Quantitative correlation of kinetic parameters and procedural factors in non-isothermal thermogravimetry-dependence of heating rate and sample mass

Dependence of kinetic parameters (activation energy and pre-exponential factor) and procedural factors (sample mass and heating rate) independent of the reversibility and the type of reactions in non-isothermal thermogravimetry have been established. Tris(ethylenediamine)nickel(II) oxalate dihydrate has been selected as a model complex and experiments were carried out at different heating rates and sample masses to study the dependence quantitatively. The kinetic parameters calculated using mechanistic and non-mechanistic equations show a systematic decrease with increase in either sample mass or heating rate for the dehydration and deamination reactions. For the decomposition reaction, the kinetic parameters are not influenced by the procedural factors. Mathematical correlations of high reliability are established between kinetic parameters and heating rate/sample mass using both mechanistic and non-mechanistic equations for dehydration and deamination reactions. The quantification follows an exponential decay of second order relation with respect to heating rate and a sigmoidal relation with regard to sample mass for both the dehydration and deamination reactions. No quantitative correlation is possible for the final decomposition stage. Thus, it is found that independent of the type of reaction (deamination or dehydration) the kinetic parameters have a particular dependence on the procedural variables. The equations for exponential decay and sigmoidal dependence can be represented as $ y = y_{0} + A_{1} {\text{e}}^{{ - x/t_{1} }} + A_{2} {\text{e}}^{{ - x/t_{2} }} $ and $ y = {\frac{{A_{1} - A_{2} }}{{1 + {\text{e}}^{{(x - x_{0} )/{\text{d}}x}} }}} + A_{2} $ respectively, where y represents kinetic parameters (E or A) and x represents the procedural variables (φ or m). Mechanism of the dehydration reaction is found to be random nucleation with the formation of one nucleus on each particle and the deamination is a phase boundary reaction. It is observed that the mechanism of these reversible reactions is not affected by the variation in sample mass and heating rate.     

Kinetic mechanism of the thermal decomposition of tobacco

Equations have been derived to describe the chemical kinetic factors that affect the rate of formation of products when a mixture of solid components (tobacco) decomposes on heating. Using these equations, a computer model of tobacco pyrolysis has been constructed which can calculate the gas formation rate/temperature profile from a given set of reaction parameters. By comparing the predictions of the model with experimental results at heating rates between 0.8 and 25 deg C s^?1, a generalised kinetic mechanism for the thermal decomposition of tobacco has been developed. For carbon monoxide and other low molecular weight gases, the mechanism is an independent formation of each gas from one solid tobacco component in each temperature region. Pyrolysis of some individual tobacco components in other studies suggests that each gas is actually produced from many components in each temperature region. This more complex mechanism is kinetically equivalent to the deduced mechanism of independent formation from one component.The region in which a given decomposition reaction takes place moves to higher temperatures as the heating rate increases. The amounts of gases formed over any temperature region from 200 to 900°C can be calculated for a given heating rate using the mechanism and the kinetic constants. The present results imply that 75–90% of the carbon monoxide produced by tobacco decomposition at temperatures up to 900°C during a puff on a cigarette corresponds to that formed in the “low temperature region” (200–450°C) defined for pyrolysis experiments at the lower heating rates of 1–10 deg C s^?1.     

Studies of Dehydration Process for Isostructural Series of Lanthanide(III) 2,6-dihydroxybenzoates

Dehydration kinetics of lanthanide 2,6-dihydroxybenzoates has been studied by thermogravimetry using the model-fitting method of calculation. Several kinetic models gavea good representation of the measured thermogravimetric (TG) curves but the calculation method allowed to propose the most probable dehydration mechanism. The activation energies of dehydration for isostructural 2,6-dihydroxybenzoates of lanthanides are similar for a given kinetic model. A change in the heating rate did not alter the dehydration mechanism. A linear correlation between the activation energy and the heating rate was observed for the systems studied. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the acid decomposition of human blood and plasma for the determination of selenium

A systematic investigation has been made of various methods for decomposition of blood and plasma for determination of selenium, and various sources of error have been identified. The method recommended is graded destruction by heating with nitric acid and perchloric acid, to a final temperature of 210 degrees .     

