Effusion measurements

Boehncke, A.. Martin. K.. Muller. M.G., and Cammenga, H.K. The vapor pressure of lindane (Y-l,2,3,4,5,6-hexachlorocyclo-hexane) - A comparison of Knudsen effusion measurements with data from other techniques, /. Chem. Eng. Data, 41 (3) 543-545,1996. 

C, Knkudsen effusion, measured range 10-30°C, Swan Mack 1925)... 

C, Knudsen effusion, measured range 59-69°C, Lenchitz Velicky 1970)... 

The deviations observed between extrapolated estimates from GLC data, and direct measurements with the effusion measurements appear to be too large to be accounted for by extrapolation uncertainties. The best estimate can probably be obtained by fitting the combined data to the Clausius-Clapeyron equation (footnote b of Table IV). The obvious implication is that where possible, extrapolation of pesticide vapor pressures obtained at elevated temperatures be converted to interpolation by including a direct measurement at room temperature. In terms of the work described here, vapor pressure measurements requiring the DTA should be supplemented with Knudsen cell measurements. This would require a temperature at which the vapor pressure was 10 3 mm. or less. 

In summary, Raman backscattering measurements showed the presence of C-Hx and N-H local vibrational modes in single crystal ZnO. Heating the specimens to temperatures of up to 950 °C caused hydrogen out diffusion. After dehydrogenation the local vibrational modes disappeared indicating that they are related to the presence of H. From H effusion measurements the... 

C, extrapolated, effusion, measured range 40-95°C, Verhoek Marshall 1939)... 

Oxides. Decomposition pressure measurements on the TbO system by Eyring and his collaborators (64) have been supplemented by similar and related studies on the PrO system (46) and on other lanthanide-oxygen systems (43, 44). Extensive and systematic studies of vaporization processes in lanthanide-oxide systems have been undertaken by White, et al. (6, 188,196) using conventional Knudsen effusion measurements of the rates of vaporization of the oxides into high vacuum. Combination of these data with information on the entropies and Gibbs energy functions of reactants and products of the reaction yields enthalpies of reaction. In favorable instances i.e., if spectroscopic data on the gaseous species are available), the enthalpies of formation and the stabilities of previously undetermined individual species are also derived. The rates of vaporization of 17 lanthanide-oxide systems (196) and the vaporization of lanthanum, neodymium, and yttrium oxides at temperatures between 22° and 2700°K. have been reported (188). 

Enthalpies of sublimation based on Knudsen effusion measurements have been determined for LaBe by Gordienko, et al. (57). The thermodynamics of formation of lanthanide hexaborides from oxides have been deduced by Portnoi, et al. (162) vaporization and stabilities have been studied by Smith (168). 

Lithium vapor contains an appreciable amount of dimer, whose enthalpy of dissociation has been selected by Evans (5), from spectroscopic and molecular beam measuresments to be 25.76 0.10 kcal mol at 0 K. This enthalpy of dissociation, together with the thermodynamic functions calculated in this work, has been used to find the partial pressures of Li(g) and Li2(g) from the measured total vapor pressures. Hartmann and Schneider (6), report values from 1204 to 1353 K while Mancherat (7) reports effusion measurements from 735 to 915 K. Mancherat s (7 ) pressures are calculated on the assumption of monatomic vapor and have been recalculated to fine the true total pressure. Effusion measurements by Lewis (8) and Bogros (9) have been disregarded. Mancherat (7) considers them to be inaccurate because of impurities In the lithium used, and Lewis (S used a doubtful calibration method. Enthalpy of sublimation to monatomic vapor calculated from the vapor pressures of Hartmann and Schneider (6) and of Mancherat (7) agree to within 2% and the average value has been adopted. The enthalpy of sublimation of the dimer was then calculated using this value. 

The selected dissociation energy, obtained from precise spectroscopic measurements (2), is supported by additional studies on thermochemical cycles (3), photoionization (4, 5) and torsion-effusion measurements (6). Earlier measurements have been reviewed by Herzberg (7), Gaydon (8), Brewer (9), and Drowart and Goldfinger (3 ). 

Selenium-Oxygen Compounds.—Knudsen effusion measurements have been performed on powdered selenium dioxide at temperatures between 374 and 427 K, and the vapour pressure over this range has been determined.295 The enthalpy of formation of Se02(g) was computed to be AHT(298.i5) — -26.20 kcal mol-1 this leads to a value for an average bond energy of 99.80 kcal moF1, which is comparable with the spectroscopic dissociation energy. A vibrational analysis of the 3130 A absorption system of selenium dioxide has been carried out.296... 

Thermodynamic properties Thermodynamic properties of [C4mim][PF6] in the ideal gas state were calculated from molecular and spectral data [40], The geometries of the cation, the anion, and the ion pair were optimized with the HF/6-31G combination. Subsequently the authors carried out MP2 single point calculations with the 6-31G basis set. The calculated thermodynamic quantities of the ideal gas state S°, Cp, and -(G°-H°(0)/T) were 657.4, 297.0, and 480.3 JIG1 mol 1 at 298 K and 843.1,424.4, and 252.8 JK 1 mol-1 at 500 K. The calculated vapor pressure obtained by combining a published value of the cohesive energy density, measured heat capacities, and thermodynamic properties in the ideal gas state was found to be 10 10 Pa. Thus the authors found a value that is much smaller than the lower detection limit for effusion measurements [40],... 

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