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91.
J. Morales L. Hernan L. V. Flores A. Ortega 《Journal of Thermal Analysis and Calorimetry》1982,24(1):23-34
Differential thermal analysis and differential scanning calorimetry techniques have been used to study the kinetics of phase transitions. The aragonite/calcite transformation was chosen as test reaction.Marked discrepancies were observed between the DTA kinetic results and those obtained from isothermal kinetic measurements. This disagreement is associated with the difficulty of determining accurately the reaction rate of the process from a DTA curve.In contrast, in DSC measurements, where the reaction rate is given by the height of the peak from the base-line, it is possible to get a satisfactory fit of the data, and the kinetic results obtained with this technique are in good agreement with the isothermal ones.
Zusammenfassung Die Differentialthermoanalyse und DSC wurden zur Untersuchung der Kinetik von Phasenübergängen eingesetzt. Die Umwandlung Aragonit/Calcit wurde als Testreaktion gewählt.Bedeutende Diskrepanzen wurden zwischen den kinetischen Ergebnissen der DTA und den durch isotherme kinetische Messungen erhaltenen Ergebnissen beobachtet. Diese Abweichungen sind damit verbunden, daß es schwierig ist, die Reaktionsgeschwindigkeit des Vorgangs aus einer DTA-Kurve genau zu ermitteln.Bei DSC-Messungen,wo die Reaktionsgeschwindigkeit durch die Höhe des Peaks von der Grundlinie gegeben ist, kann eine befriedigende Anpassung der Daten erhalten werden, und die durch diese Technik erhaltenen kinetischen Ergebnisse sind in guter Übereinstimmung mit den isothermen Ergebnissen.
. /. , . -. , , , , , .相似文献
92.
Nonaqueous CE and field-amplified sample stacking have been used in the determination of complex mixtures of polar aromatic sulfonates (AS; mainly benzene- and naphthalenesulfonates) of environmental concern. The analytical procedure consists of an on-column aqueous sample enrichment, followed by the nonaqueous electrophoretic determination of stacked aromatic sulfonates. Various organic solvents were used as separation medium, acetonitrile and N-methylformamide gave the best results. Optimum capillary electrophoresis separation is obtained with ammonium acetate (25 mM) dissolved in N-methylformamide-methanol (90:10) as background electrolyte. This combined method was applied to the analysis of surface water samples spiked with selected aromatic sulfonates derivatives. 相似文献
93.
Chaudhuri RK Freed KF Hose G Piecuch P Kowalski K Włoch M Chattopadhyay S Mukherjee D Rolik Z Szabados A Tóth G Surján PR 《The Journal of chemical physics》2005,122(13):134105
Tests have been made to benchmark and assess the relative accuracies of low-order multireference perturbation theories as compared to coupled cluster (CC) and full configuration interaction (FCI) methods. Test calculations include the ground and some excited states of the Be, H(2), BeH(2), CH(2), and SiH(2) systems. Comparisons with FCI and CC calculations show that in most cases the effective valence shell Hamiltonian (H(v)) method is more accurate than other low-order multireference perturbation theories, although none of the perturbative methods is as accurate as the CC approximations. We also briefly discuss some of the basic differences among the multireference perturbation theories considered in this work. 相似文献
94.
Canosa P Morales S Rodríguez I Rubí E Cela R Gómez M 《Analytical and bioanalytical chemistry》2005,383(7-8):1119-1126
The degradation of 2-(2,4-dichlorophenoxy)-5-chlorophenol (triclosan) in chlorinated water samples was investigated. Sensitive
determination of the parent compound and its transformation products was achieved by gas chromatography with mass spectrometry
detection after sample concentration, using a solid-phase extraction sorbent and silylation of the target compounds. Experiments
were accomplished using ultrapure water spiked with chlorine and triclosan concentrations in the low mg/l and ng/ml ranges
respectively. Chlorination of the phenolic ring and cleavage of the ether bond were identified as the main triclosan degradation
pathways. Both processes led to the production of two tetra- and a penta-chlorinated hydroxylated diphenyl ether, as well
as 2,4-dichlorophenol. The formation of 2,3,4-trichlorophenol was not detected in any experiment; however, significant amounts
of 2,4,6-trichlorophenol were noticed. All of these five compounds were also identified when triclosan was added to tap-water
samples with free chlorine concentrations below 1 mg/l. Minor amounts of three di-hydroxylated phenols, containing from one
to three atoms of chlorine in their structures, were also identified as unstable triclosan chlorination by-products. The analysis
of several raw wastewater samples showed the co-existence of important concentrations of triclosan and its most stable by-products
(2,4-dichlorophenol and 2,4,6-trichlorophenol), reinforcing the potential occurrence of the described transformations when
products containing triclosan are mixed with chlorinated tap water. 相似文献
95.
