Self-cooling effect on the kinetics of nonisothermal dehydration of lithium sulfate monohydrate

The effects of self cooling on the apparent kinetics of the nonisothermal dehydration of crushed crystals of lithium sulfate monohydrate were investigated using TG accompanied by DTA and power-compensation type DSC. Linearity of the sample heating rate on the TG-DSC system is much better than that on the TG-DTA. Kinetic obedience and Arrhenius parameters obtained from the TG-DTA deviate considerably from those obtained from the TG-DSC; the latter are the more accurate due to the better linearity of the sample heating rate.

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Zusammenfassung Mittels TG, ergänzt durch DTA und DSC mit Leistungskompensation wurde der Einfluß des Self-cooling-Effektes auf die scheinbare Kinetik der nichtisothermen Dehydratation von zerkleinerten Kristallen aus Lithiumsulfat-Monohydrat geschätzt. Die mittels TG-DTA erhaltene Kinetik und die Arrheniusschen Parameter weichen erheblich von denen ab, die mittels TG-DSC ermittelt wurden. DSC mit Leistungskompensation und TG-DSC haben gegenüber TG oder TG-DTA den großen Vorteil, das nichtisotherme kinetische Verhalten von Feststoffzersetzungen zu charakterisieren. Mittels Thermoanalyse nichtisotherm ermittelte kinetische Parameter sind ohne Anwendung von DSC eher unreell, besonders wenn sie bei größeren Aufheizgeschwindigkeiten und Probengrößen bestimmt wurden.

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We thank Dr. A. K. Galwey for reading the text and for valuable comments.     

Calculation of the kinetic parameters of dehydration of lithium peroxide monohydrate in nonisothermic regime by derivatographic method

The dehydration of lithium peroxide monohydrate was studied by derivatography under nonisothermic conditions for the calculation of main kinetic parameters in the temperature range 90–146°C. Experimental conditions (sample size and the linear heating rate) caused by thermoconductivity of the object under investigation were determined. It is shown that the process under study proceeds according to the kinetic law close to the first order one (n = 0.85±0.03). Values of the activation energy and the pre-exponential factor [E _a = 86.0±0.8 kJ mol^?1, k ₀ = (2.19±0.16)×10¹¹ min^?1] were obtained. A suggestion was formulated that the dehydration mechanism of lithium peroxide monohydrate was analogous to that of the hydrates of the alkaline earth metal (calcium, barium, and strontium) peroxides.     

Effect of thermal dehydration and rehydration on Na-magadiite structure

The effect caused by dehydration and rehydration of the synthetic Na-magadiite was investigated by TGA, XRD, SEM, and ²⁹Si NMR. Thermal analysis of Na-magadiite presented two well-defined loss mass stages between 20 and 150 °C and another between 270 and 310 °C, both related to the removal of interlayer water. The swelling behavior of Na-magadiite was studied by thermal dehydration data obtained at 150 and 300 °C, and respective rehydration by water addition. X-ray patterns showed that the dehydration of Na-magadiite at 150 and 300 °C provoked the basal spacing decrease. The XRD also showed that only the material treated at 150 °C returned to the original structure with the rehydration. ²⁹Si NMR spectra showed that after rehydration, the Q₃/Q₄ relationship presented the same value for Na-magadiite treated at 150 °C. However, this Q₃/Q₄ value decreased when the treatment was done at 300 °C. Kinetic studies of thermal decomposition showed that the dehydration of magadiite is based on a phase boundary-controlled reaction, caused by contracting areas. The exfoliation of lamellas with thermal treatment can explain this behavior, as observed in SEM images.     

Effect of thermal dehydration on surface characteristics of titania gel

The surface properties of titania gel and its thermal dehydration products were investigated by nitrogen and water vapour sorption between 110 and 600°C. Structural and phase changes were studied by X-ray diffractometry and differential thermal analysis. Up to 200°C, samples were X-ray amorphous and formed anatase at 300, 450 and 600°C. The glow phenomenon is attributed to the conversion to anatase at 350°C.Thermal dehydration of the gel between 110 and 300°C and 600°C led to the widening of the pore radii of the gel. At 450°C, marked steps appeared on the nitrogen isotherm, and was accompanied by a sharp increase in nitrogen uptake. The stepwise character is attributed to the presence of a certain porosity characteristic of the gel.Heats of immersion in water depend mainly on the hydrophilic centres on the surface, whereas heats of immersion in cyclohexane are controlled by the micropore fraction of the gel.     