The thermal decomposition of ammonium metavanadate

On heating, ammonium metavanadate (AMV) decomposes in several atmospheredependent stages. An important decomposition intermediate, ammonium hexavanadate (AHV), may also be prepared by wet-chemical methods and the kinetic parameters for the thermal decomposition of AMV and of the AHV preparation have been obtained. The kinetic study has been supplemented by surface-area measurements and by electron microscopic examination of the surfaces of reactant, intermediate and product crystallites. On the basis of the type of decomposition curve, the measured activation energies, and the effects of oxygen and water vapour on the decomposition rate, it has been concluded that in vacuum and in inert atmospheres the evolution of ammonia is the rate-determining step, while in oxidizing atmospheres evolution of water is rate determining. Comparison of the kinetic parameters with thermodynamic data for the decomposi. tion has led to suggestions as to the nature of the activated complexes involved.     

A kinetic study of water evolution from a molten hydrated salt

Meso lithium potassium tartrate dihydrate melted before dehydration and a kinetic study of this reaction has been completed. This system is of interest in establishing the kinetic characteristics of a homogeneous rate process in the absence of added solvent. Results are of interest in considering the mechanisms of solid or condensed state reactions where melting is a possibility.The evolution of the initial 1.2H₂O from the single crystal dihydrate reactants was zero order, the rate then became deceleratory and the first order expression was obeyed to 1.6H₂O. The activation energy of the process was high, 230±10 kJ mol^–1 (350–380 K). Evolution of the remaining water occurred by a slower first-order process to give the anhydrous salt. The dehydration of crushed powder reactant was initially relatively more rapid but was deceleratory throughout, obeying the first order equation. It is concluded that salt dehydration is controlled by the rate of surface release of water that is comparatively mobile within the reactant melt.G.M.L. thanks the Department of Education for Northern Ireland for the award of a Postgraduate Scholarship held during this work.     

Pseudo-phase changes in cupric sulfate pentahydrate during dehydration

A study of the dehydration kinetics of a model system, cupric sulfate pentahydrate, was made using the temperature programmed desorption technique. The three decomposition steps from the pentahydrate to the anhydrous salt were clearly resolved into distinct spectral peaks even at sample heating rates in excess of 10 K. min^?1. A linear relationship between spectral peak temperature and square root of heating rate was observed for each dehydration step. The kinetic data revealed different activation enthalpies and entropies for each dehydration sequence for heating rates above and below approximately 8 K min^?1. These latter findings are interpreted in terms of crystalline to amorphous pseudo-phase changes in the solid hydrate during decomposition which become apparent only at fast sample heating rates. Enthalpy and entropy changes associated with these structural alterations are evaluated. The results also help to clarify earlier work on the dehydration mechanism in calcium phosphates.     

Synthesis and properties of some polyesterimides

New polyesterimides have been obtained by condensation of pyromellitic dianhydride with various aromatic diamines and by esterifying, before the actual dehydration, some carboxyl groups of polyamic acid with 4,4′bis (2-hydroxyethoxy) dinaphthyl 1,1′. Some new polyesterimides containing polynaphthalene rings in the side chain have thus been obtained. Thermal properties of the new polymers were studied by thermogravimetric analysis. Investigations were carried out by determining the decomposition temperatures and measuring the weight losses on heating polymer samples under atmospheric conditions in the presence of air. The influence of the structure of the polymers on their properties was investigated.     

Thermal decomposition of oxalates. Part 17. Thermal decomposition of manganese(II) oxalate in a nitrogen atmosphere

Comparative studies of the kinetics of the isothermal and nonisothermal dehydration and decomposition of manganese(II) oxalate in an atmosphere of nitrogen are reported. Agreement between the values of the energy of activation for the isothermal and the nonisothermal dehydration at high heating rates was obtained. At low heating rate, the value obtained for the energy of activation is comparable with the enthalpy of dehydration. Values of 143 and 242 kJ mole^?1 were obtained for the energy of activation of the isothermal and nonisothermal decomposition, respectively. The difference is attributed to the condition of the anhydrous salt used in both cases. The theory of absolute reaction rate is applied and the parameters obtained are discussed.     

Pressure and laser intensity effects on kinetic rates in binary reactive gas mixtures

The irradiation of binary reactive gaseous mixture by a CW CO₂ laser at changing pressure of absorber or transparent reactant have been performed at over-all pressures of several hundreds torrs with a maximum laser intensity of 680 W.cm⁻². A radial thermodiffusion effect leading both to a transient depletion of absorber concentration in the irradiation zone and a decrease of the rate of decomposition have been shown. The maximum yield of decomposition which appears at a given intensity and pressure of absorber when the pressure of reactant is changed is general for the binary gas mixtures tested ; it may be understood in terms of laser energy deposited in the mixture and distributed between the absorber and reactant partners. A rate decomposition dependent on the square of incident laser intensity and proceeding from this distribution is also exhibited.     

