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Abstract  

This paper outlines the different ways of taking the distance from thermodynamic equilibrium into account in kinetic studies based on thermoanalytical experiments. The three main approaches are: (i) avoiding or neglecting the effect of the reverse reaction, (ii) describing the influence of distance from equilibrium on apparent kinetic parameters, and (iii) incorporating a driving force factor in the rate equation. Finally, the contradiction of the microscopic nature of the processes and the macroscopic character of the usual rate equation are briefly discussed.

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Abstract  

Identification and monitoring of gaseous species released during thermal decomposition of pure thiourea, (NH2)2C=S in argon, helium and air atmosphere have been carried out by both online coupled TG-FTIR and simultaneous TG/DTA-MS apparatuses manufactured by TA Instruments (USA). In both inert atmospheres and air between 182 and 240°C the main gaseous products of thiourea are ammonia (NH3) and carbon disulfide (CS2), whilst in flowing air sulphur dioxide (SO2) and carbonyl sulphide (COS) as gas phase oxidation products of CS2, and in addition hydrogen cyanide (HCN) also occur, which are detected by both FTIR spectroscopic and mass spectrometric EGA methods. Some evolution of isothiocyanic acid (HNCS) and cyanamide (NH2CN) vapours have also observed mainly by EGA-FTIR, and largely depending on the experimental conditions. HNCS is hardly identified by mass spectrometry. Any evolution of H2S has not been detected at any stage of thiourea degradation by either of the two methods. The exothermic heat effect of gas phase oxidation process of CS2 partially compensates the endothermicity of the corresponding degradation step producing CS2.

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Journal of Thermal Analysis and Calorimetry
Authors:
Katalin Szécsényi
,
I. Esztelecki
, and
G. Pokol

Abstract  

In order to increase the nutrition value of bread, one of the most commonly used foodstuff all over the world, different additives are used in bread processing. In this paper we describe the thermal changes in bread and that of with 0.5% crude soybean lecithin additive. Their thermal stability has been investigated by TG, DSC and EGD methods. The thermal changes were also followed of soy products, lecithin and lysine, ingredients used as bread additives in order to check if they may suffer any thermal degradation during the baking process. The data obtained can be of use only for qualitative conclusions. According to the obtained data at the usual bread baking temperature only the additives in crust may partly decompose while in the crumb, at lower temperatures the additives, due to baking, are not damaged. The thermal methods give a possibility for rapid estimation of processes induced by heat effects in additives during the baking, and they are suitable to detect the changes during the bread-making procedure. However, they are neither suitable to provide any quantitative data on these changes nor facts affecting the nutrition value and of the bread.

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Abstract  

Four sulphato and nitrato complexes of cerium(IV),viz. (NH4)4Ce(SO4)4·2H2O (1), (NH4)2Ce(SO4)3 (2), (NH4)2Ce(NO3)6 (3) and Cs2Ce(NO3)6 (4) were studied by simultaneous TG/DTA under various experimental conditions in order to establish their decomposition mechanism and to compare the results with the literature data which have been reviewed. In the case of the ammonium compounds (1, 2 and3) the decompositions are accompanied by changes in the oxidation state of cerium; the presence of Ce(III) and Ce(IV) were studied byex situ magnetic susceptibility and XPS measurements. The crystal structure of (1) was determined as well. It forms monoclinic crystals with space groupP21/c; the parameters of the unit cell are:a=12.638(18) Å,b=11.362(10) Å,c=13.607(11) Å, β=110.17(9)°,V=1834.05 Å3.

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Abstract  

Novel methods of unified evaluation of two (or more) thermogravimetric curves have been worked out on the basis of known non-linear parameter estimating procedures (Gauss-Newton-Marquardt-type regression and the direct integral method of Valkó and Vajda were adapted). Their ability to provide estimate for common kinetic parameters of several TG (m−T) or DTG (dm/dt-T) curves were tested for pairs of curves of different heating rates, and for repeated curves of the same heating rate, obtained for the decomposition of CaCO3 in open crucible. In these cases the Arrhenius terms and then-th order model functions were assumed. The fitting ability of estimations made for single curves and for pairs of curves sharing different number of parameters, was judged on the base of residual deviations (S res ) and compared to the standard deviation of the measurements. In the case of different heating rates, the two curves could not be described with the assumption of three common parameters, because of the minimum residual deviation was very high. However, sharing of activation energy and preexponential term only, and applying different exponents for the two curves, provided a satisfactory fit by our methods. Whilst in the case of repeated curves, we could find such a common three-parameter set, which has a residual deviation comparable with the standard deviation of the measurements. Because of their flexibility (taking into account the variable number of common parameters and the versatile forms of model equations), these methods seem to be promising means for unified evaluation of several related thermoanalytical curves.

