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111.
112.
Pollet BG Lorimer JP Hihn JY Phull SS Mason TJ Walton DJ 《Ultrasonics sonochemistry》2002,9(5):267-274
Ultrasound was found to increase the oxidation peak current and hence the decomposition rate of thiosulphate 50-fold compared to silent conditions. The effects of the ultrasonic frequency (20 and 38 kHz) and power upon the electrochemical oxidation of thiosulphate in aqueous KCl (1 mol dm-3) at stationary stainless steel and platinum electrodes were studied chronoamperometrically and potentiostatically (at various scan rates). No sigmoidal-shaped voltammograms were observed for the redox couple S4O6(2-)/S2O3(2-) in the presence of ultrasound. However, application of ultrasound to this redox couple provided an increase in the oxidation peak current at the frequencies employed, the magnitude of which varied with concentration, scan rate and ultrasonic power. Under sonication at 20 and 38 kHz, the oxidation peak potential shifted anodically with increasing ultrasonic power. This anodic shift in potential may be due to the formation of hydroxyl radicals, changes in electrode surface composition and complex adsorption phenomena. The large increase in oxidation peak currents and the rates of decomposition of thiosulphate, in the presence of ultrasound, are explained in terms of enhanced mass transfer at the electrode due to cavitation and acoustic streaming together with microstreaming coupled with adsorption phenomena. It is also shown that changes in macroscopic temperature throughout the experiment are insufficient to cause the observed enhanced diffusion. 相似文献
113.
Pamela de Oliveira Rossini Fabiana S. Felix Lúcio Angnes 《Central European Journal of Chemistry》2012,10(6):1842-1849
Losartan is an antihypertensive agent that lost its patent protection in 2010, and, consequently, it has been available in generic form. The latter motivated the search for a rapid and precise alternative method. Here, a simple conductometric titration in aqueous medium is described for the losartan analysis in pharmaceutical formulations. The first step of the titration occurs with the protonation of losartan producing a white precipitate and resulting in a slow increase in conductivity. When the protonation stage is complete, a sharp increase in conductivity occurs which was determined to be due to the presence of excess of acid. The titrimetric method was applied to the determination of losartan in pharmaceutical products and the results are comparable with values obtained using a chromatographic method recommended by the United States Pharmacopoeia. The relative standard deviation for successive measurements of a 125 mg L?1 (2.71×10?4 mol L?1) losartan solution was approximately 2%. Recovery study in tablet samples ranged between 99 and 102.4%. The procedure is fast, simple, and represents an attractive alternative for losartan quantification in routine analysis. In addition, it avoids organic solvents, minimizes the risk of exposure to the operator, and the waste treatment is easier compared to classical chromatographic methods. 相似文献
114.
Vignolini P Gehrmann B Melzig MF Borsacchi L Scardigli A Romani A 《Natural product communications》2012,7(7):875-877
The homeopathic tincture of Taxus baccata L. is monographed in the current German Homeopathic Pharmacopoeia (HAB 2009). However, the described identification test is a common comparative TLC procedure that might be updated. The purpose of the current work was the quali-quantitative analysis by HPLC/DAD/MS of Taxus tincture. In this study we characterized polyphenolic compounds, in particular four hydroxycinnamic derivatives (0.85 mg/mL) and four flavonoids (quercetin and kaempferol 3-O-rutinoside and xylosyl glucosides); the total polyphenol content was 1.265 mg/mL of tincture. Starting from the official German Homeopathic Pharmacopoeia method of preparation, the aim of this work was to optimize a rapid and reproducible method for the analysis of herbal drugs and tincture, directly prepared in store or the herbalist's shop, to ensure safety and efficacy of the preparation. The procedure has to ensure validation, robustness of the results, and provide a quick response about the composition of compounds in the herbal drug preparation. 相似文献
115.
In this paper we extend the ideas of Brugnano, Iavernaro and Trigiante in their development of HBVM (s,r) methods to construct symplectic Runge–Kutta methods for all values of s and r with s≥r. However, these methods do not see the dramatic performance improvement that HBVMs can attain. Nevertheless, in the case of additive stochastic Hamiltonian problems an extension of these ideas, which requires the simulation of an independent Wiener process at each stage of a Runge–Kutta method, leads to methods that have very favourable properties. These ideas are illustrated by some simple numerical tests for the modified midpoint rule. 相似文献
116.
117.
Russell G. Baughman Daniel J. Brink Jill M. Butler Pamela R. New 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(5):528-531
The first single‐crystal studies of three bis‐transoid Cu–hydroxamate salts, bis(3‐methoxy‐4,N‐dimethylbenzohydroxamato‐O,O′)copper(II), [Cu(C10H12NO3)2], bis(4‐chloro‐N‐methylbenzohydroxamato‐O,O′)copper(II), [Cu(C8H7ClNO2)2], bis(N‐methyl‐3,5‐dinitrobenzohydroxamato‐O,O′)copper(II)–chloroform (1/2), [Cu(C8H6N3O6)2]·2CHCl3, are presented. The Cu atom in each of the title compounds sits at a center of inversion and displays a nearly square‐planar geometry with the hydroxamate‐O atoms connected to it in a syn configuration. The N atoms are in a transoid configuration. Each five‐membered Cu–hydroxamate ring is planar, thus providing evidence that a planar N atom is present in each ring. The phenyl groups are twisted with respect to the hydroxamate group by ~40–54°. The angular strain of the sp2 carbonyl oxygen is significant (~10° from ideal). 相似文献
118.
Pamela B. Pierce Daniel Waterman 《Proceedings of the American Mathematical Society》2000,128(9):2593-2596
It is shown that the concept of bounded variation in the mean is not a meaningful generalization of ordinary bounded variation. In fact, it is a characterization of functions which differ from functions of bounded variation on a zero set.
119.
Pamela Knoll Alexander V. Gonzalez Zachary C. McQueen Prof. Dr. Oliver Steinbock 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(61):13885-13889
Precipitation reactions under flow in confined media are relevant to the control of pathological biomineralization, processes affecting aquifers, and challenges in the petroleum industry. Here we show that for a simple geometry, such conditions create macroscopic structures including helices, tubes, lamellae, slugs, and disordered patterns. All structures emerge when salt solution is slowly injected into thin capillaries filled with hydroxide solution. For the helices, the pitch is proportional to the pump rate revealing a constant period of 0.63 s. Different morphologies of the insoluble metal hydroxide can co-exist causing random transitions along the capillary. On average, 15 % of the final system contains residual hydroxide solution. While mechanically stable for flow speeds above 25 mm min−1, structures collapse and sediment for slower injection speeds. Some of the observed features share similarities with precipitate tubes in chemical gardens and the dynamics of liquid–liquid pipe flow. 相似文献
120.
Pamela Gorkin Raymond Mortini 《Transactions of the American Mathematical Society》2006,358(2):553-571
We compute norms and essential norms of linear combinations of endomorphisms on uniform algebras.