Molecular effusion vapor pressure measurements

Equation (2) can only be used for the pressure determination if there is molecular flow in the effusion orifice of the Knudsen cell. According to Boer-boom [30] molecular flow exists as a coarse approximation for p < 3.6/r (p in Pa, r in mm), where p is the pressure in the Knudsen cell and r is the radius of the effusion orifice. The demand for a molecular flow and the effusion orifice diameter thus determines the upper limit of the range for the vapor pressure measurement. Experimental and theoretical studies on the transition range between molecular and hydrodynamic flow were recently reviewed by Wahl-beck [89] and references quoted therein. Thermodynamic studies at the limit of the Knudsen flow region are discussed by Hiipert and Gingerich [80]. 

Knudsen effusion will be involved later as we discuss free molecular flow in channels and tubes. Knudsen effusion also finds application in the measurement of the vapor pressure of materials of low vapor pressure, typically in the... 

Ideally, the Knudsen effusion method can be applied for determination of vapor pressures if molecular flow is verified (p < 10 3 torr). Precise measurement of vapor... 

Molecular weight from vapor pressures Molecular weight from P-V-T measurements Double oven effusion with mass spectrometer Knudsen effusion with mass spectrometer Knudsen effusion with mass spectrometer Velocity distribution analysis Velocity distribution analysis... 

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. 

Zaitsau et al. [44] measured the vapor pressure of a series of [C mim NTf2] ionic liquids using the integral effusion Knudsen method and correlated the A H with the molar volumes and the surface tensions of the compounds. What is clear from this study is that the values for A H are approximately half that used in Rebelo s initial estimate and further indicated that the Eotvos-Guggenheim correlations which are suitable for molecular solvents do not apply for ionic liquids. 

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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