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Abstract  

The phase diagram for the AgNO3−KNO3 system has been determined using differential scanning calorimetry (DSC). Eutectic point has been found at 391 K andX Ag=0.580 mole fraction AgNO3. The DSC curves indicate the existence of an intermermediate compound (AgNO3·KNO3) in the KNO3-rich region of the phase diagram. This compound was identified in the solid phase by X-ray diffraction. The melting and the crystallization processes were followed with the aid of a hot stage microscope, too.

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Abstract  

Differential scanning calorimetric (DSC) measurements have been carried out on Bi-Sn based amalgam precursors to be used in compact fluorescent lamps (CFLs) to study the changes in melting and solidifying behaviour caused by In dopant. The phase and elemental compositions of the samples have been characterized by using X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX), respectively. One of the endothermic peaks of the liquid amalgam formation shifted from 121°C to 112 and 105°C, with increasing content of 2.5 and 4.8 mass% In of samples, respectively.

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Partial thermal reduction of ammonium paratungstate tetrahydrate

Evolved gas analysis (TG/DTA-MS) and solid state studies (XRD, FTIR)

Journal of Thermal Analysis and Calorimetry
Authors:
I. Szilágyi
,
J. Madarász
,
F. Hange
, and
G. Pokol

Abstract  

Thermal decomposition of ammonium paratungstate tetrahydrate, (NH4)10[H2W12O42]4H2O has been followed by simultaneous TG/DTA and online evolved gas analysis (TG/DTA-MS) in flowing 10% H2/Ar directly up to 900C. Solid intermediate products have been structurally evaluated by FTIR spectroscopy and powder X-ray diffraction (XRD). A previously unexplained exothermic heat effect has been detected at 700–750C. On the basis of TG/DTA as well as H2O and NH3 evolution curves and XRD patterns, it has been assigned to the formation and crystallization heat of γ-tungsten-oxide (WO2.72/W18O49) from β-tungsten-oxide (WO2.9/W20O58) and residual ammonium tungsten bronze.

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Abstract  

Thermochemical properties of crown ether complexes have been studied by simultaneous TG-DTA (thermogravimetric analysis-differential thermal analysis) coupled with a mass spectrometer, DSC (differential scanning calorimetry) and hot stage microscopy (HSM). The examined complexes contain benzylammonium- [BA], (R)-(+)-a-phenylethylammonium- [(R)-PEA], (R)-(+)- and (S)-(-)-a-(1-naphthyl)ethylammonium perchlorate [(R)-NEA and (S)-NEA] salts as guests. In the cases of BA and (R)-PEA an achiral pyridono-18-crown-6 ligand [P18C6], and in the case of (R)-NEA and (S)-NEA a chiral (R,R)-dimethylphenazino-18-crown-6 ligand [(R,R)-DMPh18C6] was used as host molecule to obtain four different crown ether complexes. In all cases, the melting points of the complexes were higher than those of both the host and the guest compounds. The decomposition of the complexes begins immediately after their melting is completed, while the BA and (R)-PEA salts and the crown ether ligands are thermally stable by 50 to 100 K above their melting points. During the decomposition of the salts and the four complexes strongly exothermic processes can be observed which are due to oxidative reactions of the perchlorate anion. Ammonium perchlorate crystals were identified among the decomposition residues of the salts. P18C6 was observed to crystallize with two molecules of water. The studied complexes of P18C6 did not contain any solvate. BA was observed to exhibit a reversible solid-solid phase transition upon heating. The heterochiral complex consisting of (S)-NEA and (R,R)-DMPh18C6 shows a solid-solid phase transition followed by two melting points. HSM observations identified three crystal modifications, two of them simultaneously co-existing.

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Abstract  

Identification and monitoring of gaseous species released during thermal decomposition of the title compound 1, Zn(tu)2Cl2, (tu=thiourea, (NH2)2C=S) have been carried out in flowing air atmosphere up to 800°C by both online coupled TG-EGA-FTIR and simultaneous TG/DTA-EGA-MS. The first gaseous products of 1, between 200 and 240°C, are carbon disulfide (CS2) and ammonia (NH3). At 240°C, an exothermic oxidation of CS2 vapors occurs resulting in a sudden release of sulphur dioxide (SO2) and carbonyl sulphide (COS). An intense evolution of hydrogen cyanide (HCN) and beginning of the evolution of cyanamide (H2NCN) and isothiocyanic acid (HNCS) are also observed just above 240°C. Probably because of condensation and/or polymerization of cyanamide vapors on the windows and mirrors of the FTIR gas cell optics, some strange baseline shape changes are also occurring above 330°C. Above 500°C the oxidation process of organic residues appears to accelerate which is indicated by the increasing concentration of CO2, while above 600°C zinc sulfide starts to oxidize resulting in the evolution of SO2. All species identified by FTIR gas cell were also confirmed by mass spectrometry, except for HNCS.

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