Infrared laser-induced decomposition of 2,2-dimethyloxetane

The unimolecular decomposition of 2,2 dimethyloxetance to give either isobutence and formaldehyde or ethene and acetone induced by a pulsed CO₂ laser has been investigated. Absorption characteristics and fractional decomposition have been studied as a function of laser fluence, irradiation frequency, reactant pressure, and added inert bath gas. Both absorption cross section and fractional decomposition are approximately independent of pressure of 2,2-dimethyloxetane below 50 times; 10^?3 torr and increase with pressure at higher pressures of 2,2-dimethyloxetane. At pressures sufficiently low that collisions are negligible during the laser pulse, added inert gases reduce the amount of decomposition. Calculations of the fractional decomposition have been carried out based on RRKM theory and assuming either a Boltzmann or a Poisson intermolecular energy distribution. Master equation calculations of both absorption and decomposition for 10R20 irradiation have also been performed. Agreement between observed and calculated results for 10R20 irradiation could be obtained only by assuming that most, but not all, of the molecules in the irradiated volume absorb the laser radiation. Differences between the absorptions of the 10R20 and 9P20 lines and in the resulting extents of decomposition indicate that the fraction of irradiated molecules which absorbs 9P20 radiation is smaller than the fraction which absorbs 10R20 radiation.     

The use of thermal desorption GC/MS to study weight loss in thermogravimetric analysis of di-acid salts

Thermal desorption gas chromatograph mass spectrometry (TD GC/MS) was used to study weight loss in thermogravimetric analysis (TGA). The technique of thermal desorption utilizes the same temperature heating rate as the TGA to thermally desorb volatiles from solid sample matrices. Volatiles were cryo-trapped at −60 °C. After thermal desorption is complete, the trapped volatiles are separated by a GC capillary column and identified by mass spectrometry. In this study, the TD GC/MS was applied in pharmaceutical development to understand the chemical reactions attributed to the weight loss in the thermal decomposition of two dicarboxylic acid salts of a drug substance. These two salts exhibited different thermal stabilities. The thermally induced chemical reactions obtained from these two salts included dehydration and decarboxylation. Thermal degradation compounds were identified and reaction pathways for decomposition were proposed. The stability of the salts is dependent on the identity of the dicarboxylic acids from which they were generated. The information obtained from TD GC/MS helps better understand the weight loss process in thermogravimetric analysis.     

Kinetics and mechanism of non-isothermal dehydration of nickel acetate tetrahydrate in air

The non-isothermal decomposition of nickel acetate tetrahydrate in air was studied using thermogravimetry (TG)–DTG, differential scanning calorimetry (DSC) and XRD techniques. The decomposition occurs in two major steps and the final product is NiO. The dependence of mass loss on heating rates in TG measurements imply that the dehydration and hydrolysis concur at temperature below 240 °C; the apparent activation energies calculated by Flynn, Wall and Ozawa (FWO) isoconversional method indicate the existence of a consecutive process. The kinetics of the first major decomposition step (below 240 °C) was obtained with multivariate non-linear regression of four measurements at different heating rates. According to the kinetics results from non-linear regression, the dehydration reaction (F1 type with an activation energy E of 167.7 kJ/mol) goes first. After the loss of almost half of water, the retained water and acetate are linked to each other by hydrogen bonding, so dehydration and hydrolysis concur. The pathway with a lower E is related to the hydrolysis process and the other is corresponding to the dehydration process. The simulations of reactants at different heating rates were used to verify the correctness of the reaction model. With the kinetics results, the dehydration mechanism was discussed for the first time.     

Investigation on Thermal Decomposition of Pyrite Part I

Thermochemical decomposition of pyrite in air-gas medium has been studied by means of MOM-Q 1500D Derivatograph. The derivatograms have been obtained at a heating rate of 5 K min^-1 and air flowrate of 25 1 h^-1 to final temperatures of 833, 933 and 1273 K, chosen on the basis of the stepwise oxidation of pyrite. The solid product is analyzed using X-ray powder diffraction and Mössbauer spectrometer techniques. The formation of nonstoichiometric sulfides (pyrrhotines) as intermediates has been confirmed. The second stage of decomposition is the oxidation of these pyrrhotines to hematite. The recorded weight losses are lower than the theoretically possible because of the formed nonstoichiometric iron sulfides. The complicated structure of the intermediate products has been confirmed by means of other techniques.     

