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71.
Comparison of subcritical water and organic solvents for extracting kava lactones from kava root 总被引:1,自引:0,他引:1
Subcritical water extraction of lactones from a kava (Piper metlhysticum) root was compared to a Soxhlet extraction with water, to boiling in water, and to a sonication in acetone. For ground kava (250-500 microm), 2 h of subcritical water extraction were required for a complete extraction at 100 degrees C, while at 175 degrees C, 20 min were sufficient. For a complete extraction of the unground (shredded) kava, the time of extraction was extended to 40 min at 175 degrees C. Boiling for 2 h and extraction with Soxhlet apparatus for 6 h, both of which employed water at atmospheric pressure, produced yields 40-60% lower than those obtained with subcritical water. With unground kava, 40 min of subcritical water extraction yielded essentially the same recoveries of lactones as 18 h of sonication with acetone, methylene chloride, or methanol. 相似文献
72.
73.
P. Kubíček 《Czechoslovak Journal of Physics》1976,26(3):300-305
Heterodiffusion of Cr has been studied using the method of thin layer and the radionuclide51Cr. The diffusion characteristics determined from the experimental results in the temperature range 1800 KT1970 K areD
o=1·59×10–2 cm2/s andE=22·3±1·6 kcal/mol. The experimental method is discussed in detail and the results are compared with those of other authors. 相似文献
74.
The experimental system involves the permeative preconcentration of CO(2) from an acidified sample through a tubular silicone rubber membrane into a suitable receptor. Procedures for measuring the CO(2) collected in the receptor by the absorbance change of an acid-base indicator incorporated in the receptor or by the conductance change of the receptor are compared. Previous theoretical considerations of the photometric system are augmented for numerical modeling and the theoretical behavior of the conductometric system is considered in detail. Experimental data generally conform to theoretical expectations. Based on either reproducibility or sensitivity, the conductometric measurement method using a solution of 1-10mM tris(hydroxymethylamino)methane as receptor is recommended. The interference from sulfite or sulfide can be eliminated by adding acidic permanganate to the sample; considerably more involved arrangements are necessary to eliminate interferences from cyanide. 相似文献
75.
Kapicka L Dastych D Richterová V Alberti M Kubácek P 《Magnetic resonance in chemistry : MRC》2005,43(4):294-301
The higher order high-resolution (31)P and (19)F NMR spectra of hexafluorocyclotriphosphazene (F(2)PN)(3) were measured at 183 K and interpreted using subspectral analysis and iterative fitting computation. (F(2)PN)(3) forms a rigid nine-spin system [A[X](2)](3) with D(3h) symmetry. Two complete and very similar sets of six experimental spin-spin coupling constants, (1)J(P,F), (2)J(P,P), (2)J(F,F), (3)J(P,F), (4)J(F,F)(cis) and (4)J(F,F)(trans), were determined for the first time. Theoretical DFT calculations of chemical shifts and coupling constants were performed to assess their predictive value. The PP/aug-cc-pVDZ treatment rendered the best agreement with experimental data. 相似文献
76.
77.
It is shown that covalent bonding between high density polyethylene (HDPE) and glass spheres can have a significant influence on the stress relaxation behaviour and the creep properties of the corresponding composites at room temperature. The bonding is obtained by reacting the glass spheres with an azide functional alkoxysilane which is capable of bonding to the HDPE-chain. The internal stress, evaluated from relaxation experiments, increased markedly as a result of this treatment, and it is suggested that the internal stress level reflects the properties of the interphase region between the filler and the bulk matrix and its effect on the viscoelastic properties. 相似文献
78.
79.
Nicola D’Antona Raffaele Morrone Paolo Bovicelli Giovanni Gambera David Kubáč Ludmila Martínková 《Tetrahedron: Asymmetry》2010,21(20):2448-2454
A new route to synthesize cyclophellitol and epi-cyclophellitol from racemic starting materials in enantiopure forms has been developed. The synthesis involves a multi-enzymatic biotransformation pathway of the novel cyano-cyclitol (1R,4S,5R,6R)/(1S,4R,5S,6S)-4,5,6-trihydroxycyclohex-2-enecarbonitrile by a cooperative use of lipase, nitrile hydratase, and amidase. 相似文献
80.
For many solid materials the stress relaxation process obeys the universal relationF = – (d/d lnt)max = (0.1 ± 0.01) (
0 —
i
), regardless of the structure of the material. Here denotes the stress,t the time,
0 the initial stress of the experiment and
i
the internal stress. A cooperative model accounting for the similarity in relaxation behaviour between different materials was developed earlier. Since this model has a spectral character, the concepts of linear viscoelasticity are used here to evaluate the corresponding prediction of the dynamic mechanical properties, i.e. the frequency dependence of the storageE () and lossE () moduli. Useful numerical approximations ofE () andE () are also evaluated. It is noted that the universal relation in stress relaxation had a counterpart in the frequency dependence ofE (). The theoretical prediction of the loss factor for high-density polyethylene is compared with experimental results. The agreement is good. 相似文献