Constant stoichiometry

For example in the Fe-Cr-B system (Fig. 5.5) Cr2B extends across the diagram at a constant stoichiometry of boron as (Cr,Fe)2B (Villars and Calvert 1985). Also shown is the corresponding extension of the tetragonal Fe2B compound. Such phases are commonly called Tine compounds . Generally, sublattice occupation can be described as... 

In Eq. 1.3, i A = -1 for any A and uB = +1 for any B. Since Eq. 1.3 is an overall reaction, the assumption of constant stoichiometry underlying the definition of is not trivial, as discussed in Section 1.1. For example, at high pH, Eq. 1.28 would not always be applicable because of the influence of the reactions in Eqs. 1.1 and 1.5. On the other hand, at equilibrium, when the hydration reaction is described by Eq. 1.10, the application of Eq. 1.28 is possible. This fact serves to emphasize the difference between equilibrium chemical species that figure in thermodynamic parameters (e.g., Eq. 1.11) and kinetic species that figure in the mechanism of a reaction. The set of kinetic species is in general larger than the set of equilibrium species for any overall chemical reaction. 

Example, The decomposition of di-tertiary butyl peroxide (dtBP) has been found to follow this constant stoichiometry in the gas phase. ... 

Variance in the Conversion Percentage. Considerable variance in the fraction of nitrogen converted was observed among repetitions of runs with constant stoichiometry, net power input, and pressure. The data points plotted in Figure 4 include the maximum observed conversion levels for each stoichiometry. Sources of error in the experimental procedure which could have scattered the data down from maximum conversion were ... 

We also showed by pulse radiolysis that C03 accelerates the decay of Br2 (formed from OH f Br ), but further work is required to characterize the rate constant, stoichiometry, and product spectrum of the Br2 + 003 interaction (R2). 

In many situations, the PEFC is run at a constant inlet flow rate of oxygen rather than at a constant stoichiometry. In that case, it is convenient to rewrite Eq. (6.37) as... 

In Section 4.2.1 we will show that at constant stoichiometry A the local current density in the cathode channel of a PEFC varies as ... 

In the regime with constant stoichiometry, the variation of flow velocity along the channel is independent of the mean current in the cell. The function (4.21) is illustrated in Figure 4.1 for several values of parameter a. ... 

At constant stoichiometry and variable flow velocity, the oxygen mass conservation equation reads... 

The generic structure is indicative of random incorporation of dienyl iron tricarbonyl moieties, corresponding to 1,3 octadiene-, 2,4 octadiene- and 3 5 octadiene-iron tricarbonyl. We have found that a derivative of constant stoichiometry is obtained ia the thermal decomposition of an excess of Fe(C0)5 dilute xylene solution of c/5-poly(butadiene). The polymer is a bright yellow, air-stable, solid which was conveniently isolated from benzene by lyophilization (see Experimental). The purification procedure detailed in the Experimental Section is a key factor in the stability of the product. 

The case is thus made. C/5-polybutadiene reacts with an excess of Fe(C0)5 to yield poly(1,3-octadienyliron tricarbonyl) a derivative of constant stoichiometry which can only be described as an alternating copolymer incorporating i -butadienyliron tricarbonyl moieties bearing trans/trans and trans/cis tetramethylene moieties. 

Single cells with active areas of 5 to 50 cm are often used to evaluate the performance and durability of MEAs. The result represents the average performance of the entire cell. For cells with active areas of around 5 cm, the flow rates of the reactants are very low at a given stoichiometry, and may not be easy to control accurately. Very often, a constant flow rate of a few times stoichiometry corresponding to the highest current density is used. In such cases, mass transport resistance is made artificially low and does not represent the situation in which a constant stoichiometry is used for the entire current density region. 

FIGURE 10.35 Temperature and RH for constant stoichiometries (2, 4, 8). (Reprinted from /. Power Sources, 184, L. A. M. Riascos. Relative humidity control in polymer electrolyte membrane fuel cells without extra humidification, 204-211, Copyright (2008), with permission from Elsevier.)... 

Constant Stoichiometry versus Constant Oxygen Flow... 

Fig. 18 Fuel cell performance for the p-PBI membranes from the sol-gel process. Polarization curves of fuel cells under H2/air (squares) and H2/O2 (circles)) without any feed gas humidification. The membrane PA doping level was approximately 32 mol PA/PRU. The catalyst loading in both electrodes was l.Omgcm" Pt, and the cell was operated at 160 °C at constant stoichiometry of 1.2 stoic and 2.5 stoic at the anode and the cathode, respectively...

The BIAcore technology can be used to study binding constants, stoichiometry, and thermodynamics (discussed in detail in [5]) of protein-DNA interactions however, experimental design has to be considered to produce meaningful results. 

This convenient reduction occurs for constant-stoichiometry operation only, since the flow rate and generation are both linearly proportional to current and thus cancel each other out. In a general transient case, there would also be a mass storage/depletion term in Eq. (6.17). 

Chapter 7 is devoted to important physicochemical —basically mechanistic — aspects of the direct enantioseparations, carried out by using either CSP or mobile phase. In such cases, the diversity of the involved separation mechanisms is much greater than the most of other chromatographic modes (and, particularly, when compared with the relatively simple physicochemical rules governing adsorption or partition liquid chromatography). Thus, the author of this chapter discusses enantioseparation in terms of the solute-chiral selector complexation constants, stoichiometry and selectivity of complexation, the nature of the binding sites on the stationary phase surface, and, finally, the supramolecular mechanisms of complexation. 

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