Determination of Surface Area

Abstract

The purpose of this Manual is two-fold: first to draw attention to the problems and ambiguities which have arisen in connection with the reporting of gas adsorption (physisorption) data; second to formulate proposals for the standardisation of procedures and terminology which will lead to a generally accepted code of practice. The proposals are based on, and are in general accordance with, the Manual of Symbols and Terminology for Physicochemical Quantities and Units (1979) and Parts I and II of Appendix II (1972 and 1976).

The first stage in the interpretation of a physisorption isotherm is to identify the isotherm type and hence the nature of the adsorption process(es): monolayer-multilayer adsorption, capillary condensation or micropore filling. The BET method is unlikely to yield a value of the actual surface area if the isotherm is either Type I or Type III; but both Type II and Type IV isotherms are, in general, amenable to the BET analysis, provided that the value of C is neither too low nor too high and that the BET plot is linear in the region of the isotherm containing Point B.

The computation of mesopore size distribution is valid only if the isotherm is of Type IV, but in view of the complexity of most pore systems little is to be gained by the application of an elaborate method of computation. If a Type I isotherm exhibits a nearly constant adsorption at high relative pressure, the micropore volume is given by the amount adsorbed at the plateau. At present there is no reliable procedure available for the computation of the micropore size distribution from a single isotherm.

A check list is recommended to assist authors in the measurement of adsorption isotherms and the presentation of the data in the primary literature.