Page 124 - Gas Adsorption Equilibria

110 Chapter 2

amounts – say several milligrams – of the sorbent are available,
gravimetric measurements are strongly recommended.

2. Approach to equilibrium
Gas adsorption processes may last for seconds, hours or – sometimes –
even days. Therefore one never can be sure whether mermodynamic
equilibrium in a volumetric experiment has been realized. Hence the
time which should elapse between opening the expansion valve and
reading of instruments, especially thermometer and manometer has to
be chosen according to experience or accompanying gravimetric
measurements which – contrary to volumetry / manometry – also
provide information on the kinetics or the sorption process, cp. Chap. 3.

3. Wall sorption
Upon expansion from the storage vessel the sorptive gas may not only
be adsorbed on the surface of the sorbent material but also on the walls
of the adsorption vessel and the tube connecting both vessels. This may
cause additional uncertainties in measurement. These often but not
always can be reduced by performing complementary experiments with
gas expansion to the empty adsorption chamber including no sorbent
material at all. To reduce wall adsorption electropolishing of all inner
surfaces is recommended. An experiment allowing to determine wall
adsorption is described in Chapter 4, Sect. 3.6.

4. Uncertainties in step-up experiments
In step-up pressure experiments, i. e. gas expansion and adsorption
processes with remnant gas in the adsorption chamber, cp. Figs. 2.1,
2.6, the uncertainties of the adsorbed mass accumulate due to the
algebraic structure of the sorptive gas mass balance equation

Here indicates the mass of gas added to the storage vessel in
step (n+1) at closed expansion valve. The quantities are the
masses of the gas in the sorptive phase and of the adsorbed phase at the
end of the n-th experiment respectively. Note that for step-up
adsorption experiments whereas for step-down desorption
experiments In practice the total number of steps should
not exceed 3-4 as then uncertainties of about 20 % and more typically
occur.