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31.
Hiltrud Breitenfelder-Manske Friedrich Kohler 《Monatshefte für Chemie / Chemical Monthly》1985,116(11):1247-1261
The second virial coefficients of homonuclear three-centerLennard-Jones molecules are calculated with various parameters of the isosceles triangle connecting the three sites. A special effort is made to establish the reducedBoyle temperaturesT
B and the values of the second virial coefficients atT/T
B=0.3 for the sake of comparison with one- and two-centerLennard-Jones molecules. It is shown that it is possible to find parameter values of the interaction potential of one- and two-centerLennard-Jones molecules which give very similar values of second virial coefficients forT/T
B0.3, and the equivalence conditions are established. These conditions might not only give a basis for a microscopic scaling of state variables, but also some restrictions for the validity of the group contribution concept.Presented in part at the DFG-Colloquium at Paderborn, 19th April 1982, and at the 5th Conference on Mixtures of Nonelectrolytes and Intermolecular Interactions, April 18–22, 1983, at Halle (GDR). 相似文献
32.
Electroformed nickel tubing was deactivated by chemical vapor deposition of silicon from silane gas and subsequent treatment with cyclooctamethyltetrasiloxane (D4). Standard activity tests performed on the uncoated tube and also on columns coated with crosslinked and uncrosslinked, nonpolar stationary phase, show that good quality flexible columns can be prepared from nickel tubing. The inner surface of the silicon coated tube was characterized by Auger depth profile analysis. 相似文献
33.
Enol and keto tautomers of methyl 3-oxo pentanoate could be separated on a HP-5 capillary column. The chromatographic peaks
were identified by examining characteristic mass ions arose from the corresponding enol and keto molecular ions. The study
showed that the area percentage of enol tautomer is a function of temperature of the column. Treating the column as a reactor,
the energy of activation for the on-column tautomerization could be extracted (35.1 kJ mol−1) by monitoring the loss of the enol tautomer, because the reaction is found to obey pseudo first-order kinetics. The enthalpy
and the entropy changes (ΔH = −3.98 kJ mol−1, ΔS = −7.89 J K−1mol−1) for the enol-to-keto reaction in the stationary phase were also obtained. 相似文献
34.
Rabindra N. Roy James J. Gibbons Guilio LaCross Jr. Charles W. Krueger 《Journal of solution chemistry》1976,5(5):333-343
The two thermodynamic dissociation constants of glycine at 11 temperatures from 5 to 55°C in 50 mass % methanol-water mixed solvent have been determined from precise emf measurements with hydrogen-silver bromide electrodes in cells without liquid junction. The first acidic dissociation constant (K
1)for the process HG+H++G± is expressed as a function ofT(oK) by the equation pK
1 = 2043.5/T – 9.6504 + 0.019308T. At 25°C, pK
1is 2.961 in the mixed solvent, as compared with 2.350 in water, with H°=1497 cal-mole–1, G°=4038 cal-mole–1, S°=–8.52 cal-°K–1-mole–1, and C
p
o
=–53 cal-°K–1-mole–1. The second acidic dissociation constant (K
2)for the process G±H++G– over the temperature range studied is given by the equation pK
2 = 3627.1/T – 7.2371 + 0.015587T. At 25°C, pK
2is 9.578 in MeOH–H2O as compared with 9.780 in water, whereas H° is 10,257 cal-mole–1, G° is 13,063 cal-mole–1, S° is –9.41 cal-°K–1-mole–1, and C
p
o
is –43 cal-°K–1-mole–1. The protonated glycine becomes weaker in 50 mass % methanol-water, whereas the second dissociation process becomes stronger despite the lower dielectric constant of the mixed solvent (=56.3 at 25°C). 相似文献
35.
36.
Thermal and thermo-oxidative degradation of polyethylene and polypropylene is discussed. The products of thermal and thermo-oxidative degradation are analysed using a GC-MS after previous adsorption in a column containing a sorbent of styrene divinyl benzene copolymer type. Aldehydes are the products mostly resulting from the thermo-oxidation of polyethylene and methyl ketones are those resulting from the thermo-oxidation of polypropylene.
