Applicability of the BET theory

The remaining two isotherms, types IV and V, are modifications of the type II and type III isotherms due to the presence of pores. 

Rarely, if ever, does the BET theory exactly match an experimental isotherm over its entire range of relative pressures. In a qualitative sense, however, it does provide a theoretical foundation for the various isotherm shapes. Of equal significance is the fact that in the region of relative pressures near completed monolayers (0.05 P/Pq 0.35) the BET theory and experimental isotherms do agree very well, leading to a powerful and extremely useful method of surface area determination. 

The fact that most monolayers are completed in the range 0.05 P/Pq 0.35 reflects the value of most C constants. As shown in Table 4.2, the value ofW/W equals unity in the previous range of relative pressures for C values between 3 and 1000, which covers the great majority of all isotherms. 

The sparsity of data regarding type III isotherms, with C values of 2 or less, leaves open the question of the usefulness of the BET method for determining surface areas when type III isotherms are encountered. Often in this case it is possible to change the adsorbate to one with a higher C value thereby changing the isotherm shape. Brunauer, Copeland and Kantro, however, point to considerable success in calculating the surface area from type III isotherms as well as predicting the temperature coefficient of the same isotherms. 

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Throughout the preceding section we have tabulated the measure of sorption expressed in terms of that obtained by the application of the BET theory. In general, the theory and data are compatible over the classical range limited to about 0.05 Pq to 0.35 Pq. Such a correlation should not be construed to serve as proof of the BET mechanism or even to measure a true specific surface area. The BET theory is, by its very nature and derivation, a theory for multilayer formation. Such a process is probably in play for nitrogen sorption but is questionable for water and... 

For the adsorption of argon on boron carbide, Knowles and Moffat [182] found that the application of the BET theory gave more consistent results using liquid aigon pressure rather than the solid-vapor pressure. 

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