The concept of a standard isotherm

From a consideration of the nature of the forces bringing about physical adsorption (cf. Chapter 1), it is evident that the detailed course of the isotherm of a given gas on a particular solid at a given temperature must depend on the nature of both the gas and the solid each adsorbent-adsorbate system will have a unique isotherm. Nevertheless for a given gas. 

Well as various samples of nonporous but amorphous silica. They found that the points fitted on to a common curve very closely, which may be plotted from Table 2.14. A corresponding curve, though based on fewer samples, was put forward for y-alumina. The two curves are close to one another, but the divergence between them is greater than that between different samples of the same substance. Standard isotherm data for argon (at 77 K) on silica have been obtained by various workers.  

The problem of standard isotherms has engaged the attention of Brunauer and his associates,who have put forward five standard isotherms for nitrogen, oxygen and water, characterized by different ranges of the BET c-constant 2(X) c 50 c = 23 14-5 c 10 c = 5-2 and (for relative pressures above 0-5), 200 c 10. 

Standard data for the adsorption of nitrogen at 77 K on nonporous hydroxylated silica  

As will be demonstrated in Chapter 4, however, the presence of micropores distorts the Type II isotherm in a sense which is reflected in a much increased value of the constant c. In such cases the value of c is no guide at all to the course of the isotherm on the external surface. Consequently the appropriate criterion for choosing the correct f-curve for a particular system is the similarity in chemical properties and not in c-values l etween the solid under test and the reference solid. 

---