Equilibrium-Based Selectivity

There are two general classes of selectivity equilibrium-based and kinetically based. Both types depend on specific interactions of the analyte molecule with the selective layer. In addition, there is also physical selectivity based on the highly specific interaction of the molecule with the electrostatic and electromagnetic fields. The topic of selectivity is broad enough to deserve its own chapter. We examine issues pertaining to selectivity in Chapter 2. 

Because there is no obvious reason for selecting equilibrium constants from runs in which the initial solution contained either solid KCI or solid KBr (runs A or B), the calculations that follow are based on average solid compositions and average equilibrium constants of A and B runs for each total composition reported (Table IV). The equilibrium constants for Equation 5 are shown as a function of KBr mole fraction in Figure 1. 

The foundation of equilibrium-selective adsorption is based on differences in the equilibrium selectivity of the various adsorbates with the adsorbent While all the adsorbates have access to the adsorbent sites, the specific adsorbate is selectively adsorbed based on differences in the adsorbate-adsorbent interaction. This in turn results in higher adsorbent selectivity for one component than the others. One important parameter that affects the equilibrium-selective adsorption mechanism is the interaction between the acidic sites of the zeolite and basic sites of the adsorbate. Specific physical properties of zeolites, such as framework structure, choice of exchanged metal cations, Si02/Al203 ratio and water content can be... 

Although most of the commercial adsorptive separation processes are operated under the selective-equilibrium adsorption mechanism, adsorptive separation may also be based on diffusion rates through a permeable barrier which are designated as rate-selective adsorption processes. In some instances there may be a combination of equilibriums as well as rate selective adsorption. A rate-selective adsorption process yields good separation when the diffusion rates of the feed components through the permeable barrier differ by a wide margin. 

The reaction to form the palladium complex is similar to that reported for amine salts, although here, because a bidentate chelating ligand is used, no chlorine atoms are retained in the complex, and the system is easy to strip. Also, as both reactions involve initial ion pair extraction, fast kinetics are observed with 3-5 min contact time to reach equilibrium at ambient temperature. The extraction conditions can be easily adjusted in terms of acidity to suit any relative metal concentrations and, because the reagent is used in the protonated form, good selectivity over base metals, such as iron and copper,... 

Selectivity. Selectivity in a physical adsorption system may depend on differences in either equilibrium or kinetics, but the great majority of adsorption separation processes depend on equilibrium-based selectivity. Significant kinetic selectivity is. in general, restricted to molecular sieve adsorbents—carbon molecular sieves, zeolites, or zeolite analogues. 

For an equilibrium-based separation process a convenient measure of the intrinsic selectivity of the adsorbent is the separation factor an, defined by analogy with relative volatility as ... 

WATEQ2 consists of a main program and 12 subroutines and is patterned similarly to WATEQF ( ). WATEQ2 (the main program) uses input data to set the bounds of all major arrays and calls most of the other procedures. INTABLE reads the thermodynamic data base and prints the thermodynamic data and other pertinent information, such as analytical expressions for effect of temperature on selected equilibrium constants. PREP reads the analytical data, converts concentrations to the required units, calculates temperature-dependent coefficients for the Debye-HKckel equation, and tests for charge balance of the input data. SET initializes values of individual species for the iterative mass action-mass balance calculations, and calculates the equilibrium constants as a function of the input temperature. MAJ EL calculates the activity coefficients and, on the first iteration only, does a partial speciation of the major anions, and performs mass action-mass balance calculations on Li, Cs, Rb, Ba, Sr and the major cations. TR EL performs these calculations on the minor cations, Mn, Cu, Zn, Cd, Pb, Ni, Ag, and As. SUMS performs the anion mass... 

The texture properties of the ultrathin porous glass membranes prepared in our laboratory were initially characterized by the equilibrium based methods nitrogen gas adsorption and mercury porosimetry. The nitrogen sorption isotherms of two membranes are shown in Fig. 1. The fully reversible isotherm of the membrane in Fig. 1 (A) can be classified as a type I isotherm according to the lUPAC nomenclature which is characteristic for microporous materials. The membrane in Fig. 1 (B) shows a typical type IV isotherm shape with hysteresis of type FIl (lUPAC classification). This indicates the presence of fairly uniform mesopores. The texture characteristics of selected porous glass membranes are summarized in Tab. 1. The variable texture demanded the application of various characterization techniques and methods of evaluation. 

