Page 52 - Gas Adsorption Equilibria

38 Chapter 1

or a single-armed suspension balance (Rubotherm GmbH, Bochum), cp.
Chap. 3. The sorbent material is placed in a bowl at the balance. Then the
balance- or adsorption-vessel is evacuated and after this filled with gas at
certain pressure (p) and temperature (T). It can be shown, Chap. 3, that
after certain calibration measurements finally the so-called reduced mass

can be determined from the various balance readings. Here is the mass
of gas adsorbed on the sorbent of mass is the density of
the sorptive gas and is the volume of the sorbent material (s) filled with
sorbate (a). If for is a linear function of the sorptive gas density
can be determined from (1.5) by differentiation

Once is known, can be calculated from Eq. (1.5).
Such a situation is very common for gravimetric measurements of helium
gas adsorption equilibria. An example is sketched in Figure 1.8 showing
gravimetric adsorption data of activated carbon Norit R1 Extra exerted to
He (5.0). As can be seen, for high pressures the reduced mass data
easily can be linearly correlated, i. e. adsorption of helium has reached a
state of saturation. Hence, can be determined via Eq. (1.6) and also the
mass of helium adsorbed initially at low gas pressures can be calculated
from Eq. (1.5) as:

leading in our example to

Similar measurements at higher temperatures have shown that helium is
adsorbed in ACs to a certain extent even at temperatures about 500 K. The
amount of helium adsorbed increases considerably at low temperatures
This phenomenon has been investigated thoroughly by
K. Kaneko and co-workers, Chiba University, Chiba, Japan, who also have
shown that helium (diameter: 2 Å = 0.2 nm) even at very low pressures is
adsorbed in so-called supramicropores (diameter 2 r < 1 nm). These authors
also could demonstrate that helium in the adsorbed phase in submicropores