Page 173 - Gas Adsorption Equilibria

3. Gravimetry 159

and connecting tubes and valves is also assumed to be known from the
relation

Here the volume of the adsorption chamber etc. has to be determined –
for example – from helium or nitrogen expansion experiments using a sample
mass of well known volume (ballast or tare), cp. Chap. 2. For the volume of
the sorbent mass and the adsorbed phase one has again to introduce a
model assumption. We here restrict the discussion to the Gibbs excess mass of
all components adsorbed. Hence we choose the so-called helium
approximation for

being the volume of the sorbent measured by exposing it to helium at
ambient temperature and low pressure (1 atm) and assuming helium neither to
be adsorbed nor absorbed by the sorbent sample.

From the mol number and the concentrations the total mass of
sorptive gas can be calculated by

indicating the molar mass of component i = 1…N. Now the total mass of
sorptive gas (m*) originally supplied to the sorption vessel can be calculated
from the mass balance

as the total mass adsorbed can be calculated from the reduced mass
measured at the microbalance, cp. eq. (3.5, 3.14),

and the density of the sorptive gas given by (3.42), which can be rewritten
as