Relationships between three extraction parameters—solvent composition, temperature, and particle size—can be determined from simple beaker tests. Raw material ground to the desired particle size is contacted with the chosen solvent at the chosen temperature in a beaker and mixed over time. Thief samples are removed at various time intervals and analyzed for the marker. At the completion of the test, the marc is separated from the extract, dried, and analyzed. (In instances when the phytochemical is heat sensitive, the marc can be analyzed wet and the assay calculated on a dry basis). Based on this information, the amount of marker remaining in the dry marc can be determined after correcting for the marker in the absorbed extract in the wet marc. From these data, the equilibrium relationship defined in Equation 11.1 can be determined, as well as the time it takes to reach maximum extraction. Conditions are changed to give a high equilibrium constant and a short extraction time to reach equilibrium. Based on this information, one can select the appropriate extraction equipment as well as the operating conditions to operate the equipment. 

Here Xa, Ya are strictly equilibrium mole fractions for component A in the adsorbed phase and adsorbate (fluid) phase, respectively as are Xb, Fb for component B. For equilibrium-based adsorptive separation process, the adsorbent selectivity is the same as the separation factor as defined in Eq. (1). Apparently, this definition is not applicable to other processes based on kinetic and steric effects. In a kinetically controlled adsorption process, the adsorbent selectivity depends on both equilibrium and kinetic effects. A simplified definition for adsorbent separation factor is given by Ruthven et al. ... 

In contrast to equilibrium-based sensing such as described above, it is also possible to use the zeolite film as a membrane controlling molecular access to an appropriate transduction mechanism. In this case, Pd-doped semiconductor gas sensors were used as a fairly non-selective sensor platform. After coating these sensors with a thin film of MFI-type or LTA-type zeolites, they were examined with respect to gas phase sensing of different analytes such as methane, propane and ethanol, at different humidity levels (Fig. 14).[121] The response of a zeolite-coated sensor towards the paraffins was strongly reduced compared to the non-coatcd sensor device, thus resulting in an increase of the sensor selectivity towards ethanol. 

Table IX-6 Selected equilibrium constants of different thorium-sulphate reactions based on SIT model at 25°C unless otherwise identified. The uncertainties may be somewhat larger than those listed as a result of the assumption for some species that the values of A.G° /RT at 16 and 30°C are the same as those at 25°C.

The authors review the solubility and hydrolysis of tetravalent actinides using experimental data and semi-empirical estimates among the series of tetravalent actinides Th(lV), U(1V), Np(lV) and Pu(lV). They present a set of selected equilibrium constants that are based preferentially on experimental data using trace concentration of the actinides. The rationale for this is based on experience of studies of colloid formation using laser-induced breakdown detechon [1999KNO/NEC], [2000BUN/KNO],... 

Kinetic methods can be more selective than equilibrium methods if rcagenis and conditions are chosen to maximize differences in the rales at which the analyte and potential inierferenis react. In equilibrium-based methods, selectivity is realized by maximizing differences in equilibrium constants. 

A buffered solution is one in which changes in pH by the addition of H or OH are resisted. A buffer system consists of a conjugate acid-base pair whose selection is based on the requirement of what value of pH is desired to be maintained constant. Our analysis begins with a restatement of the ionization constant of a weak acid which we will denote as HA. The ionization process is HA =i H -l-A , where A is the conjugate base of the acid HA, and the equilibrium constant expression is... 

Equation (9) illustrates that in comparison to the selectivity of a simple liquid-vapor equilibrium based on the ratio of the respective volatilities, here expressed in terms of the Henry coefficients of solutes i andj, respectively, the membrane introduces a further selectivity given by the ratio of the permeabilities of the respective compounds i andj (assuming all the above assumptions to be valid). In other words, whilst the driving force for solute transport is identical in processes based on the vapor-liquid equilibrium and in vapor permeation or pervaporation, the latter can exceed the vapor-liquid equilibrium selectivity whenever... 

No generally meaningful equilibrium constants can be derived for such materials for inclusion in this compilation. However, some of the studies with these materials are sufficiently impor-tanti that they are made available to the interested reader in Appendix 1 Their selection is based on judgement by the compilers, and is neither complete nor wholly unbiased. 

A selected equilibrium transition temperature of 90.5 K is based on the value of 90.54 K obtained by Pecharsky et al. (1996) which was confirmed by values of 90.7 K from the measurements of Jayasuriya et al. (1985b) and 90.4 K from the measurements of Astrbm and Benediktsson (1988). The values given in Table 129 have been obtained for the enthalpy of transition. 

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