Knudsen mass spectrometric method

Do of 1.40 eV for AI2 is within the error bounds of the experimental value of 1.55 0.15 eV determined by Stearns and Kohl (46) using a Knudsen cell mass spectrometric method and assuming a ground state. 

The Knudsen effusion mass spectrometric method will be briefly described and Illustrated. The influence of various assumptions about the geometry and electronic structure of the molecules on the third law values of the reaction enthalpies obtained will be discussed. The development and use of empirical models for calculating dissociation and atomization energies will be Illustrated and their value as a bridge between experimental values and semi-empirical and theoretical calculations demonstrated. 

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. 

We have developed several new measurement techniques ideally suited to such conditions. The first of these techniques is a High Pressure Sampling Mass Spectrometric method for the spatial and temporal analysis of flames containing inorganic additives (6, 7). The second method, known as Transpiration Mass Spectrometry (TMS) (8), allows for the analysis of bulk heterogeneous systems over a wide range of temperature, pressure and controlled gas composition. In addition, the now classical technique of Knudsen Effusion Mass Spectrometry (KMS) has been modified to allow external control of ambient gases in the reaction cell (9). Supplementary to these methods are the application, in our laboratory, of classical and novel optical spectroscopic methods for in situ measurement of temperature, flow and certain simple species concentration profiles (7). In combination, these measurement tools allow for a detailed fundamental examination of the vaporization and transport mechanisms of coal mineral components in a coal conversion or combustion environment. 

Plante, E. R. "Vapor Pressure Measurements of Potassium Over K20-Si02 Solutions by a Knudsen Effusion Mass Spectrometric Method", 1979 p. 265, ibid, (8). 

Vaporization rates based on weight loss at T were repoted by Noguchi (8) for MgO and six other refractory oxides. Vaporization of MgO near 2000 K has been studied by Langmuir-torsion (IJ ), Langmuir-weight-loss (, 1 ), Knudsen-mass-spectrometric (14), Knudsen-effusion (15) and transpiration (1, 16) methods. Vaporization near 2000 K is primarily to the elements (14). Hg(g) and presumably 0(g) and 02(g), rather than to MgO(g). Quantitative interpretation of the vaporization species is uncertain. The adopted tables (1 ) predict that MgO(g) is insignificant near 2000 K but should become one of the significant components of the vapor near... 

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. 

The primary experimental methods used in this study are the Knudsen Effusion Mass Spectrometric (KMS) and Transpiration Mass Spectrometric (TMS) methods, as described elsewhere [KMS, (9) ... 

Porter and Schoonmaker ( ) analyzed the vapor effusing from a Knudsen cell at 1121 K mass spectrometrically and derived the partial pressures of NaF(g) and NagF Cg) at that temperature. Using these reported partial pressures we calculate, by the 3rd law method, the value Aj,H (298.15 K) = 64.1 kcal mol for reaction (A). 

Coppens et al. (4) used a combination of mass spectrometric and Knudsen cell techniques to determine the partial pressures necessary to define the equilibrium in the system V0(g) + Ge(g) = GeO(g) + V(g). The authors analyzed eight data points (1927-2145 K) by the 3rd law method and reported A H°(0 K) = 7.60 0.38 kcal mol" which actually applies to the reverse reaction (14). 

The vapor pressure over vanadium was measured by Edwards et al. (j ) using the Knudsen method and by Farber and Snvastava (6) using effusion-mass spectrometric techniques. Our 2nd and 3rd law analysis is tabulated below, where reaction (A) refers to the sublimation V(cr) = V(g) and reaction (B), the vaporization V(t) = V(g). 

This is a thorough study of the evaporation of a-MnSe in the temperature range 1225 to 1780 K using the mass-spectrometric Knudsen-cell method. The predominant vaporisation process is... 

UY/DRO] Uy, O. M., Drowart, J., Determination by the mass spectrometric Knudsen cell method of the atomization energies of the gaseous aluminium chalcogenides, AI2, AlCu, AlCuS and AICUS2, Trans. Faraday Soc., 67, (1971), 1293-1301. Cited on pages 228, 229, 230, 532. 

SMO/DRO] Smoes, S., Drowart, J., Determination of the dissociation energy of selenium monoxide by the mass-spectrometric Knudsen-cell method, J. Chem. Soc. Faraday Trans., 80, (1984), 1171-1180. Cited on pages 118, 119, 562, 563, 564. 

The standard free energy of formation (kcalmoF ) of liquid lithium hydride is given by (13.47 x lO T -16.55). The thermodynamic properties of the Li-LiH system have been determined by a mass spectrometric Knudsen effusion method in the plateau region from 973 to 1146 K. The enthalpy, AHitjs, of the reaction... 

We have studied the interactions of HNO3, HCl and HBr with liquid sulfuric acid surfaces using both time dependent uptake and equilibrium vapor pressure methods in a Knudsen cell reactor equipped with mass spectrometric detection. Measured solubilities will be presented as Henry s law coefficients along with thermochemical parameters. 

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