The dehydration of nickel sulfate

Nickel sulfate was recrystallized to obtain the 7 H₂O, β6 H₂O and various habits of α6 H₂O. Dehydration and phase transitions were studied using X-ray analysis and DSC with effluent gas analysis. NiSO₄ · 7 H₂O dehydrates spontaneously via 7 → 6β → 6α at room temperature, while the dehydration pathway of NiSO₄ α6 H₂O is 6α → 6γ → 4 → 1. The effect of time and storage on the 6α—6β phase transition was investigated.     

Spectrophotometric and thermal analytical studies on the dehydration of copper(II) sulfate and its double salts

Spectrophotometric (diffuse reflection) and TG-DTA data on the dehydration of CuSO₄ · 5H₂O, Na₂Cu(SO₄)₂ · 2H₂O, M₂Cu(SO₄)₂ · 6H₂O(M⁺ = K⁺, Rb⁺, Cs⁺ and NH⁺₄) and Co₂Cu(SO₄)₃ · 18H₂O are given. Although complete dehydration of CuSO₄ · 5H₂O brings about a striking color change from blue to white, it was found that there is only a slight decrease in the v?_max. of its d-d band in the course of this change, and the total light absorption in the visible-UV region increases at the same time. The dehydration of the alkali metal and ammonium double salts, most of which contain [Cu(OH₂)₆]²⁺ aquo ions (in contrast to the [Cu(OH₂)₄]²⁺ in CuSO₄ · 5H₂O), occurs generally more easily than that of CuSO₄ - 5H₂O, and their v?_max. increases slightly in the change, leading to blue or green anhydrous products, although the formation of a white modification was observed with the potassium salt. It was also found that the v?_max. of the Cu²⁺ ion in the dehydrated cobalt(II) double salt is still lower than that in anhydrous CuSO₄, i.e., the ligand field and/or tetragonality around it is decreased by the presence of Co²⁺ ions.     

The dehydration of copper(II) acetate monohydrate

The thermal dehydration of copper(II) acetate hydrate has been studied between 353 and 406 K, over a range of humidities. The dehydration is controlled by nucleation-and-growth kinetics at low temperatures, with an activation energy of 154 kJ·mol⁻¹, which changes to contracting-disc kinetics at higher temperatures with a lower activation energy of 76 kJ·mol⁻¹. Frequency factors have also been derived; the value for the high temperature process is low (10⁷s⁻¹) and that for the low temperature step is high (10¹⁷s⁻¹). Optical microscopy has been used to clarify the bulk kinetics; there is evidence for a reactive layer at the surface of the decomposing solid. In celebration of the 60^th birthday of Dr Andrew K. Galwey     

Synthesis,characterization and thermogravimetric decomposition of 2-ethylpyridinium metatungstate monohydrate. Kinetic study of its thermal dehydration

2-Ethylpyridinium dodecatungstate(VI) monohydrate was synthetized and characterized as-(C₅H₄NHC₂H₅)₅H[H₂W₁₂O₄₀] · H₂O by chemical analysis and electronic, IR and¹H NMR spectral studies.The UV spectra showed the characteristic maximum absorption at 260 nm for the metatungstate anion. The two central protons in the anion were also characterized via IR and¹H NMR spectroscopy.The thermal behaviour of this compound was investigated by thermogravimetric and derivative thermogravimetric methods. The TG curves revealed four main steps of decomposition, viz. dehydration, partial organic base decomposition and pyrolysis of the organic base, leading to the formation of WO₃ in the final stage.The intermediates obtained at the end of the various thermal decomposition steps were studied via the IR spectral data on the residues obtained by isothermal heating of the compound at different temperatures.Kinetic parameters for the dehydration reaction were obtained through two methods of analysis.

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Zusammenfassung 2-Äthylpyridindodekawolframat(VI) monohydrat wurde hergestellt und durch chemische Analyse, Elektronenspektrum, IR und¹HNMR Spektraluntersuchungen als-(C₅H₄NHC₂H₅)₅H[H₂W₁₂O₄₀] · H₂O charakterisiert. Das UV Spektrum zeigte bei 260 nm ein für das Metawolframat Anion charakteristisches Absorptionsmaximum. Die zwei Zentralprotonen des Anions wurden auch durch IR und¹HNMR Spektroskopie charakterisiert. Das thermische Verhalten dieser Verbindung wurde mittels TG und DTG untersucht. Die TG-Kurven zeigten vier Hauptzersetzungsschritte u. z. Dehydratisierung, partielle Zersetzung der organischen Base, · Pyrolyse der organischen Base und Bildung von WO₃ als letzte Stufe. Die nach verschiedenen thermischen Zersetzungsschritten erhaltenen Zwischenprodukte wurden durch die Spektraldaten der Rückstände der isothermisch auf verschiedene Temperaturen erhitzten Verbindung untersucht. Die kinetischen Parameter der Dehydratisierung wurden nach zwei Analysenmethoden bestimmt.

