Adsorption general procedure

A powerful technique in studying both adsorption and desorption rates is that of programmed desorption. The general procedure (see Refs. 36, 84) is to expose a clean metal filament or a surface to a known, low pressure of gas that flows steadily over it. The pressure may be quite low, for example, 10 mm Hg or less, so that even nonactivated adsorption can take some minutes for... 

M. J. M. Wells, General procedures for the development of adsorption trapping methods used in herbicide residue analysis, in Proceedings of the 2nd Annual International Symposium on Sample Preparation and Isolation Using Bonded Silicas, Analytichem International, Harbor City, CA, 1985, pp. 63-68. 

There is a multitude of choices for shakers (reciprocal or not) and centrifuges with annexed spares, with several variations to the above described general procedure. Widely different soiksolution ratios (R, kg litre-1) have been used to investigate the adsorption and desorption behaviour of trace-metal ions by soils, from 1 2 up to 1 200 soiksolution dilutions (Table 10.3). The background electrolyte solution and... 

Use of the Langmuir isotherm has made this computational scheme appear quite simple, because direct solution for P° (step 2) is possible. However, most equations for the adsorption isotherm are less tractable, and this calculation must be done numerically. This significantly increases the computational task, but does not alter the general procedure. 

Williams et al. [62] also studied the copolymerization of D- and L-enantiomorphs in dioxan solution and obtained transition points similar to those shown for the L-NCA in Fig. 8. However, in these systems the transition occurred at lower P (Fig. 9), a result which is contrary to that expected if a coil-helix transition were responsible for the change in rate. Kinetic analysis was carried out in terms of simultaneous polymerization in both phases following prior adsorption of monomer. (The general procedure was similar to that of Biihrer and Elias [63] (p. 615 et seq.) and was used by them). It was found that only in the associated phase is selectivity essentially complete with no cross-over reactions. The analysis leads to an estimate of Kl d / l l = 1-6 0.6, in reasonable agreement with the value determined directly (p. 615). 

Gas adsorption. The general procedures for the determination and interpretation of adsorption and desorption isotherms have been reviewed in earlier volumes of this series (129,276). An adsorption-desorption isotherm permits estimation of the surface area, pore volume, average pore diameter, and approximate pore-size distribution. 

The specific component of free energy of adsorption is generally determined by the subtraction the dispersive component from the total free energy of adsorption. Several procedures of the calculation of the specific component have been presented in the literature. The differences between the respective procedures lie in the choose of the reference state of the adsorbed molecule. 

Different types of Amberlite XAD resins with unique physical and chemical properties have been available. Their use for adsorption of polar organic molecules directly out of biological samples has been demonstrated with pharmaceuticals [263], plant nucleotides [264], plant growth hormones [265], and various steroids [262,268]. In the last case, ample evidence is now available that the use of these resins causes substantially better recoveries of more polar steroid metabolites [262,268] than the previously employed solvent extractions. Setchell et al. [262] used the organic resins and modified dextranes for a complete fractionation of urinary steroid conjugates. Their general procedure (Fig. 3.15) involves the initial sample adsorption and several... 

The previous section gives an overview of the experiments necessary to determine the model parameters. This section describes general procedures for evaluating parameter values from experimental data. Model parameters are deflned in Chapter 2, while Section 6.2 shows how they appear in the model equations. Based on the assumptions for these models it follows that all plant effects as well as axial dispersion, void fraction, and mass transfer resistance are independent of the adsorption/desorption processes occurring within the column. 

The experimental procedure for measuring the extent of adsorption generally involves admitting a gas to a chamber that contains the sample and waiting for a fixed period of time, during which equilibrium is supposedly achieved. Figure 6.1 indicates one common type of... 

Chromatographic separation methods cannot be described here. However, in general, it is necessary to develop specific procedures for specific separations. Consequently the choice of the adsorption, solvent and elution agents etc. will be dictated by the nature of the materials to be separated. The general procedures and essentials of chromatography are described and discussed in books dealing with that subject. In addition there are available... 

A simpler and more general technique involves alternate adsorption of monolayers of biomolecules and polyelectrolytes. This method was used by Lvov and coworkers to make ultrathin films of a variety of proteins and oppositely charged polyions [77-79]. Figure 13 illustrates the general procedure. In this... 

The general glassy carbon (GC) electrode modification is shown in Fig. 11b. A summary of the general procedure is as follows OMC is ultrasonically dispersed in DMF to give a homogenous dark suspension. The suspension is then used to coat the GC electrode by adsorption and the solvent dried under infrared lamp to form GC/OMC electrode [47]. GC/OMC electrode is then coated by pipetting a solution of DDAB in chloroform and allowed to dry to form GC/OMC/DDAB electrode. The GC/OMC/DDAB electrode is then immersed into BiCoPc to form GC/OMC/DDAB/BiCoPc/DDAB composite modified electrode. 

Most analytical scale column adsorption chromatography is now performed with so-called solid phase extraction (SPE) columns. These are small, prepacked, single-use columns designed for automated or semi-automated cleanup of samples for analysis by gas or liquid chromatography. A note of caution recovery is generally less than 100% with any adsorption chromatography procedure. Recovery should be determined as part of the development of the methodology. 

A second general type of procedure, due to McBain [29], is to determine n by a direct weighing of the amount of adsorption. McBain used a delicte quartz spiral spring, but modem equipment generally makes use of a microbalance or a transducer. An illustrative schematic is shown in Fig. XVII-6. 

Mercury porosimetry is generally regarded as the best method available for the routine determination of pore size in the macropore and upper mesopore range. The apparatus is relatively simple in principle (though not inexpensive) and the experimental procedure is less demanding than gas adsorption measurements, in either time or skill. Perhaps on account of the simplicity of the method there is some temptation to overlook the assumptions, often tacit, that are involved, and also the potential sources of error. 

A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

None of the procedures outlined can claim any strict justification. Indeed, the deviations of experimental curves from the calculated ones based on simple assumptions can be due in general to a number of causes, some of which were dealt with in Section II.A. A principal ambiguity lies in the choice of whether to treat such departures in terms of either variable Ed or kd, and in the former case often whether the changes in Ed are to be attributed to nonequivalence of adsorption sites, or to lateral interactions between the adsorbed particles, or to yet some other factor (98). 

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