the isolated samples scanning electron microscopy was carried out. The intermediate products were found to be dolomite, calcite and periclase, while the final products were calcium oxide and periclase. Using these results a mechanism of thermal decomposition for dolomite is proposed.