0.05 to 0.35 p/p° is the relative pressure range in which mono/multi layer adsorption takes place on non-porous materials, for example on the external surfaces of the grains of a powder. Also for porous materials with sufficiently large pores, the classical range of 0.05-0.35 is valid (generally pore widths > 10 nm).

For microporous materials the BET range is shifted substantially towards lower pressures. It is common practice nowadays to make a so called 'Rouquerol plot'. This is a plot of n(1 - p/p°) vs p/p°, with n the amount adsorbed. In the BET range, the Rouquerol plot continuously increases, so the maximum p/p° of the BET fitting range is given by the maximum of the plot. The lower end of the range is less crucial. All measured points in the low p/p° range can be included. Some people recommend to exclude lower points until an optimal R^2 is obtained for the the BET regression. This is particularly relevant when the intercept on the BET plot is very low.

The reasoning behind all this is very nicely explained in this paper:

Article Is the BET Equation Applicable to Microporous Adsorbents?

regards,

Pieter

I think the relative pressure might be between 0.05 to 0.30. The monolayer is formed at very low pressure and multilayering starts around 0.05 relative pressure. For relative pressure above 0.3, there may exist capillary condensation, which is not amenable to multilayer analysis.