Hello When the BET measurement curve becomes vertical at relative pressures close to 0.995, it indicates that the adsorption is approaching saturation. This means that the surface of the material being measured is becoming fully covered with the adsorbate, and further adsorption is limited. This point is known as the monolayer capacity, and it provides important information about the surface area and porosity of the material. The point at which the curve becomes vertical can be used to calculate the specific surface area of the material using the BET equation. You can perform the aspiration process by reducing the pressure in order to see the shape and type of the hysteresis loop, then determine the nature of the sample, whether it is nails or not, from the equal-degree adsorption curve.

I have to correct Ali Alnazza Alhamad . The monolayer (from which the surface area can be calculated using BET equation) is formed around 0.1 p/0, or even lower for micropore-containing samples.

The vertical rise at p/p0 ~1 has nothing to do with the sample... its simply the adsorbate condensing in the sample cell Christian Weinberger spotted that. Often comes about by incorrectly "assuming" a value for p0 and not using the correct value by having the instrument measure it. In this case, the p0 value being used by the instrument is higher than the true value. For N2/LN2 p0 is around 10 torr higher than ambient atmospheric pressure... you can do a quick sanity check on the data using that to see how much the p0 was in error.

Thank you very much for the clarification, my dear professor.

When you get that close to Pvap, you start to get condensation in other parts of you setup. If you have a volumetric system, the glass may have a higher energy of adsorption than you sample. Of you might be getting macropore filling of the spaces between the particles.

It used to be thought that there might be something interesting in this high pressure region. It's my opinion that it may take some experiments in the space station to answer what is going on here, because the energy of adsorption (bold E) is very close to the surface tension energies. At the isotherm inflection point the E is about 625 joules per mole versus say 10 to 20 kilojoules per mole ar the start of the isotherm. After the inflection point is gets even lower. Typical surface tension energies are below about 500 J/mol for typical mesopores, so at those pressures you mention very close to Pvap, even large spaces might start to fill.

The BET according to the present IUPAC convention is valid only between ~0.05 and ~0.35 P/P(vap). The later value can be much smaller than 0.35 and depends upon the Rouquerol criterion. This to use values only up to the maximum of the expression n(ads)*(1-P/P(vap)).

This, however, is not the solution to “fix” BET or the Langmuir analysis. Both of these should be abandoned since observations have disproved them, not just statistically but they fail critical experimentation when tested.) I you do use the BET you will get an answer about 2 to 3 times too high in the IUPAC range. At low pressure you probably will get and answer about 1/3 the correct answer. In transition between its off by a factor of 10 to 100.

I suggest you get a hold of some of my publication of some by Juegen Adolphs. What we have presented a some hypotheses that are based on quantum mechanics (QM) in one case and upon fluid dynamic in the second case. The approach is more basic in both cases and they are consistent. For the novice, I suggest that my treatment using the results of the QM (you don’t need to know QM, just use the results.) The results shows a few ways to plot the data that will make more sense.

If you have microporosity, then the log-law will give you a answer quickly. If you have mesoporosity then the chi-plot will give you a better form of the answer if you supplement the base equation. I have written up both QM and the Adolphs’ approach in the second edition of the book I wrote (ISBN 978-0-12818785-2.) It should be available on interlibrary loan.