General conclusions determination of specific surface from adsorption isotherms

14 General conclusions determination of specific surface from adsorption isotherms 

The BET method for calculation of specific surface A involves two steps evaluation of the monolayer capacity n from the isotherm, and conversion of n into A by means of the molecular area a . 

To obtain a reliable value of from the isotherm it is necessary that the monolayer shall be virtually complete before the build-up of higher layers commences this requirement is met if the BET parameter c is not too low, and will be reflected in a sharp knee of the isotherm and a well defined Point B. For conversion of into A, the ideal adsorptive would be one which is composed of spherically symmetrical molecules and always forms a non-localized film, and therefore gives the same value of on all adsorbents. Non-localization demands a low value of c as c increases the adsorbate molecules move more and more closely into registry with the lattice of the adsorbent, so that becomes increasingly dependent on the lattice dimensions of the adsorbent, and decreasingly dependent on the molecular size of the adsorbate. 

When other adsorptives, such as those detailed in Section 2.9, are employed for surface area determination, calibration against nitrogen or argon is strongly recommended, so long as the specific surface exceeds lm g . For areas below this figure the calibration becomes too inaccurate, and an alternative adsorptive, usually krypton, has to be used. 

As explained in Section 2.13, the use of iz,-plots makes it possible to avoid the involvement of either n or when an alternative adsorptive is being used for evaluating the surface areas of a set of related solids. It is then no longer necessary to exclude the use of isotherms having a low value of c, consequently the method is applicable even if the isotherm of the alternative adsorptive is of Type III (cf. Chapter 5). Calibration of one sample by nitrogen or argon adsorption is still required. 

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