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2- (VI) - (C₅H₄NHC₂H₅)₅H[H₂W₁₂O₄₀] - , , - , 250 , -. - . . : , , . , , . , , .

     

Vibrational spectra of 6Li/7Li and H/D substituted single crystal lithium sulfate monohydrate

Infrared and Raman spectra of several isotopic species of Li₂SO₄·H₂O were investigated. Bands with a lithium-isotopic shift of 3–8% were observed in the spectral region from 300–580 cm⁻¹. The spectra of the deuterated single crystals indicate a coupling between the lithium vibrations and the librational modes of the water molecules. The assignment of the bands with appreciable ⁶Li/⁷Li isotope effect to translational modes of the lithium atoms is confirmed by a rough normal coordinate calculation.     

Effect of deuteration on the diameter-effect curve of liquid nitromethane

The detonation properties of liquid nitromethane [CH(3)NO(2)] are probably the most thoroughly studied of any condensed-phase explosive. Because it is homogeneous (i.e., lacks hot-spot phenomena), it provides a window into the underlying chemical processes induced by a passing shock or detonation wave-such information is submerged in the complex fluid mechanics when heterogeneous explosives are detonated. In this paper, we provide experimental data and data analysis of the effect that deuterating nitromethane's methyl group has on some aspects of the processes that occur in the detonating liquid material. In the experimental part of this study, we report diameter-effect curves (i.e., inverse charge internal radius vs steady detonation speed) for pure CH(3)NO(2) and pure CD(3)NO(2) confined in right-circular cylinders of C-260 brass. Large differences in the infinite-medium (i.e., plane wave) detonation speed and in the failure diameter of the two materials are observed. Interpretations of the observations based on physical and chemical theory are given. The observed large decrease in deuterated nitromethane's infinite-medium detonation speed, relative to the protonated material, is interpreted in terms of the Zeldovitch, von Neumann, and Doering theory of steady-state detonation. We also estimate the relative size of the steady plane-wave reaction-zone length of the two materials. We interpret the observed increases in NM's failure diameter and its steady one-dimensional chemical-reaction-zone length due to deuteration in terms of the quantity of NM aci ion present. The new results are placed in the context of earlier work on detonating liquid nitromethane.     

Explosive thermal dehydration of solids

Selected examples of dehydration, accompanied by violent disintegration or bloating of the decomposing solid substance are presented. This phenomenon has been explained as a specific case of internal decomposition of solids. A sealed box with compressed gaseous products is proposed as a model of the process. The practical applications of this phenomenon are discussed.

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Zusammenfassung Es werden ausgewählte Beispiele für die Dehydratation, begleitet von heftiger Zersetzung oder Aufblähen der zersetzten Feststoffe dargelegt. Diese Erscheinung wird als ein Spezialfall der internen Zersetzung von Feststoffen beschrieben. Als Modell für diesen Prozeß wird ein abgeschlossener Behälter mit unter Druck stehenden gasförmigen Produkten vorgeschlagen. Praktische Anwendungen dieser Erscheinung werden diskutiert.

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Dedicated to Professor Dr. H. J. Seifert on the occasion of his 60^th birthday

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The work was financially supported by the Ministry of National Education, grant No P/03/257/90-z. The authors wishes to express his thanks to all who contributed to this support.     

