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121.
This study presents results on the kinetics of kaolinite dehydroxylation. The accuracy of various methods of determining the values for the kinetic parameters and their sensitivity in detecting the mechanism of reaction is investigated. In particular, the differential order of reaction method of Baker, the general method of Achar et al., the integral method of Boy and Bohme, and the method of Coats and Redfern as modified by Fong and Chen are considered.
Kaolinites from well-known sources are used to study the influence of crystallinity on the values of kinetic parameters. The statistical significance of the various mathematical methods for the assessment of the data obtained from non-isothermal thermogravimetry is determined by comparison with experimental and theoretical data using a computer programme developed for this purpose. The study demonstrates that the kinetic parameters can be used to quantify the degree of crystallinity of kaolinite and also confirms other findings that the dehydroxylation of kaolinite is a second-order reaction. 相似文献
122.
In this work, a flow analysis procedure for the determination of copper, chromium, iron and lead in lubricating oils using flame AAS as detection technique is described. The flow manifold was designed to implement the multicommutation approach and it comprised three 3-way solenoid valves controlled by a personal computer. The flow system presented allowed to process the oil samples to determine wear metals without any prior preparation. Aiming to assess accuracy the results were compared with those obtained by manual procedure using flame AAS. Applying the joint-confidence ellipse test, no significant difference at the 95% confidence level was observed. Other profitable features such as a sample throughput of 50 determinations per hour; relative standard deviations (n = 5) below 2% for Cu, and below 8% for Cr, Fe and Pb; and linear responses in the range 0–40 ppm (w/w) (Cu, Fe) and 0–15 ppm (w/w) (Cr, Pb) were also achieved. 相似文献
123.
Rosa Brito V. A. Rodríguez Janet Figueroa Carlos R. Cabrera 《Journal of Electroanalytical Chemistry》2002,520(1-2)
In this work we studied the self-assembled monolayer (SAM) formation of 3-mercaptopropyltrimethoxysilane [HS(CH2)3Si(OCH3)3] (MPS) and 3-aminopropyltrimethoxysilane [H2N(CH2)3Si(OCH3)3] (APS) on platinum surfaces. We used X-ray photoelectron spectroscopy (XPS), specular reflectance FTIR, and electrochemistry to study the composition and structure of the self-assembly. XPS results of the Pt modified electrodes showed the presence of sulfur and nitrogen binding energy peaks indicating the presence of MPS and APS. The platinum substrates modified with MPS and APS showed IR absorption bands corresponding to the different stretches present in both compounds. Platinum disk electrodes, before and after being submerged in 1 mM solutions of MPS and APS for 24 h, were characterized voltammetrically by using 2.5 mM K4Fe(CN)6 in 0.1 M KCl, as the redox active couple. The MPS/Pt system is free of mass transfer effects, which are characteristic of pinhole defects in an MPS monolayer. 相似文献
124.
Alberto Escarpa Concepcin Prez‐Cabrera María Cristina Gonzlez 《Journal of separation science》2000,23(11):637-643
A fast high‐performance liquid chromatography method was used for analysis of prominent flavan‐3‐ols and flavonols in vegetables. Gradient elution with phosphoric acid‐acetonitrile mixtures and phosphoric acid‐methanol mixtures allowed fast and complete separation of the studied phenolic compounds within analysis times less than 10 min. The development of two elution gradients using methanol and acetonitrile as modifiers proved to be an excellent approach for the verification of the real polyphenolic composition in vegetables samples because the two optimized methods allowed the separation of the same number of compounds in the same elution order. Diode‐array detection was employed for the provisional identification of phenolic compounds that were not available as standards. We preferred methanol as a modifier because it was less toxic and cheaper than acetonitrile. Detection limits ranged between 0.12 and 0.59 μg mL–1. High recoveries of phenolics from fresh vegetables were measured in all studied cases, independent of the phenolic structure, matrix, and vegetable in question. High levels of procyanidins between 150 and 450 mg kg–1 were found in all studied vegetables. Quantification of quercetin and kaempferol glycosides was only possible in marrow and onion, respectively. 相似文献
125.
