The BET equation, widely used in specific surface area analysis, is based on several key assumptions. First, it assumes that the solid surface is uniform and that multilayer adsorption occurs. Second, it suggests that the heat of adsorption for layers beyond the first is equivalent to the heat of liquefaction of the adsorbate. These assumptions form the foundation for both thermodynamic and dynamic derivations of the BET model. In the derivation process, the second layer of adsorption is treated as a physical aggregation of the adsorbate molecules rather than an interaction with the solid surface itself. This simplification allows for the development of the two-constant and three-constant BET equations, which are essential in both thermodynamic and kinetic analyses. Experimental studies have shown that variations in the solid surface do not significantly affect the adsorption behavior of the second and subsequent layers. For low-temperature nitrogen adsorption methods, nitrogen is typically used as the adsorbate. The BET equation is valid within a relative pressure range of 0.05 to 0.35. Below 0.05, the adsorption is too weak to form a monolayer, and above 0.35, capillary condensation becomes dominant, making the linear relationship less accurate. This range ensures reliable and consistent results for surface area calculations.
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