Page 182 - Gas Adsorption Equilibria

168 Chapter 3

4. Wall sorption
In gravimetric adsorption measurements of pure gases adsorption of
the gas on walls of tubes and vessels does not pose a problem as no
mass balances of the gas are necessary. However, in multicomponent
adsorption measurements it may influence the sorptive gas
concentration to a certain extent, especially at low gas pressures and
temperatures (T < 77K), and if– for example in a binary gas mixture –
one component is strongly, the other only weakly adsorbed.
Electropolishing of inner surfaces of all tubes and vessels, preferably
made of stainless steel, can reduce this problem considerably.

5. Extreme pressures
Contrary to manometry/volumetry, very high and very low pressures
of the sorptive gas do not pose a serious problem in gravimetric
adsorption measurements. This is a due to the fact that in gravimetry,
the adsorbed mass is determined by its weight, i. e. a quantity which in
principle is physically independent of the gas pressure.

Actually, it is possible to use a microbalance equipped with a dense
sinker of accurately known mass and volume as a manometer: by
measuring its weight in a gas, one can calculate from the buoyancy
term the density of the gas and from it and the gas temperature via the
thermal equation of state of the gas, its pressure. This method already
proved to be useful for low pressures (p < 10 Pa) [3.1, 3.23-3.25].
However, it always should be checked whether radiation equilibrium
between the (normally gold coated) sinker and the (black) walls of the
gas vessel is maintained. If not, creeping gas flows may occur which
may cause serious uncertainties in the balance’s measurements.

It also is possible to use two balances in parallel, one bearing a sinker
and acting as a gas manometer, the other including the sorbent sample
and serving as sorption measuring instrument.

However, it should be mentioned that even gravimetric sorption
measurements may become uncertain at very low pressures due to gas
adsorption at the vessel or basket including the sorbent or other parts
of the microbalance. Therefore it is recommended always to perform
calibration measurements, i. e. measurements with the empty balance
bearing no sorbent material but being exposed to the same sorptive
gas.