The reactions of pyrrolidine with O-ethyl S-(X-phenyl) dithiocarbonates (X = 4-methyl, 4-methoxy, H, 4-chloro, 4-nitro, 2,4-dinitro, and 2,4,6-trinitro) are subjected to a kinetic study in 44 wt% aqueous ethanol, 25.0°C, and ionic strength 0.2 M (maintained with KCl). Pseudo-first-order kinetics are found under amine excess. Linear plots of the pseudo-first-order rate coefficient against concentration of free-base pyrrolidine are obtained for all the reactions, the nucleophilic rate coefficient (kN) being the slope of such plots. The Bronsted-type plot (log kN vs. pKa for the leaving group) is linear with slope βlg = − 0.2, which is consistent with a mechanism through a tetrahedral intermediate (T±) where its formation is rate determining. The βlg value is very similar to that found in the same reactions in water. There is a great difference in the mechanism of the reactions of O-ethyl S-phenyl dithiocarbonate with pyrrolidine (order one in amine) and piperidine (complex order in amine) in aqueous ethanol, and this is attributed to a greater nucleofugality from T± of piperidine rather than pyrrolidine. © 1997 John Wiley & Sons, Inc. 相似文献
126.
Sr5(VO4)3(CuO) was prepared via solid state reactions from mixed powders of the metal oxides or carbonates in corundum crucibles in air (1173–1740 K). The compound is transparent and stable in air. The color changes with the preparation temperature from light gray (1173 K) to gray (1740 K). The crystal structure (space group P63/m, No. 176; Z = 2; a = 10.126 Å, c = 7.415 Å) is a derivative of the apatite Ca5(PO4)3OH, and is characterized by isolated [VO4]3– anions (d(V–O) = 1.710 Å) and infinite linear 1∞[CuO]1– chains (d(Cu–O) = 1.854 Å) inserted in the channels parallel to the hexagonal axis. The compound prepared at 1740 K contains vacancies at the copper and oxygen positions of the linear chains (about 10% and 5%, respectively). 相似文献
127.
E. F. Herrera Peraza O. Díaz Rizo M. E. Montero Cabrera A. T. Hernández R. Contreras Folgar M. Ixquiac Cabrera O. Hernández Aguilar M. V. Manso Guevara I. Alvarez Pellon R. A. López Duménigo G. Padrón González M. C. López Reyes 《Journal of Radioanalytical and Nuclear Chemistry》1999,240(2):437-443
This paper explains chronologically the implementation of thek 0-standardization and other parametric methods in Cuba. Fundamental and applied results in the determination of the most important parameters are obtained, using the IBR-2 at Dubna and Triga Mark III at Mexico reactor facilities. Some modifications in the Stoughton-Halpering convention, and its comparison with the modified Westcott and Høgdahl method are presented. The application of the described methods at the CS-ISCTN low flux facility, using the big samples techniques, is presented. 相似文献
128.
J. M. Aceves G. A. Arroyo Y. M. Vargas R. Miranda A. Cabrera F. Delgado 《Heteroatom Chemistry》2004,15(1):71-76
The reactions between 1,2-ethandithiol with several carbonylic compounds to form the corresponding 1,3-dithiolanes were performed using a natural clay as promotor. The target molecules are used as reagents to obtain fine chemicals, herbicides, fungicides, and pharmaceutical compounds among others applications. A kinetic study of camphorquinone and 1,2-ethandithiol showed a first-order dependence with 1,2-ethandithiol as well as with camphorquinone. The substrate transformation in percent was found to be dependent with the reaction time, the amount of catalyst, and the reagents concentration. © 2003 Wiley Periodicals, Inc. 15:71–76, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/.10215 相似文献
129.
Cabrera Pérez Laura C. Padilla-Martínez Itzia I. Cruz Alejandro Correa Basurto José Miliar García Ángel Hernández Zavala Argelia A. Gómez López Modesto Rosales Hernández Martha C. 《Molecular diversity》2020,24(4):1-14
Molecular Diversity - 11-Beta hydroxysteroid dehydrogenase type 1 (11β-HSD1) regulates cortisol levels mainly in adipose, hepatic and brain tissues. There is a relationship between the high... 相似文献
130.
An unusually large attenuation (approximately 50%) in the low temperature nuclear orientation directional gamma anisotropy from unannealed thermal neutron irradiated single crystal (160Tb)TbNiAl4 has been observed in the region of saturation of the second order nuclear orientation coefficient, B2. It is suggested that the form of the remanent neutron activation recoil damage in unannealed (160Tb)TbNiAl4 is fundamentally electric quadrupolar in origin, as opposed to magnetic dipolar. 相似文献