Calculation of the BET Compatible Surface Area from Any Type I Isotherms Measured below the Critical Temperature

It may occur in practice that the nitrogen isotherm should be measured at 77 K only in order to determine the Brunauer-Emmett-Teller (BET) specific surface area [as(N2, 77)]. This fact has given cause for an elaborate method to calculate the value of as(N2, 77) from Type I isotherms measured on any adsorbents at any temperature. Since Type I isotherms are measured most often in practice the proposed method makes it possible to calculate the value of as(N2, 77) from isotherms of adsorptives which are the actual topics of the investigations. Thus, in these cases the determination of nitrogen isotherms at 77 K can be omitted. The proposed method is based on the Tóth (T) equation and on its modified and extended forms. In these equations are present the parameters chim, chio, and t with the following physical meanings: chim and chio are integral constants originating from the Gibbs equation integrated between definite limits of pressure and coverage and t is a parameter characterizing the heterogeneity of the adsorbents. The parameters chim and chio assure the thermodynamic consistence of these relationships. It is proven that the parameters (chim)1/t and (chio)1/t depend only on the structure of adsorbents (micro-, mezoporous, or smooth surfaces). These parameters, calculated from Type I isotherms measured under the critical temperature of the adsorptives, are the bases of the calculation of the BET compatible surface areas. Copyright 1999 Academic Press.