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Seminar on Adsorption : Adsorption

Measurement of Free Space

The meaning of free space (dead volume) is that it is not the geometrical volume of the system but it is the volume of convenience used for the calculation of adsorption amount. In case of nitrogen adsorption measurement, the sample is cooled down by liquid nitrogen. Upon the cooling, there will be two different temperature zones in the sample cell. The gas density of cooled zone is higher and it is influenced by the coolant temperature and by the change in the coolant level. The volumetric method counts the number of un-adsorbed gas molecules (density), and in the process, the free space plays an important role. The conventional volumetric measurement procedure is to cool down the sample by liquid nitrogen, measure the free space by non-adsorbing He gas at the liquid nitrogen temperature, and then, measure the nitrogen or other adsorptive gas isotherm. During the adsorption measurement, the coolant level needs to be maintained.

What' is the dead volume?

The free space is expressed by the following equation. It is the apparent volume of cooled space that is converted from the volume at the coolant temperature to the volume at the apparatus temperature.

On the other hand, our new method, AFSM, can continuously measure the change of free space at each data point without maintaining the coolant level. This new volume calibration method (AFSM) has introduced us more accurate adsorption measurement.


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