Specific surface area positive adsorption methods

POSITIVE adsorptiqn METHODS. The measurement of specific surface area with a positive adsorption method requires the satisfaction of three basic experimental conditions ... 

Although the positive adsorption methods offer the advantage of convenient determination in heterogeneous samples, they suffer from the uncertainty involved in the calibration of the packing area (which usually must be done by comparison with results from a physical method using reference clay materials) and from the fact that the monolayer parameter is model-dependent through Eqs. 1.6 and 1.7. It must also be remembered that the specific surface area determined by positive adsorption is ultimately a function of the reaction between surface functional groups and some probe molecule. If the experimental conditions of the reaction are close to those under which the surface behavior of a sample is of interest, then this estimate of specific surface area has surface chemical relevance. 

S. J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity. Academic Press, London, 1982, TTie first two chapters of this well known monograph present a thorough discussion of the concept of the packing area and the measurement of specific surface area by positive adsorption methods. 

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. 

Specific surface area determination is most commonly conducted by positive adsorption studies (Section 7.6.4.2), and here the election of the probe molecule is very important. The most traditional and widely used method is N2 adsorption at 77 K, using the BET isotherm to evaluate monolayer coverage (Section 7.6.4.2) sometimes other inert gases, such as argon, are employed in the same conditions (Sposito 1984). In soil characterization, other substances are also used in the BET... 

The solution depletion method for studies of adsorption in solution uses material in the form of powder or pellets with specific dimensions and surface areas. This material may be physically or chemically treated to produce hydrophobic, or positively or negatively charged hydrophilic surfaces which is exposed to the solution. The decrease in concentration of the adsorbate from the bulk solution is then measured. The precision of this method is dependent on the analytical technique used. However, for biological material, the technique does not allow measurement of subtle changes in surface denaturation. The technique has been used by a number of researchers. ... 

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