Calculation of the BET Compatible Surface Area from Any Type I Isotherms Measured below the Critical Temperature Jo ´zsef To ´th,* ,1 Ferenc Berger,† and Imre De ´ka ´ny† *Research Institute for Applied Chemistry, University of Miskolc, H-3515 Miskolc-Egyetemva ´ros, P.O. Box 2, Hungary; and †Department of Colloid Chemistry and Nanostructured Materials Research Group of the Hungarian Academy of Sciences, Attila Jo ´zsef University, Szeged, Hungary Received August 4, 1998; accepted December 30, 1998 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 [a s (N 2 , 77)]. This fact has given cause for an elaborate method to calculate the value of a s (N 2 , 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 a s (N 2 , 77) from isotherms of adsorptives which are the actual topics of the investigations. Thus, in these cases the deter- mination of nitrogen isotherms at 77 K can be omitted. The proposed method is based on the To ´th (T) equation and on its modified and extended forms. In these equations are present the parameters  m ,  o , and t with the following physical meanings:  m and  o are integralconstants 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  m and  o assure the thermodynamic consistence of these relationships. It is proven that the parameters ( m ) 1/t and ( o ) 1/t depend only on the structure of adsorbents (micro-, mezo- porous, or smooth surfaces). These parameters, calculated from Type I isotherms measured underthe critical temperature of the adsorptives, are the bases of the calculation of the BET compatible surface areas. © 1999 Academic Press Key Words: BET method; its substitution; omitting of nitrogen isotherms; isotherms Type I; BET compatible surfaces; T equation. INTRODUCTION The IUPAC Commission on Colloid and Surface Chemistry verified in 1985 (1) “that the Brunauer-Emmett-Teller (BET) gas adsorption method has become the most widely used standard procedure for the determination of the surface area of finely-divided and porous materials.” It has also been stated that “at present time, nitrogen is generally considered to be the most suitable adsorptive for surface area determination and it is usually assumed that the BET monolayer is close packed, giving a m (N 2 ) = 0.162 nm 2 at 77 K ( a m is the molecular cross-sectional area occupied by the adsorbate molecule in the complete monolayer). The existence of a strictly constant value of a m (N 2 ) is unlikely, therefore, with other adsorptives arbi- trary adjustments of the a m value are required to bring the BET area into agreement with the nitrogen value.” For example, a m (Kr) has to vary between 0.17– 0.23 nm 2 and a m (Xe) be- tween 0.17– 0.27 nm 2 (1). In spite of these problems the Commission summarized their opinion with the following sen- tence: “. . . it is recommended that nitrogen should continue to be used for the determination of both surface area and meso- pore size distribution.” This recommendation has been ac- cepted and the BET(N 2 , 77) method continues to be used in all physico-chemical laboratories around the world. In spite of this wide and successful application of the BET(N 2 , 77) method it has some disadvantageous properties. These are as follows: (i) In most cases we have to investigate quite different isotherms measured at different temperatures. The tempera- tures of these isotherms may be much greater than 77 K; the molecular sizes, molecular masses, and a m values of the inves- tigated adsorptives may essentially differ from those of nitro- gen. (ii) In case (i) separate measurements of the N 2 isotherm at 77 K are required for determination of the surface area; there- fore, liquid nitrogen (low-temperature adsorption equipments) has to be used in any case. (iii) The BET equation is an oversimplified relationship and its practical applicability is based on the fact that the different simplifications (homogeneous surface, the equality of the con- densation, and adsorption heat in the second layer, etc.) luckily compensate one another. (iv) The calculation of the specific surface area is possible only if the BET equation is applied to a limited part of the N 2 isotherms, usually not outside the relative pressure range of 0.05– 0.30; i.e., the BET relationship cannot explain and de- scribe the whole measured isotherm. Taking the advantages and disadvantages of the BET(N 2 , 77) method into account it was obvious to elaborate on such a practical method which eliminated the disadvantages men- tioned above but included the possibility of calculation of the specific surface area approximately equal to the values of 1 To whom correspondence should be addressed. Journal of Colloid and Interface Science 212, 402– 410 (1999) Article ID jcis.1998.6072, available online at http://www.idealibrary.com on 402 0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.