Negative Adsorption Methods

In Equation 7.26, is the solid area per unit of mass where the ion is excluded and is the exclusion distance, the (effective) distance from the surface where the solution is depleted of X. It is a function of concentration and can be evaluated 

a plot of versus will be a straight line, and can be evaluated from the slope. The negative adsorption method has been applied to some phyllosilicates (Bolt and Warkentin 1958 Edwards, Posner, and Quirk 1965a, b) but has received less attention than its positive adsorption counterpart. 

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NEGATIVE ADSORPTION METHODS. Negative adsorption refers to the phenomenon in which a charged solid surface confronts an ion of like charge in an aqueous suspension and the ion is repelled from the surface by... 

The development of a negative adsorption method for measuring specific surface area is based on the additional definition ... 

The determination of specific surface area of soils and soil coUoids is of great importance in characterizing the reactivity of a sample, among other factors. However, it is actually an operational concept, because the A5 value depends on the experimental method employed, as it will be shown in Section 7.6.4.I. The underlying fact is that the effective area available for a particular reaction or process is dependent on the reactants and/or external factors involved. The experimental methods can be broadly classified into three categories (Sposito 1984) physical methods, positive adsorption methods, and negative adsorption methods. 

There are several variations to this technique. The negative adsorption method is based on the exclusion of co-ions from the electrical double layer surrounding charged particles [76]. 

The area is an important surface parameter for catalytic studies. It is needed to evaluate the rate constant of the surface reaction from the kinetics as well as to allow a fair comparison to be made of the effectiveness of different catalysts. Areas are commonly determined by nitrogen or krypton gas adsorption interpreted by the Brunauer-Emmett Teller (BET) isotherm [30, 32], A number of other methods has been proposed and utilised including microscopy, isotopic exchange, chromatography, gas permeability, adsorption from solution, and negative adsorption (desorption) of co-ions [30, 33]. 

Negative adsorption of Cl from 0.001 M NaCl and substantial uptake of Na from 0.001-0.1 M NaCl was found by a radiotracer method [1675]. 

Bolt, G. H., and B. P. Warkentin, 1956. Influence of the method of sample preparation on the negative adsorption of anions in montmorillonite suspensions. Proc, 6th Int, Congr, Soil Sci. B 33-40. Broadbent, F. E., R. H. Jackman, and J. McNicoll, 1964. Mineralization of carbon and nitrogen in some New Zealand allophanic soils. Soil ScL 98 118-128. 

An additional method for increasing particle size deserves mention. When a precipitate s particles are electrically neutral, they tend to coagulate into larger particles. Surface adsorption of excess lattice ions, however, provides the precipitate s particles with a net positive or negative surface charge. Electrostatic repulsion between the particles prevents them from coagulating into larger particles. 

Although the p.z.c. is difficult to determine experimentally, and although the values obtained vary with the method used, it is of fundamental significance in electrochemistry, since it provides information on adsorption of ions and molecules, i.e. if the potential is negative with respect to the p.z.c. cations will tend to be adsorbed and anions repelled, and vice versa. The p.z.c. appears to be a natural reference point for a rational scale of potentials defined by... 

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