Analyse der thermischen Oxidationsprodukte von Polypropylen und Polyäthylen durch Gas-Chromatographie/ Massenspektrometrie
Zusammenfassung Der thermische und thermisch-oxidative Abbau von Polyäthylen und Polypropylen wird diskutiert. Die Abbauprodukte wurden mit Hilfe der GC-MS nach vorhergehender Adsorption an einer Säule aus Styrol-Divinylbenzol-Copolymer analysiert. Die thermischen Oxidationsprodukte des Polyäthylens sind meistens Aldehyde, die des Polypropylens Methylketone.相似文献
37.
Liquid and solid cyclic peroxides derived from aliphatic ketones are explosive materials so their enthalpies of vaporization
and other thermodynamic or condensed-phase properties cannot be measured directly. In this work the enthalpies of vaporization
of peroxides at 298.15 K were estimated simply from gas chromatographic retention times measured at different temperatures.
The technique correlates changes in the retention times of compounds whose enthalpies of vaporization are known (called the
reference series), with those of the compounds of interest. If t
R′ is the adjusted retention time (retention time of each compound minus the retention time of unretained diethyl ether, used
as solvent) a plot of ln t
R′ against 1/T for each compound (reference compounds and cyclic peroxides) results in a straight line (r
2 > 0.99 for all compounds). The enthalpy of transfer from solution to the vapor state (Δsolg
H
m) can be obtained by multiplying the slope by the gas constant (R). A second plot correlates the enthalpies of transfer from
solution to the vapor state (Δsolg
H
m), as measured by gas–liquid chromatography (GLC), with enthalpies of vaporization of reference materials (Δvap
H
m at 298.15 K) available in the literature. C9–C15 fatty acid methyl esters and hydrocarbons were used as reference compounds. The enthalpies of vaporization of the cyclic
organic peroxides were calculated from the equation of the line obtained in this second correlation, the slope of which was
Δvap
H
m (at 298.15 K)/Δg
sol
H
m. The experiments were performed under isothermal conditions with a DB-5 capillary column, flame-ionization detection (FID),
and nitrogen as carrier gas. The column temperature was varied over a range of at least 30–70 K between 403 and 473 K, with
chromatograms being acquired at 10 K intervals. Enthalpies of vaporization of cyclic organic peroxides are not available in
the literature, and the values given in this paper, obtained by gas chromatography, are the first to be reported. 相似文献
38.
A simple method is described for the quantitative determination of organochlorine pesticides and polychlorinated biphenyls (PCB's) in water at the sub-ppb level. A micro gas-phase extractor advantageously replaces other preconcentration and purification techniques. The extract is analyzed by capillary gas chromatography without further enrichment. The recovery at the ppb level was nearly 100% for organochlorine pesticides and more than 80% for PCB's. The complete procedure including sample preparation, steam distillation-extraction, and capillary gas chromatographic analysis is carried out in less than four hours. 相似文献
39.
Summary An efficient and economical GC method for rapid determination of FAMEs in rapeseed-mustard is described. The seeds were transmethylated
with acetylchloride, using microwave heating and separation achieved on a 3 m column packed with a mixture of 2% SP-2300 and
3% SP-2310 on Chromosorb ‘W’. The method is compared with the conventional heating method and extended efficiently for half-seed
analysis. The fatty acid composition of the FAMEs mixtures prepared by both methods was similar with highly significant correlation
coefficients (P<0.001). 相似文献
40.
GC headspace analyses of various aromatizing herbs have been compared with those relating to the essential oils, obtained by steam distillation, of the same plants. In this way it was possible to establish the most significant differences between the composition of a herb flavor and that of its essential oil. In particular, we observed some very volatile compounds in the headspace samples which were absent from the essential oil; these components may make an important contribution to the herb flavor. The identification of these substances is still in progress. 相似文献