Properties of the BET Equation

The BET equation filled an annoying gap in the interpretation of adsorption isotherms, and at the time of its appearance in 1938 it was also hailed as a general method for obtaining surface areas from adsorption data. The equation can be put in the form 

The very considerable success of the BET equation stimulated various investigators to consider modifications of it that would correct certain approximations and give a better fit to type II isotherms. Thus if it is assumed that multilayer formation is limited to n layers, perhaps because of the opposing walls of a capillary being involved, one 

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The Type II isotherms obtained experimentally often display a rather long straight portion (BC in Fig. 2.9), a feature not strictly compatible with the properties of the BET equation which, as we have seen, yields a point of... 

Type II isotherms (see Fig. 3) displ often almost linear segment in a wide range of moderate relative pressures, a feature not strictly compatible with the properties of the BET equation, which has been used to estimate the inflection point. The point, at which tliis linear segment starts, was termed by Emmett and Brunauer [66] as point B (see Fig. 6) and used to estimate the completion of monolayer. So, the adsorption at point B, as, was assumed to represent the monolayer capacity. In [67] Brunauer and Emmett suggested that point A - the... 

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