96.
97.
Basallote MG Estevan F Feliz M Jesús Fernández-Trujillo M Hoyos DA Llusar R Uriel S Vicent C 《Dalton transactions (Cambridge, England : 2003)》2004,(4):530-536
The novel incomplete cuboidal cluster [W3Se4H3(dmpe)3](PF6), [1](PF6), has been prepared by reduction of [W3Se4Br3(dmpe)3](PF6) with LiBH4 in THF solution. The trihydroxo complex [W3Se4(OH)3(dmpe)3](PF6), [2](PF6), was obtained by reacting [W3Se4Br3(dmpe)3](PF6) with NaOH in MeCN-H2O solution. The complexes [1](PF6) and [2](PF6) were converted to their BPh4- salts by treatment with NaBPh4. Recrystallisation of [1](BPh4) in the presence of traces of water affords the mixed dihydride hydroxo complex [W3Se4H2(OH)(dmpe)3](BPh4). The crystal structures of [1](BPh4), [2](BPh4) and [W3Se4H2(OH)(dmpe)3](BPh4) have been resolved. Although the [1]+ trihydride does not react with an excess of halide salts, reaction with HX leads to [W3Se4X3(dmpe)3]+ (X = Cl, Br). The kinetics of this reaction has been studied at 25 degrees C in MeCN-H2O solution (1:1, v/v) and found to occur with two consecutive kinetic steps. The first step is independent of the nature and concentration of the X(-) anion but shows a first order dependence on the concentration of acid (k1 = 12.0 mol(-1) dm(3) s(-1)), whereas the second one is independent of the nature and concentration of both the acid and added salts (k2 = 0.024 s(-1)). In contrast, the reaction of [2]+ with acids occurs in a single step with kobs = 0.63 s(-1)(HCl) and 0.17 s(-1)(HBr). These kinetic results are discussed on the basis of the mechanism previously proposed for the reactions of the analogous [W3S4H3(dmpe)3]+ cluster, with special emphasis on the effects caused by the change of S by Se on the rate constants for the different processes involved. 相似文献
98.
Elena E. Stashenko William Torres Jairo Ren Martinez Morales 《Journal of separation science》1995,18(2):101-104
Volatile secondary metabolites from Colombian ylang-ylang flowers were obtained by combined steam distillation - solvent extraction. The samples were analyzed by high resolution gas chromatography with flame ionization, nitrogen/phosphorus, or mass spectrometric detection. The chemical composition of the oils extracted from flowers at different stages of development differed both qualitatively and quantitatively. The generation of total volatile metabolites, light oxygenated compounds in particular, increased markedly during flower maturation. In this work the quality of the ylang-ylang essential oils was studied as a function of flower maturity. 相似文献
99.
L. Morales A. Caneiro D. VegaR. Zysler H. LanzaR.C. Mercader 《Journal of solid state chemistry》2002,168(1):100-109
The incorporation of Sn into LaMnO3 perovskite and its influence on magnetotransport properties were studied in samples synthesized at low temperature. Single-phase materials for two series of samples with La/(Sn+Mn)=1 and La/(Sn+Mn)<1 ratios were obtained by substitution of up to 10% of the Mn ions by Sn4+. The effect of Sn substitution was monitored through measurements of thermal, “M(T)”, and magnetic field, “M(H)”, dependences of magnetization, as well as of resistivity, “ρ(T)”, at 0 and 70 kOe. These showed that this effect depends strongly on the perovskite cation site ratio (A/B). For La/(Sn+Mn)=1, M and TC were depressed as Sn content was increased. The magnetization data suggest the presence of magnetic clusters with superparamagnetic behavior. No evidence of magnetoresistance (MR) was found. For La/(Sn+Mn)<1 ratio, the samples showed ferromagnetic behavior and MR and both M and TC raised as Sn content increased. The results are discussed in terms of A site vacancies. 相似文献
100.
L. F. Brum Malta Marta Eloisa Medeiros 《Journal of Thermal Analysis and Calorimetry》2005,81(1):149-152
Summary This work reports the room-temperature stabilization of the Bi4V2-xFexIIO11-1.5x γ ‘ phase, a promising ionic conductive material that finds application in solid oxide fuel cell and oxygen sensor devices. The Fe(II) cation proved to be a better stabilizer than Fe(III), which was previously used, since a lower substitution degree of V5+ is needed for the former. Powder X-ray diffraction, Fourier-transform infrared spectroscopy and differential scanning calorimetry were used in these experiments. 相似文献