Knudsen cells temperature measurement

The enthalpy of formation of See(g) has been determined from mass spectro-metric investigations of selenium vapour using Knudsen cells. The measurements can be separated into the approximate temperature ranges 420 to 494 K for equilibria with... 

The enthalpy of formation of Seg(g) has been determined from mass spectro-metric investigations of selenium vapour using Knudsen cells. The measurements can be separated into the approximate temperature ranges 420 to 494 K for equilibria with solid selenium and 494 to 700 K for equilibria with liquid selenium. The various investigations have used different methods and auxiliary data for deriving enthalpies of formation at 298.15 K from the measurements. For the purpose of this review the measurements were recalculated (see Appendix A) using the adopted values for heat capacities and entropies of the gaseous species and condensed phases. The values of the enthalpies of formation of Seg(g) are summarised in Table V-21. 

Ion intensities and Knudsen cell temperatures are the quantities measured in the course of an investigation by the Knudsen effusion mass spectrometric method. Partial pressures are computed from these quantities for the vapor species identified. 

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 investigate high-temperature equilibria, the gaseous species are identified from their parent ions and the relative intensities of ions as a function of temperature help to define the reactions proceeding in the Knudsen cell. The ion current-electron-accelerating voltage curves determine the appearance potential at which the ion is first observed, and intensities are measured at 1-3 eV above this value to prevent ion fragmentation. 

Knudsen cells (effusion cells) are exclusively used for vapor pressure measurements (see vapor pressure) in the pressure range from 1 torr to 10-6 torr. In the low temperature range (—20° — +400 °C) pyrex glass cells are applicable. Especially the vapor pressures of dyes, organic compounds can be measured in such cells, because metal cells may sometimes cause catalytic decompositions of the investigated materials. 

Another high-temperature cell (Fig. 8 g, up to 2400 °C, can be produced from tungsten. Tungsten Knudsen cells are used primarily for high-temperature vapor pressure measurements, e.g. for metal oxides. They are suitable also for metals when graphite linings are applied to the inner surface. The vapor pressure can be determined... 

An important requirement for such vapor pressure measurements with Knudsen cells is the temperature distribution in and around the cell. If the temperature is not homogeneous within the cell, condensation and crystallization of the vaporized species may occur in the colder regions. This was observed e.g. with gold. In other cases the orifice was reduced in diameter up to complete blockage when vaporized metals like aluminium where oxidized due to the partial pressure of oxygen. To prevent these deposits, ultra high vacuum is necessary or the use of graphite cells instead of alumina cells. 

The enthalpies of phase transition, such as fusion (Aa,s/f), vaporization (AvapH), sublimation (Asut,//), and solution (As n//), are usually regarded as thermophysical properties, because they referto processes where no intramolecular bonds are cleaved or formed. As such, a detailed discussion of the experimental methods (or the estimation procedures) to determine them is outside the scope of the present book. Nevertheless, some of the techniques addressed in part II can be used for that purpose. For instance, differential scanning calorimetry is often applied to measure A us// and, less frequently, AmpH and AsubH. Many of the reported Asu, // data have been determined with Calvet microcalorimeters (see chapter 9) and from vapor pressure against temperature data obtained with Knudsen cells [35-38]. Reaction-solution calorimetry is the main source of AsinH values. All these auxiliary values are very important because they are frequently required to calculate gas-phase reaction enthalpies and to derive information on the strengths of chemical bonds (see chapter 5)—one of the main goals of molecular energetics. It is thus appropriate to make a brief review of the subject in this introduction. 

Low cost is one of the main advantages of the vapor pressure methods, as compared with calorimetric techniques. An apparatus to measure the vapor pressures of low boiling temperature liquids can be built easily in an undergraduate chemistry laboratory. However, the same is not quite true if we want to measure the vapor pressures of low-volatility substances, such as most solids. In these cases, Knudsen cells are usually the method of choice, but they require more expensive high-vacuum equipment [36]. 

In the Knudsen effusion method a substance is enclosed in a sealed container into which a very small hole is drilled. This hole must be knife-edged and the mean free path of the vapour must be 10 times the diameter of the hole. In its simplest form an experiment proceeds as follows. The Knudsen cell, with sample in it, is carefully weighed and then heated in a vacuum at the requisite temperature for a set time. The cell is then re-weighed and the weight loss is measured. However, it is now more usual to continuously measure the weight of the cell. If the molecular weight and surface area of the sample is known the vapour pressure can be found. 

A variety of experimental techniques have been used for the determination of uptake coefficients and especially Knudsen cells and flow tubes have found most application [42]. Knudsen cells are low-pressure reactors in which the rate of interaction with the surface (solid or liquid) is measured relative to the escape through an aperture, which can readily be calibrated, thus putting the gas-surface rate measurement on an absolute basis. Usually, a mass spectrometer detection system monitors the disappearance of reactant species, as well as the appearance of gas-phase products. The timescale of Knudsen cell experiments ranges from a few seconds to h lindens of seconds. A description of Knudsen cell applied to low temperature studies is given [66,67]. 

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. 

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. 

The Knudsen cell mass spectrometric method is well established and has been described in many reviews, as can be seen in references (6-10). It is an important method for equilibrium vapor studies of high temperature systems for temperatures up to approximately 3000 K. There is no other method presently available that permits the measurement of bond energies of minor molecular vapor components at such high tein)eratures. 

The vapour pressure of Ga2Se3(cr) was measured in the temperature range 970 to 1180 K using mass spectrometry and Knudsen cells. It was found that the vaporisation reaction is... 

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