Study on electro-conductance and thermal analysis of lithium carnallite

Lithium carnallite is a hydrated double salt containing lithium and magnesium chlorides. The processes of thermal dehydration and thermal decomposition of lithium carnallite during heating has been studied by the conductance-thermoanalysis which was designed according to Berg and-Wendlandt, and an interpretation of the conductance-thermoanalytical curve has been suggested.     

竹材非等温热解动力学

利用热重分析技术对竹材在高纯N2条件下,从室温至1273K进行了非等温热解分析,研究了升温速率(5、10、20和40K/min)对热解过程的影响,探讨了其热解机理。研究表明,竹材非等温热解过程主要分为失水干燥、快速热解和缓慢分解三个阶段组成,其中第二阶段是整个过程的主要阶段,析出大量挥发分造成明显失重。升温速率对热解过程有显著影响,随着升温速率的增加,最大热解速度增大,对应的峰值温度升高,热滞后现象加重,热解各阶段向高温侧移动。热解机理满足一维扩散Parabolic法则,反应机理函数为g(α)=α2。不同升温速率下活化能为75.32-82.99kJ.mol-1,指前因子为1.17×105-1.12×106min-1。     

An LX-10 kinetic model calibrated using simulations of multiple small-scale thermal safety tests

A new chemical kinetic model for the beta-delta transition and decomposition of LX-10 (95% octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, 5% Viton A binder) is presented here. This model implements aspects of previous kinetic models but calibrates the model parameters to data sets of three experiments: differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and one-dimensional time to explosion (ODTX). The calibration procedure contains three stages: one stage uses open-pan DSC and TGA to develop a base reaction for formation of heavy gases, a second stage features closed-pan DSC to ascertain the autocatalytic behavior of reactant gases attacking the solid explosive, and a final stage adjusts the rate for the breakdown of heavy reactant gases using ODTX experimental data. The resultant model presents a large improvement in the agreement between simulated DSC and TGA results and their respective experiments while maintaining the same level of agreement with ODTX, scaled thermal explosion, and laser heating explosion times when compared to previous models.     

On the apparent compensation effect found for two parallel reactions

The authors present a critical analysis of the use of an overall single reaction rate equation instead of the true rate equations corresponding to the decomposition of a substance according to two parallel reactions. Isothermal as well as nonisothermal decomposition are considered. An apparent compensation effect has been evidenced in both cases. It has been assigned to the dependence of the kinetic parameters on temperature (for the isothermal case), conversion, and heating rate (for nonisothermal one). © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 673–681, 1998     

Thermal decomposition of scandium(III) benzenetricarboxylates in air and nitrogen atmospheres

The conditions of thermal decomposition of scandium(III) hemimellitate, trimellitate and trimezinate in air and nitrogen atmospheres have been studied. On heating, the benzene-tricarboxylates of Sc(III) decompose in two stages. First, the hydrated complexes lose crystallization water; heating in air finally yields Sc₂O₃, and heating in a nitrogen atmosphere Sc₂O₃ and C. The dehydration of the complexes is associated with strong endothermic effects. The decomposition of benzenetricarboxylates in air is accompanied by an exothermic effect and in nitrogen by an endothermic effect. The activation energies of the dehydration and decomposition reactions have been calculated for the Sc(III) benzenetricarboxylates.     

Kinetic of non-isothermal dehydration of equilibrium swollen poly(acrylic acid-co-methacrylic acid) hydrogel

The kinetic of non-isothermal dehydration of equilibrium swollen poly(acrylic acid-co-methacrylic acid) hydrogel (PAM) has been investigated. Thermogravimetric and conversion dehydration curves were recorded at various heating rates 5–30 K min^?1. The conversion dehydration curves at all investigated temperatures can be mathematically described using the logistic regression function in entire. It was found that activation energy complexly changes with the increasing dehydration degree. Physical meaning of the parameters of logistic function (b and w) is given. It was established that, during the dehydration, changes in the fluctuating hydrogel structure occur, and that limiting step on the kinetics of hydrogel dehydration have rate of structural rearrangement of hydrogel (actual relaxation mechanism). A procedure for determining the dependence of effective activation energy on temperature and dehydration degree, based on logistic function, is exposed. Possible explanation for the existence of negative values of activation energy in the certain range of temperature, is given.