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961.
J. C. Rouchaud M. Fedoroff G. Revel 《Journal of Radioanalytical and Nuclear Chemistry》1977,38(1-2):185-191
A new method of silicon determination in molybdenum by the30Si(n, γ)31Si was developed. All the problems occurring during this analysis: standardization, quantitative dissolution, silicon sorption
on vessels, reproducibility of β-counting...were carefully studied and new answers were brought to them. The chemical speratation
of silicon was performed with a column of anion exchange resin in HCl-HF-H2O2 solution and a column of alumina at pH 9. Accuracy and reproducibility were controlled on standard samples prepared by fusion
of inactive molybdenum and radioactive silicon in a plasma furnace. 相似文献
962.
Heating a toluene solution of dicarhomethoxyacetylenebis(triphenylphosphine)platinum(0) at 130°C gives the ortho-metalated complex (Ph3P)(Ph2t-trans-(COOMe)CCHCOOMe. 相似文献
963.
The conformational equilibria of 3-chloro- and 3-bromotetrahydropyran were measured by NMR and IR spectroscopy using model 2-alkyl-5-halotetrahydropyrans. The 3-chloro compound was 76·2% equatorial in carbon tetrachloride and 58·5% in acetonitrile and the 3-bromo compound 85% equatorial in carbon tetrachloride. The conformational equilibrium is discussed in terms of dipole, electronic and van der Waals effects. 相似文献
964.
The photochemical behaviour of twelve phenylmethylthiazole isomers is identical to that of non-methylated products. Thus, in order of decreasing reactivity we have: 2-phenylthiazole or 5-phenylisothiazole > 5-phenylthiazole or 3-phenylisothiazole > 4-phenylthiazole or 4-phenylisothiazole. The two latter undergo photoisomerization only to a slight extent.Most of the rearranged compounds (95%) are formed either by a valence bond isomerization mechanism via bicyclic intermediates where the phenyl group is conjugated to the cycle, or by the Kellogg mechanism, through a 180° rotation around the bonds adjacent to the sulphur atom.These results have furthermore allowed us to confirm the selective action of iodine in these isomerization reactions. 相似文献
965.
Theoretical study of the effect of frit quality on chromatography using computational fluid dynamics
Summary Frits at both ends of a chromatographic column, especially for a preparative column, have significant influence on the flow
distribution within the column and thus the column efficiency. However, frits have received little attention from chromatographers
in the past. Here a theoretical study was conducted with the aid of CFD software FLUENT to investigate the effect of frits
on the performance of homogeneous and heterogeneous chromatographic columns. A dimensionless number,FQ, was applied to characterize frit quality. This study visualized how frit quality affects the flow distribution and the concentration
band, the shape of eluted pulse at the colum exit and column efficiency. Simulation results show that the development length
of the flow distribution is related toFQ but has nothing to do with the packing heterogeneity. The curvature of the concentration band in a column depends onFQ and packing quality. This study shows column efficiency can be improved significantly by increasingFQ and/or frit permeability. 相似文献
966.
Bunyard WC Kadla JF DeYoung J DeSimone JM 《Journal of the American Chemical Society》2001,123(30):7199-7206
The thermal decomposition of the free-radical initiator bis(perfluoro-2-N-propoxyprionyl) peroxide (BPPP) was studied in dense carbon dioxide and a series of fluorinated solvents. For the fluorinated solvents, the observed first-order decomposition rate constants, k(obs), increased with decreasing solvent viscosity, suggesting a single-bond decomposition mechanism. The k(obs) values are comparatively larger in dense carbon dioxide and similar to the "zero-viscosity" rate constants extrapolated from the decomposition kinetics in the fluorinated solvents. The decomposition activation parameters demonstrate a compensation behavior of the activation enthalpy with the activation entropy upon change in solvent viscosity. Comparison of the change in activation parameter values upon change in solvent viscosity for BPPP with two additional initiators, acetyl peroxide (AP) and trifluoroacetyl peroxide (TFAP), further suggests that carbon dioxide exerts a very minimal influence on the decomposition mechanism of these initiators through solvent-cage effects. 相似文献
967.
The solvation parameter model is used to construct equations for the estimation of the non-specific toxicity of neutral organic compounds to five organisms used for short-term toxicity testing. For the bacteria Vibrio fischeri (Microtox test) and Pseudomonas putida, the protozoan Tetrahymena pyriformis (Tetratox test), the green alga Scendesmus quadricauda and the brine shrimp Artemia salina, the main factors resulting in increased non-specific toxicity are size (dominantly) and lone-pair electron interactions, with hydrogen-bond basicity the most important solute property reducing toxicity. Species differences in relative non-specific toxicity are largely related to differences in cohesion and hydrogen-bond acidity of the biomembranes. The models for non-specific toxicity are proposed as an alternative to the octanol-water distribution constant for the determination of baseline toxicity. Failure of the octanol-water distribution constant to model non-specific toxicity is quantitatively explained by its inability to adequately characterize the sorption properties of the biomembranes for compounds with varied properties. 相似文献
968.
An automated technique for the determination of analytes in an ultrasonically levitated sample of 2 to 5 microL volume has been developed. Contactless dosing of reagents or solvents into an ultrasonically levitated drop was realized via piezoelectric micropumps. Drop size was continually controlled with a programmed CCD camera. A diode array spectrometer designed for the use with optical fibers was used for absorption and fluorescence measurements. Determinations via direct absorption measurements following the method of standard addition and acid-base titrations with an absorption indicator and a fluorescent indicator were carried out. The mean consumption of sodium hydroxide added via a piezoelectric micropump in five successive titrations with 18 nmol of sulfuric acid inside a levitated drop (indicator bromothymol blue) had a relative standard deviation of 0.7% and differed only by 0.2% from the expected value. 相似文献
969.
A. Boughriet C. Cordier L. Deram B. Ouddane H. Chamley M. Wartel 《Fresenius' Journal of Analytical Chemistry》1995,352(3-4):341-353
A study of managanese and iron associated with particulate matter in seawaters derived from the English Channel (and North Sea). Sequential extraction procedures have been used to identify geochemical processes that govern Mn and Fe coprecipitation/accumulation/distribution in different particulate phases (carbonate; Fe-Mn(hydr)oxides; organic matter and residual phase). The oxidation states of Mn and Fe, some mechanisms of bonding and the magnetic characteristics of particles have been investigated by Mössbauer spectroscopy, electron spin resonance(ESR) spectroscopy, and mass susceptibility balance. The effect of pH and redox potential on stability of different dissolved and particulate Mn species was also examined. In the these calcareous seawaters, manganese (II) is predominantly bound to the carbonate phase (calcite) in the form of MnxCa1-xCO3. However the persistent occurrence of Mn in oxidation states > 2 in these particles has suggested the existence of manganese(III) in the polymeric FeOOH structures as an intermediate entity during Mn(II) oxidation in the sea. The activity coefficient of the entity MnOOH in FeOOH has been evaluated and p-pH diagrams of Mn have been proposed under marine conditions. 相似文献
970.