Page 181 - Gas Adsorption Equilibria

3. Gravimetry 167

5. PROS AND CONS OF GRAVIMETRY

This section is devoted to the advantages and disadvantages of gravimetric
measurement of gas adsorption equilibria. It reflects experiences we made in
our experimental work in this field during the last 15 years using both two
beam and magnetic suspension balances.

5.1 Advantages

1. Accuracy
All microbalances which are commercially available today, cp. Tab.
3.1, [3.7-3.9] normally exhibit high reproducibility, sensitivity, and
accuracy (up to of measurements. Hence they allow one to
determine adsorbed gas masses on porous solids much more
accurately than manometric measurements. The gain in accuracy often
is about one order of magnitude; at very low pressures it may be two
orders of magnitude or even more.

2. Amount of sorbent material
For highly sensitive microbalances (Thermo Cahn, Hiden, Mettler-
Toledo, Rubotherm, Setaram, TA Instruments, VTI) only tiny amounts
of sorbent materials are needed to measure gas adsorption equilibria.
This is advantageous for investigations in newly developed sorbent
materials where often only small amounts are available. However,
results may be misleading as the sorbent sample used does not
represent a “statistically averaged” sample of a technical sorbent and
thus may exhibit considerable statistical deviations.

3. Approach to equilibrium
Microbalances with alphanumerical display and electronic data
recording systems allow one to observe the approach to equilibrium
for gas adsorption processes in porous sorbent samples. Typical
relaxation times can be one or several seconds, minutes, hours, and –
sometimes – even days, cp. helium adsorption data Sect. 2 of Chap. 1.
Hence gravimetric measurements do allow one to check whether an
adsorption system actually has reached its equilibrium state, i. e. these
measurements deliver in principle also information concerning the
kinetics of the adsorption process, represented for example by
(phenomenological) diffusion coefficients, cp. Sect. 2.3 and Sect. 4.4
and [3.27, 3.48).