Nitrogen as the standard adsorbate

For surface area determinations the ideal adsorbate should exhibit BET C values sufficiently low to preclude localized adsorption. When the adsorbate is so strongly tied to the surface as to be constrained to specific adsorption sites, the adsorbate cross-sectional area will be determined more by the adsorbent lattice structure than by the adsorbate dimensions. This type of epitaxial adsorption will lead to decreasing measured surface areas relative to the true BET value as the surface sites become more widely spaced. 

On the other hand, if the C value is sufficiently small, the adsorbate lateral mobility on the surface will tend to disrupt any tendency for an organized structure to develop and the adsorbed layer might appear more as a two-dimensional gas. 

In either of the preceding cases, very high or very low C values, any attempt to calculate the effective adsorbate cross-sectional areas from the bulk liquid properties will be subject to considerable error. Nitrogen, as an adsorbate, exhibits the unusual property that on almost all surfaces its C value is sufficiently small to prevent localized adsorption and yet adequately large to prevent the adsorbed layer from behaving as a two-dimensional gas. 

Kiselev and Eltekov established that the BET C value influences the adsorbate cross-sectional area. They measured the surface area of a number of adsorbents using nitrogen. When the surface areas of the same adsorbents were measured using n-pentane as the adsorbate the cross-sectional areas of n-pentane had to be revised in order to match the surface areas measured using nitrogen. It was found that the revised areas 

A similar relationship between n-butane cross-sectional areas and the BET C constant was found by Lowell and Shields. A plot of the revised cross-sectional areas of n-butane versus the BET constant is given in Fig. 6.3. 

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