Adsorption by solids

3 Also called secondary , irreversible , and, if it takes place slowly at temperatures not too far above room tomperatures, activated adsorption. Chemisorption, dofined as co-valent combination of the adsorbate with the surface atoms, is very often irreversible, in the sense that the gas originally adsorbed comes off in a different state (e.g. as atoms instead of molecules, or as compounds with the atoms of the substratum). It need not necessarily be irreversible, however. 

Adsorption may occur through simple electrostatic forces,1 e.g. alkali metals on tungsten, and silver or halide ions on silver halides. 

There is little doubt that if the adsorption is of the van der Waals type, the molecules of the adsorbed gases are not dissociated on the surface. It 

8 McBain s term Sorption applies to all cases when a gas or solute is taken up by a solid. Adsorption means strictly the formation of a layer on an impermeable surface a sorption to penetration, or solution, into the interior. With the zeolites, there are pores of quite regular shape and size, only a few A. across. With charcoal there are irregular pores of similar size, as well as larger ones. Foreign substances, finding their way into these very fine pores may be said to be adsorbed on the walls of the pores, but as they permeate the whole structure of the solid more or less uniformly, they may equally be said to be absorbed by the solid. McBain avoids this difficulty simply by omission of the prefix. 

London, Z. physical. Chem. B, 11, 222 (1930) Lennard Jones, Trans. Faraday Soc., 28, 333 (1932) Bora, Atomic Physics, 254, 339 (Blackie. 1935). 

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The main techniques that have been used to dispose of industrial effluents include chemical precipitation, ion exchange, electrochemical processes, and adsorption onto various adsorbents and/or membrane filtration. Although all of these techniques are capable of removing heavy metals to some extent, adsorption by solid substrates is preferred because of its high efficiency, easy handling and cost as well as the availability of adsorbent. 

The mechanisms which are responsible for adsorption by solids from liquids may be classified thus ... 

Detailed discussion of earlier work by other investigators would unduly lengthen the present review. Many references given in the papers listed below may be consulted for information on this matter. Briefly, however, it may be said that hydrogen bonding in adsorption by solids froth solution has been reasonably well established for the following systems ... 

For further data on specific adsorption by solids, from solutions, see Chap. Ill, 13. 

Scheme 2.2 After adsorption by solid sodium fluoride complex and sensitive organic compounds can be cleanly perfluorinated [9],...

Moisture adsorption by solid dosage forms can result not only in increased chemical drug degradation but also in changes in the functional stability of dosage forms. 

Adsorption isotherms describe solute adsorption by solids at constant temperature and pressure. An adsorption isotherm shows the amount of adsorbate sorbed as a function of its equilibrium concentration. A variety of isotherm shapes are possible, depending on the affinity of the adsorbent for the adsorbate (Fig. 9.9). 

The following parameters of triflu-promazine hydrochloride and other phenathiazine have been studied Adsorption by solids such as talc, kaolin and activated charcoal, surface activity at the air-solution interface oil-water partitioning, photo induced interaction with a lecithin monomolecular film, and interaction with bovine serum albumin. ... 

C. H. Giles and S. D. Forrester, Studies in the early history of surface chemistry, Chemistry and Industry, Nov. 8, 1969 Jan. 17, 1970 Jan. 9, 1971 July 24, 1971 Nov. 13,1971 April 15,1972. This history of surface phenomena is a must for any serious student of the subject. Parts V and VI, on solute adsorption by solid surfaces, give and well-illustrated, lively account of the role of soil chemists in the development of the adsorption isotherm concept. 

In adsorption by solids from liquid phases, substances in solution at very low concentrations are often preferentially adsorbed the presence of trace quantities of water and other impurities in the solution may therefore have an effect on adsorption. Harkins and Dahlstrom [31], for example, reported that extremely small quantities of water in benzene increas the energy of immersion of oxides to about three times the value obtained wiUi pure benzene. In order to obtain liquids of sufficient purity it may be necessary to fractionate and then store over metallic sodium or other drying agent such as silica gel, calcium sulphate, alumina and so on. 

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