Other methods for determining surface area

Nitrogen gas adsorption at liquid nitrogen temperature is the most widely used method of surface area determination [1]. Krypton at liquid nitrogen temperature is favored for low surface area powders. Most gases can and have been used and these include water vapor at 

Permeametry is often used for control purposes due to its simplicity 

Surface areas may also be determined from heats of adsorption and the technique has been greatly simplified with the introduction of sensitive flow micro-calorimeters. These can be used with liquid or gas 

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The overall aim of this book was to provide a critical exposition of the use of adsorption data for the evaluation of the surface area and the pore size distribution of finely divided and porous solids. The intended audiences were workers in academic institutions and in industrial laboratories. Considerable space is devoted to the BET method (see Chapter 3 in this book) for determining the specific surface and the use of the Kelvin equation for the calculation of pore size distribution. However, attention is also given to other methods for estimating surface area from adsorption measurements, namely those based on adsorption from solution, on heats of immersion, on chemisorption and on the application of the Gibbs adsorption equation to gaseous adsorption. Each of the eight chapters is extensively referenced. 

Other important parameters in particle analysis are surface area, pore size, and volume. The basic method for measuring surface area involves determining the quantity of an inert gas, usually nitrogen, required to form a layer one molecule thick on the surface of a sample at cryogenic temperature. Many techniques are used for pore size measurement impregnation with molten metal, particle beam transmission, water absorption, freezing point depression, microscopy, mercury intrusion, and gas condensation and evaporation. The last three techniques are most often utilized. 

Tt is the purpose of this paper to describe methods for determining and A interpreting dye spectra in aqueous dispersions of silver halides and other substrates. Such spectra can be utilized for the direct measurement of surface concentrations of dyes from which, in turn, the surface area of the substrate can be derived. The techniques involved are not limited to a specific dye class but will be illustrated in this paper by the behavior of cyanine dyes. 

Over the past 50 years the BET method has become an extremely popular method for determining the surface area of adsorbents, catalysts and various other finely... 

Despite the lack of consistency of the theory, however, the model has persisted and the BET equation continues to be one of those most commonly used. Moreover, the BET method for determining the surface area of solids is internationally accepted and is routinely used in the determination of that parameter. This is because the method is able to provide fairly acceptable results concerning the surface area of many solids, despite the defects inherent to the theory. This and other paradoxes of the BET theory remain to be elucidated and it is because of this that we were prompted to make a review of the BET plot, proposing other ways to check the model that serve to clarify some aspects of the theory. 

The specific surface area and the pore size distribution of sorbents and catalysts are of central importance for their properties. For most cases it is sufficient to use the BET theory for the determination, although it will face limitations at very low surface areas and in the presence of micropores. The most successfully used gas is N2 but, for low surface area materials, the use of a more easily condensable gas, such as Ar, has advantages. It is strongly recommended, however, that these methods be calibrated against a weU-known sample, as the packing density of the various gases cannot be extrapolated directly from their molar volumes. The concept of surface area is not applicable for micropores, i.e., when the pore size and the size of the sorbed molecule approach each other. 

Important methods for the determination of the specific surface area and of the pore size distribution are based on the measurement of the gas adsorption isotherm [1,2]. The gas adsorption method and the evaluation according to Brunauer, Emmett and Teller using the two-parameter BET equation has been standardized in several countries for a number of years and an ISO standard just appeared. To establish the pore size distribution the method of Barrett, Joyner and Halenda (BJH) is generally accepted. Other methods for this purpose make use of the flow resistance of air through the compressed sample. The Blaine test and other flow tests used to characterize building materials are standardized world-wide. 

The surface areas determined by the BET method are apparent surface areas only since the BET adsorption equation is, in principle, not valid when micropore filling occurs. The determination of the true surface area in the micropores depends on the method used for the evaluation of the adsorption isotherms arul on the model used for the shape of the micropores (cylindrical, slit-shaped or other). 

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