Effusion rates, vapor pressure

Figure 6.15 Specific evaporation rate (vapor pressure) versus pore fiiiing degree of supported [EMIMJINTfj] (141 °C, HV, Ni-catalyst, crushed to about 200pm) and comparison with simulations by Model V effusion from face surface cf sample is r.d.s. but...

The vapor pressure of a molten metal can be measured with a device called a Knudsen cell. This is a container closed across the top by a thin foil pierced by a small, measured hole. The cell is heated in a vacuum, until the vapor above the melt streams from the small hole (it effuses). The weight of the material escaping per second tells the rate at which gaseous atoms leave. 

Vapor pressures were determined by using the Knudsen effusion technique. Effusion rates through and orifice contained in each sample cell were measured as a function of temperature by use of a mass spectrometer/target collection... 

To determine the effect of orifice size on vapor pressure, five different orifices were used to measure the effusion rates at 25 °C. of one compound, 2-chlorcMt-aminopyrimidine. Carson, Cooper, and Stranks (6) have shown that the net rate of effusion is directly proportional to the flux (rate of effusion per unit area) if other factors are constant. That is, a graph of... 

Knudsen121 Effusion Gauge. To measure the vapor pressure of solids or liquids indirectly, a Knudsen cell is a cylindrical cell containing the sample. A small opening at the top of the cell allows molecules to evaporate at a fixed rate, proportional to the vapor pressure inside the cell. The mass loss of the cell is proportional to the pressure and is measured after a fixed time, for several temperatures. 

For very low vapor pressures (below about 10 Pa), the vapor pressure can be measured by the Knudsen method, in which the rate of escape of vapor through a small hole is measured. The kinetic theory of gases (with correction factors for hole geometry, etc.) is used to relate the rate of effusion to the vapor pressure. The attainable accuracy of such methods is around 1% to 10% at best, but it is often the only experimental option for substances of very low volatility. 

If even at the lowest heating rate only decomposition occurs (no change of TG signal if different carrier gases are used), the vapor pressure caimot be determined by ambient pressure TGA, and complimentary HV effusion... 

From this discussion, the effect of orifice shape on the effusate distribution needs to be removed before the relationship between vapor pressure and sample composition and temperature can be studied accurately. The calibration of orifices is easily achieved with a multiplecell configuration by putting the same material in different cells and comparing the rate or ion production at the one temperature. Hiis calibration procedure is referred to as a GFR measurement ... 

Estimate the rate at which Hg effuses out a hole of area 0.10 mm at 295 K.The vapor pressure of Hg at this temperature is 0.0014 mmHg. 

Knudsen effusion cells are used to determine vapor pressures of high-temperature materials. For example, a Knudsen cell is filled with tungsten and heated to 4500 K in a vacuum. Measurements show that the cell loses mass— assumed to be W vapor—at the rate of 2.113 grams per hour out of a hole that is 1.00 mm in area. Calculate the vapor pressure of W at 4500 K. 

The heat of vaporization depends on the measurement of vapour pressure. The low volatility of the metals and their salts and oxides restricts us to two methods of measurement, the Knudsen method and the rate of evaporation method. The principle of the Knudsen method, which depends on the rate of effusion of the vapour from a small orifice in a heated cell containing the material, has been discussed in Section 3.3. The rate of evaporation method, due to Langmuir 293 depends upon the assumption that there is no activation energy opposing condensation on this view the rate of... 

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). 

The apparent inapplicability of the method to measurements (by the Arrhenius approach) of rate constants k, which at first glance are not related to the absolute value of equilibrium pressure of the primary product Pgqp-However, none of the above reasons can account for the fact that this method is ignored in measurements of the vaporization rate from a free surface (after Langmuir), while it is employed widely in effusion studies (after Knudsen). 

Misconception and its Interpretation The low magnitudes of the vaporization coefficients for many substances (effusion method, which is used for estimations of the maximum decomposition rate or of the equilibrium pressure of the product. The ratio of the equilibrium pressure inside the cell, P q, to the effusion pressure, P g, is governed by [3] ... 

Arsenic(III) sulfide sublimes readily, even below its melting point of 320 "C. The molecules of the vapor phase are found to effuse through a tiny hole at 0.28 times the rate of effusion of Ar atoms under the same conditions of temperature and pressure. What is the molecular formula of arsenic(III) sulfide in the gas phase ... 

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