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Abstract  

Geometries and energies of isolated CaC2O4H2O, CaC2O4, CaCO3, CaO, H2O, CO and CO2 were determined at the ab initio level using effective core potential valence basis sets of doublezeta quality, supplemented with polarization functions. The effects of electron correlation were taken into account at the second order Mller-Plesset level of theory. For CaC2O4H2O, the correlation for the basis set superposition error was also included. Common routines were employed to evaluate entropies, heat capacities, as well as enthalpies and free enthalpies of formation of all entities. The enthalphies and free enthalpies of consecutive dehydration of CaC2O4H2O, decarbonylation of CaC2O4 and decomposition of CaCO3 towards CaO and CO2 were determined on the basis of avialable data from the literature or those predicted thoretically. Assuming that upon all the above mentioned processes the system maintains equilibrium, the fractions reacted, enthalpy changes and differential dependencies of thesevs. temperature were derived and compared with experimental thermoanalytical data.

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Abstract  

A novel parameter of the relative rate of thermal decomposition has been defined on the basis of an analysis of equation relating the logarithm of the conversion degree on the temperature. The dependence of this parameter on temperature in the dynamic conditions has been analyzed and discussed. The dependence of the relative rate of thermal decomposition is a linear relationship involving two coefficients. These coefficients can be related to the enthalpy and activation energy. The parameter developed has been used for the analysis of a series of consecutive reactions of thermal decomposition of calcium oxalate monohydrate.

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Abstract  

A new method of the multiple rate iso-temperature was used to define the most probable mechanism g(α) of a reaction; the iterative iso-conversional procedure has been employed to estimate apparent activation energy E a, the pre-exponential factor A was obtained on the basis of E a and g(α). In this new method, the thermal analysis kinetics triplet of dehydration of calcium oxalate monohydrate is determined, which apparent activation energy E a is 82.83 kJ mol-1, pre-exponential factor A is 1.142105-1.235105 s-1, the most probable mechanism belongs to phase boundary reaction Rn with integral form g(α)=1-(1-α)n and differential form f(α)=n(1-α)1-(1/n), where accommodation factor n=2.40-1.40.

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Abstract  

The differential and integral isoconversional methods for evaluation the activation energy, described in the first note of this series, were applied for: a) simulated data for two successive reactions; b) dehydration of calcium oxalate monohydrate. It was shown that for these systems the activation energy depends on the conversion degree as well as on the method of evaluation.

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Abstract  

Reliable kinetic information for thermal analysis kinetic triplets can be determined by the comparative method: (1) An iterative procedure or the KAS method had been established to obtain the reliable value of activation energy E a of a reaction. (2) A combined method including Coats-Redfern integral equation and Achar differential equation was put forward to confirm the most probable mechanism of the reaction and calculate the pre-exponential factor A. By applying the comparative method above, the thermal analysis kinetic triplets of the dehydration of CaC2O4H2O were determined, which apparent activation energy: 813 kJ mol-1, pre-exponential factor: 4.51106-1.78108 s-1, the most probable mechanism function: f(α)=1 or g(α)=α, which the kinetic equation of dehydration is dα/dt=Ae-E a /RT.

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Abstract  

Sodium tartrate dihydrate, lactose monohydrate, potassium citrate monohydrate, and calcium oxalate monohydrate are commonly used as primary standards for methods that determine water content. Identification of the type of water of hydration (channel, lattice, or ion associated) is provided herein for each standard based upon thermogravimetry/differential thermal analysis (TG/DTA) and dynamic vapor sorption data. Sodium tartrate dihydrate was found to be a mixed hydrate, containing 1 mol of channel water and 1 mol of lattice water. Lactose monohydrate and calcium oxalate monohydrate were both determined to contain lattice water of hydration. Potassium citrate was shown to exist as an ion-associated hydrate. Also provided is a discussion of how hydrate type, thermal properties and hygroscopicity may impact suitability of each compound for use as a standard.

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Journal of Thermal Analysis and Calorimetry
Authors: A. Kohutova, P. Honcova, V. Podzemna, P. Bezdicka, E. Vecernikova, M. Louda, and J. Seidel

Abstract  

Thermal decomposition and structural characterization of three human kidney stones (KS1–KS3) extracted from patients of Eastern Bohemia have been carried out using X-ray powder diffraction systems (XRD), scanning electron microscope with energy dispersive X-ray micro analyser (SEM-EDX) and differential thermal analysis (DTA). The samples KS1 and KS2 solely consisted of calcium oxalate monohydrate (a.k.a. whewellite, CaC2O4·H2O). The third sample, KS3, was formed from calcium oxalate dihydrate (weddellite, CaC2O4·2H2O), calcium oxalate monohydrate, and hydroxyapatite (HA, Ca10(PO4)6(OH)2). Thermal measurements were carried out in the range between room temperature and 1,230 °C. XRD analysis was utilized to investigate the change of phases at 800 and 1,230 °C.

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Abstract  

Reactions that have an initial acceleratory period are common in both organic and inorganic systems. The Šesták-Berggren equation, dx/dt= -kx n(1-x)m[-ln(x)]p, with p set to zero (also called the extended Prout-Tompkins (PT) equation) is an excellent empirical kinetic law for many of these systems. In this work, it is shown to fit both isothermal and constant heating rate pyrolysis data for a well-preserved algal kerogen in a petroleum source rock and two synthetic polymers (polycarbonate and poly-ether-etherketone), dehydration of calcium oxalate monohydrate, decomposition of ammonium percholorate, and diffusive release of gas implanted in materials. Activation energies derived by non-linear regression to multiple experiments are consistent with those derived by simple isoconversional methods. Errors caused by misapplication of first-order kinetics to single-heating-rate data are discussed briefly.

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Abstract  

For kinetic evaluation by TG and DSC of simple and complex reactions we used a direct fit on the basis of TG differential equations and DSC non-linear equations. The advantages of an overall evaluation, i.e. the simultaneous inclusion of all data sets at different heating rates, are described. TG and DSC measurements of the dehydration of calcium oxalate monohydrate and the decomposition of calcium oxalate are used as examples.

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by temperature T eq , or linear relationship ( 11 ) with a negative slope, expressed by coefficient a 2 . Calcium oxalate monohydrate Thermokinetics Thermal decomposition of CaC 2 O 4 ·H 2 O

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