It depends if you are using the classic pycnometry based on liquids. If yes, then there is no way you can determine the apparent density correctly, since no liquid will fully penetrate into micropores at normal atmospheric conditions. If you have a high pressure system, you could try to do something similar to mercury porosimetry, but you will need to find a way to precisely measure the weight and volume of the "sample holder" when all pores are filled with Hg, which usually happens only at high pressures.

For a more simple, yet more or less accurate determination you should check gas pycnometry. Check out the wikipedia page of "gas pycnometer".

Hi philippe.

What exactly do you mean with "ultramicropore" ? The porosity is a scale issue, and many definition exist. You can use the cardell's one (2003), who class it by dominant transfer forces. Mercury porosimetry is really usefull to determine the poral size distribution, and gas pycnometer for entire opened porosity. But in all cases, you don't have the trap porosity.

Maybe, by pycnometry, you mean the pycnometry describe by the european norm EN-1936 which is a destructive method, and consist in crushing the material in order to gain access to the trap porosity, and after reach the solid density.

However, which granulometry do you need when you crush the material ? It's dependend of the porosity you target, and its location. Il you're interested by the intra-granular porosity, for example, you will need crush the grains themselves.

Don't forget with this method, when you have microporosity and infraporosity, to degas the water and the material.

This problem is frequently encountered in gas addsorption work on microporous materials when trying to determine the volume displaced by the sample. Helium pycnometry can be attempted, but many activated carbons will adsorb helium even at room temperature, yielding a falsely high density (even a negative displaced volume). Raising the temperature of the measurement would be helpful in principle, but also increases the experimental difficulties.

What is the goal of the density measure? Definition of apparent density should be related to the application of the material and the process you want to describe.

The goal is to determine accurately the dead volume of an adsorption cell before running adsorption of gases or vapours. If ultramicropores do adsorb helium at room temperature, as well as at 77K, what else can be done ?

Could this be done alternatively by a tracer experiment?

I forgot to mention...

IUPAC currently defines "micropores" as pores accessible to adsorption in the range of width up to 2 nm, divided into two subranges: ultramicropores below 0.7 nm and supermicropores from 0.7 to 2 nm. [Pure and Appl Chem, 57, 1985, p603}

In the case where helium adsorption is suspected, as with microporous carbons, the sample volume can best be calculated from the degassed sample weight and an assumed densityfor graphite/graphene: 2.24 g/cc. Fortunately, some error is tolerable: since these high surface area materials adsorb at quite low pressures and small sample weights are usually used, the moles of gas displaced by the sample are very small compared to the amount adsorbed.

see the link

http://www.microanalysis.com.au/techniques/pycnometry-absolute-density-bulk-tamped-density-analysis/