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A. van Tets und H. Jucker 《Fresenius' Journal of Analytical Chemistry》1966,223(5):344-350
Zusammenfassung Verschiedene Endpunkt-Bestimmungsmethoden wurden auf thermodynamischer Basis anhand ihrer unterschiedlichen Ansprechempfindlichkeit miteinander verglichen. Die Grundlage des Vergleichs beruht auf der Annahme, daß das Umschlagsgebiet genau beim Äquivalenzpunkt beginnt und daß die Konzentrationsänderungen in eindeutigem Zusammenhang mit mindestens einer meßbaren Änderung vom Titrations-system stehen.Je ein potentiometrisches, photometrisches, visuelles, coulometrisches, konduktometrisches und thermometrisches Beispiel der Endpunktsbestimmungen wurden gegeben. Die ersten drei Beispiele beziehen sich auf die Reaktion einer verdünnten Lösung von Benzoesäure in Wasser mit Lauge (Tab. 1) und die anderen drei auf einfache Zinkbestimmungen (Tab. 2).Es wurde gezeigt, daß die Anzeigemechanismen, mit Ausnahme des visuellen und des thermometrischen Beispiels, hochempfindlich auf Konzentrationsänderungen reagieren. Titrationsresultate sind jedoch mit den heutigen Mitteln selten genauer als 0,1%. In vielen Fällen wird die Reproduzierbarkeit einer Endpunktbestimmung nicht durch die Empfindlichkeit des Meßinstrumentes, sondern durch die Abstimmung der gesamten Titrationsverhältnisse begrenzt sein.
Summary Several endpoint methods have been compared, based on common thermodynamic properties and different analytical sensitivities. From a general point of view it is assumed that the endpoint region starts with the equivalent point and that the change in concentrations is in a definite and reproducible correlation with at least one measurable modification in the titration system.An example is given of potentiometric, photometric, visual, coulometric, conductometric, and thermometric endpoint detection methods. The first three examples are connected to the reaction of a dilute, aqueous solution of benzoic acid with hydroxide (Table 1). The other three are involving simple reaction systems with zinc (Table 2). It is shown that the response of the detection systems can be very sensitive, the examples of visual and thermometric indication being an exception.Titrations are seldom more accurate than 0.1% according to contemporary literature and chemical experience. It has been concluded that in many cases the precision of a titration will be limited—not by lack of instrumental sensitivity—but by the reproducibility of the titration system.相似文献
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B.A. Jucker H. Harms S.J. Hug A.J.B. Zehnder 《Colloids and surfaces. B, Biointerfaces》1997,9(6):1205-343
Adhesion of bacterial cells to solid surfaces is often largely affected by bacterial surface polymers. In this study, we investigated the adsorption of three different O-antigens isolated from bacterial lipopolysaccharides on TiO2, Al2O3, and SiO2. The O-antigens of Escherichia coli 08 DSM 46243 and Citrobacter freundii PCM 1487 had high affinity for TiO2 and low affinity for Al2O3, whereas the O-antigens of Stenotrophomonas maltophilia 70401 had low affinities for both surfaces. Adsorption on SiO2 was low for all polysaccharides. The dependence of the adsorption on the molecular mass of polysaccharides was investigated with dextrans of various chain lengths. The affinity increased with the molecular mass. The affinity of the dextrans was reduced compared with the O-antigen of E. coli, which had similar chemical composition and molecular mass. The adsorption of the E. coli and C. freundii O-antigens on Al2O3 and TiO2 was irreversible, whereas for the S. maltophilia O-antigen it was partially reversible. The reversibility of dextran adsorption decreased with increasing molecular mass.
Infrared spectroscopy showed that all bacterial O-antigens and the dextrans formed hydrogen bonds with surface hydroxyl groups or interacted with surface-bound water of TiO2, Al2O3, and SiO2. A concentration-dependent mechanism of adsorption was observed with TiO2. At low polysaccharide concentrations, the surface water molecules ware replaced by the polysaccharides, and at increased concentration the surface hydroxyl groups were involved in the formation of hydrogen bonds. At higher surface coverages, the adsorbed polysaccharides formed loops between the few adsorbed units. 相似文献
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Canny MD Jucker FM Kellogg E Khvorova A Jayasena SD Pardi A 《Journal of the American Chemical Society》2004,126(35):10848-10849
The hammerhead ribozyme is a small RNA motif that catalyzes the cleavage and ligation of RNA. The well-studied minimal hammerhead motif is inactive under physiological conditions and requires high Mg(2+) concentrations for efficient cleavage. In contrast, natural hammerheads are active under physiological conditions and contain motifs outside the catalytic core that lower the requirement for Mg(2+). Single-turnover kinetics were used here to characterize the Mg(2+) and pH dependence for cleavage of a trans-cleaving construct of the Schistosoma mansoni natural hammerhead ribozyme. Compared to the minimal hammerhead motif, the natural Schistosoma ribozyme requires 100-fold less Mg(2+) to achieve a cleavage rate of 1 min(-1). The improved catalysis results from tertiary interactions between loops in stems I and II and likely arises from increasing the population of the active conformation. Under optimum pH and Mg(2+) conditions this ribozyme cleaves at over 870 min(-1) at 25 degrees C, further demonstrating the impressive catalytic power of this ribozyme. 相似文献
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The uncapacitated plant location problem under uncertainty is formulated in a mean-variance framework with prices in various markets correlated via their response to a common random factor. This formulation results in a mixed-integer quadratic programming problem. However, for a given integer solution, the resulting quadratic programming problem is amenable to a very simple solution procedure. The simplicity of this algorithm means that reasonably large problems should be solvable using existing branch-and-bound techniques. 相似文献
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Feldsine PT Green ST Lienau AH Stephens J Jucker MT Kerr DE 《Journal of AOAC International》2005,88(5):1334-1348
A multilaboratory collaborative study was conducted to compare the Assurance GDS for E. coli 0157:H7 method and the reference culture methods for the detection of E. coli 0157:H7 in orange juice, raw ground beef, and fresh lettuce. A separate companion assay, the Assurance GDS for Shigatoxin Genes method was also evaluated with the same test portions. Fifteen laboratories participated in the study. A Chi square analysis of each of the 3 food types at the high, low, and uninoculated control levels was performed. For all foods, the Assurance GDS for E. coli O157:H7 method and the Assurance GDS for Shigatoxin Genes method were equivalent to or better than the reference methods. 相似文献
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