Characterization of dehydration behavior of untreated and pulverized creatine monohydrate powders

Creatine, which is well known as an important substance for muscular activity, is synthesized from amino acids such as glycine, arginine and ornithine in liver and kidney. It then accumulates in skeletal muscle as creatine phosphoric acid. The aim of this study was to understand the dehydration behavior of untreated and pulverized creatine monohydrate at various temperatures. The removal of crystal water was investigated by using differential scanning calorimetry (DSC), X-ray powder diffraction and scanning electron microscopy (SEM). The X-ray diffraction pattern of untreated and pulverized creatine monohydrate agreed with reported data for creatine monohydrate. However, the diffraction peaks of the (1 0 0), (2 0 0) and (3 0 0) planes of pulverized creatine monohydrate were much stronger than those of untreated creatine monohydrate. On the other hand, the diffraction peaks of the (0 1 2) and (0 1 3) planes of untreated creatine monohydrate were much stronger than those of pulverized creatine monohydrate. The dehydration of untreated and pulverized creatine monohydrate was investigated at various storage temperatures, and the results indicated that untreated and pulverized creatine monohydrate were transformed into the anhydrate at more than 30 °C. After dehydration, the particles of untreated and pulverized creatine anhydrate had many cracks. The dehydration kinetics of untreated and pulverized creatine monohydrate were analyzed by the Hancock–Sharp equation on the basis of the isothermal DSC data. The dehydrations of untreated and pulverized creatine monohydrate both followed a zero-order mechanism (Polany–Winger equation). However, the transition rate constant, calculated from the slope of the straight line, was about 2.2–7.7 times higher for pulverized creatine monohydrate than for untreated creatine monohydrate. The Arrhenius plots (natural logarithm of the dehydration rate constant versus the reciprocal of absolute temperature) of the isothermal DSC data for untreated and pulverized creatine monohydrate were linear. The activation energies of dehydration in the 40–60 °C range for untreated and pulverized creatine monohydrate were 15.02 and 10.1 kJ/mol, respectively. Dehydration of untreated creatine monohydrate had a pronounced effect on the particle size of the powder. Compared with pulverized creatine monohydrate, the particle size of untreated creatine monohydrate was significantly decreased by dehydration.     

Effect of deuteration on the infrared spectra of crystalline metal formates

The laser Raman and IR spectra of tribromoacetylchloride have been recorded and vibrational assignments are proposed.     

Influence of deuteration on lithium acetate dihydrate studied by inelastic X-ray scattering, density functional theory, thermal expansion, elastic and thermodynamic measurements

The influence of deuteration on the properties of lithium acetate dihydrate has been investigated by thermal expansion measurements, ultrasound spectroscopy and calorimetry. Inelastic X-ray scattering has been employed to investigate if the low temperature structural phase transition can be detected by a change in the vibrational spectrum. Density functional theory, DFT, calculations have been employed to complement the experimental investigations. The thermal expansion coefficients and the specific heat of the deuterated compound differ significantly from the protonated form. The differences in the elastic stiffness coefficients are just above the detection limit of the technique employed here. Temperature dependent inelastic X-ray spectroscopic measurements show no significant change of the vibrational spectrum when crossing the transition temperature. The DFT calculations show that the methyl group dynamics are best described in the framework of coupled rotators of opposing methyl groups. One of the coupled rotational modes corresponds to a hindered rotator with a barrier of 15 meV, while the other is a free rotator.     

Studies on thermal dehydration of hydrated chromic oxide

Hydrated chromic oxide was prepared by the reduction of potassium dichromate in an autoclave using sucrose. The thermal dehydration of the parent gel in different atmospheres was studied comparatively using the techniques of thermal gravimetry, differential thermal analysis, X-ray diffraction analysis, FT-IR spectroscopy, and gas adsorption. The results of thermogravimetric and differential thermal analysis show that the temperature at which the glow phenomenon occurs is dependent on the gaseous atmosphere in the heat-treatment process and increases in the following order: air, nitrogen, and carbon dioxide. When heated in CO(2), the parent gel is transformed into crystalline alpha-Cr(2)O(3) going through the phase of amorphous Cr(2)O(3). In air, however, it is decomposed directly into crystalline alpha-Cr(2)O(3) without going through the amorphous Cr(2)O(3) phase. It is also found that microcrystallites of alpha-Cr(2)O(3) have been formed before the glow phenomenon occurs at 390 degrees C in air and at 590 degrees C in CO(2), respectively. The mechanism for thermal dehydration in air and CO(2) is discussed.     

New crystals in the lithium sulfate family

The preparation and structures of caesium lithium sulfate, Cs_1.15Li_2.85(SO₄)₂, and caesium lithium rubidium sulfate, Cs_0.90Li_2.88Rb_0.22(SO₄)₂, are described and discussed in the context of simple and double sulfate polymorphism. The latter structure is related to the former through the substitution of Rb for Cs. In both crystals, the sulfate ions occupy two non‐equivalent sites, but the ions are disordered in Cs_1.15Li_2.85(SO₄)₂.