Page 139 - Gas Adsorption Equilibria

3. Gravimetry 125

Also from eqs. (3.2), (3.3) we have the relation

This can be used to calculate the exact values of balance’s beams if
measurements are performed with well known calibration masses
It also can be used to determine the sorbent mass
if already is known, cp. [3.1, 3.20, 3.22].

From eq. (3.5) the mass adsorbed can be calculated if a model for the
sorbent/sorbate volume is introduced and all the necessary
measurements have been performed, i. e. is known according to eq.
(3.10). As outlined in Chap. 1, is often approximated by the so-called
helium volume,

determined from either volumetric or gravimetric experiments performed with
helium at room temperature. Indeed, assuming helium not to be adsorbed, i. e.
one has from (3.5),

with the reduced mass to be calculated from helium experiments via
eq. (3.6) or (3.10) respectively. Inserting (3.12) in (3.5) we get the so-called
Gibbs excess mass adsorbed as

where and are calculated via eqs. (3.10), (3.13) respectively.

If the void volume in (3.5) is approximated by