Pressure, partial, determination

In this section, we describe time-resolved, local in-situ measurements of chemical potentials /, ( , f) with solid galvanic cells. It seems as if the possibilities of this method have not yet been fully exploited. We note that the spatial resolution of the determination of composition is by far better than that of the chemical potential. The high spatial resolution is achieved by electron microbeam analysis, analytical transmission electron microscopy, and tunneling electron microscopy. Little progress, however, has been made in improving the spatial resolution of the determination of chemical potentials. The conventional application of solid galvanic cells in kinetics is completely analogous to the time-dependent (partial) pressure determination as explained in Section 16.2.2. Spatially resolved measurements are not possible in this way. 

Partial Pressure Determination. For the KMS method, conversion of mass spectral ion intensities to species partial pressures is made through the basic relationships,... 

Important contributions to the area of defect chemistry of the acceptor-doped Lnj.xAxBOs, perovskites, where B is selected from Cr, Mn, Fe or Co, have been made by a number of investigators. Particular reference is made to reviews provided by Anderson [159,160] and Mizusaki [161]. The substitution of divalent alkaline-earth ions on the A-site increases the concentration of oxygen vacancies. Temperature and oxygen partial pressure determine whether charge compensation occurs by an increased valency of the transition metal ion at the B-site or by the formation of ionized oxygen vacancies. Thermogravimetric studies have indicated that in, for example, LaCrOs, YCrOs and LaMnOs the... 

If an ion has two different neutral precursors, it is necessary for the partial pressure determination to know the contribution of each precursor to its intensity. The technique used for example to determine the contributions to the Na ion intensity generated by electron impact ionization from the precursors Nal(g) and NaA(g) (A = Snij, Pblj, Felj, Dyl4 or SCI4) are described by Hilpert and coworkers [61-64]. The use of a two compartment Knudsen cell can be very useful in connection with this technique [62, 64]. Other techniques used for the quantitative determination of the aforementioned contributions are described by Pittermann and Weil [65] as well as Grimley and Forsman [43]. Techniques for the assignment of ions, of the mass spectra of binary salt systems with a common cation are described by Kapala and Skudlarski [66, 67] and Miller [68],... 

The UO2 partial pressures determined over the different solutions at 1400 K are directly proportional to the U3O8 concentration in the condensed phase. 

How are partial pressures determined A manometer can measure only the total pressure of a gaseous mixture. To obtain the partial pressures, we need to know the mole fractions of the components, which would involve elaborate chemical analyses. The most direct method of measuring partial pressures is using a mass spectrometer. The relative intensities of the peaks in a mass spectrum are directly proportional to the amounts, and hence to the mole fractions, of the gases present. 

Under equilibrium conditions the electrical conductivity of many oxide phases, e.g., CugO, FeO, CoO, NiO, or ZnO at elevated temperatures is a function of the oxygen partial pressure in the ambient gas phase 12). The oxygen partial pressure determines the metal excess or deficit in the metal oxide and thereby the concentration of electrical carriers especially excess electrons and electron holes. Thus, after proper calibration, the steady-state oxygen activity ao(st) may be deduced from measurements of the conductance of a metal oxide foil used as catalyst while an oxygen transfer reaction, e.g., CO2 + H2 = CO -)- HjO or 2N2O = 2N2 + O2 proceeds at the surface of the metal oxide 13). 

A heat treatment in a gas with the proper partial pressure determines the stoichiometry of the solid and the concentrations of the defects involved in gas exchange. 

Subtract the vapor pressure from the total P of gas mixture to give the partial pressure of needed gas. Using Dalton s law of partial pressures, determine the partial pressure of H2. 

Limbaugh CC (1985) The infrared emission-absorption method for temperature and species partial pressure determination in flames and plumes. In Wormhoudt... 

When Che diameter of the Cube is small compared with molecular mean free path lengths in che gas mixture at Che pressure and temperature of interest, molecule-wall collisions are much more frequent Chan molecule-molecule collisions, and the partial pressure gradient of each species is entirely determined by momentum transfer to Che wall by mechanism (i). As shown by Knudsen [3] it is not difficult to estimate the rate of momentum transfer in this case, and hence deduce the flux relations. 

Ac Che limic of Knudsen screaming Che flux relacions (5.25) determine Che fluxes explicitly in terms of partial pressure gradients, but the general flux relacions (5.4) are implicic in Che fluxes and cheir solution does not have an algebraically simple explicit form for an arbitrary number of components. It is therefore important to identify the few cases in which reasonably compact explicit solutions can be obtained. For a binary mixture, simultaneous solution of the two flux equations (5.4) is straightforward, and the result is important because most experimental work on flow and diffusion in porous media has been confined to pure substances or binary mixtures. The flux vectors are found to be given by... 

The value has been extrapolated because, at temperatures above 170°C, the rate of reaction 2 rapidly iucreases and it is difficult to determine the carbamate vapor pressure owiag to the formation of water and urea and the consequent lowering of the partial pressure of ammonium carbamate. 

Low temperatures strongly favor the formation of nitrogen dioxide. Below 150°C equiUbrium is almost totally in favor of NO2 formation. This is a slow reaction, but the rate constant for NO2 formation rapidly increases with reductions in temperature. Process temperatures are typically low enough to neglect the reverse reaction and determine changes in NO partial pressure by the rate expression (40—42) (eq. 13). The rate of reaction, and therefore the... 

The commercially important normal to branched aldehyde isomer ratio is critically dependent on CO partial pressure which, in propylene hydroformylation, determines the rate of interconversion of the -butyryl and isobutyryl cobalt tetracarbonyl intermediates (11). 

These temperatures, partial pressures, and dimensions were used in Example 6 to determine gas emissivity and absorptivity, .q = 0.145 OCci = 0.210. The approximate effective emissivity of the boundary is (0.8 -I-1)72 = 0.9. Then from Eq. (5-155), modified to allow for sink emissivity and for the presence of a small amount of refractory boundary,... 

Relative humidity and dew point can be determined for other than atmospheric pressure from the partial pressure of water in the mixture and from the vapor pressure of water vapor. The partial pressure of water is calculated, if ideal-gas behavior is assumed, as... 

Although quite useful when it can be apphed, this law should be checked experimentally to determine the accuracy with which it can be used. If Heniy s law holds, the solubility is defined by stating the value of the constant H (or H ) along with the temperature and the solute partial pressure for which it is to be employea. 

Membrane System Design Features For the rate process of permeation to occur, there must be a driving force. For gas separations, that force is partial pressure (or fugacity). Since the ratio of the component fluxes determines the separation, the partial pressure of each component at each point is important. There are three ways of driving the process Either high partial pressure on the feed side (achieved by high total pressure), or low partial pressure on the permeate side, which may be achieved either by vacuum or by introduc-... 

A signihcant problem in tire combination of solid electrolytes with oxide electrodes arises from the difference in thermal expansion coefficients of the materials, leading to rupture of tire electrode/electrolyte interface when the fuel cell is, inevitably, subject to temperature cycles. Insufficient experimental data are available for most of tire elecuolytes and the perovskites as a function of temperature and oxygen partial pressure, which determines the stoichiometty of the perovskites, to make a quantitative assessment at the present time, and mostly decisions must be made from direct experiment. However, Steele (loc. cit.) observes that tire electrode Lao.eSro.rCoo.aFeo.sOs-j functions well in combination widr a ceria-gadolinia electrolyte since botlr have closely similar thermal expansion coefficients. 

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