Page 99 - Gas Adsorption Equilibria
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2. Volumetry / Manometry 85
It should be noted that for and for
which reflects the occurrence of a liquid phase outside the sorbent material, i.
e. a vapor-liquid-state of the sorptive fluid.
Auxiliary remarks:
1. The quantity in Eq. (2.1) on principle also includes the mass of
gas adsorbed on the walls of the adsorption chamber and possibly
desorbed from the walls of the storage vessel upon gas expansion
These quantities should be determined by calibration
experiments without using a sorbent sample, i. e. cp. Chapter
4. If the surface of the walls of the adsorption chamber and the
storage vessel is small compared to the BET surface of the sorbent
sample, one may assume i. e. neglect
compared to
2. The volumes should be determined experimentally by
using a sample, preferably a cylinder of calibrated volume
made of dense material (Ti, Au) and performing gas expansion
experiments as described above. The amount of gas adsorbed on the
surface of the sample normally can be neglected.**
3. Waiting times for thermodynamic equilibrium in the gas adsorption
system are based on experimental experience which comes from
gravimetric measurements, cp. Chap. 3. No general rules for these
times are available today.
2.3 Uncertainties or Errors of Measurements
An important part of any kind of experimental work is to consider errors
or uncertainties of measurements [2.16 – 2.18]. To keep these down, it is
always recommended to repeat measurements of any experimental quantity at
** Suppose the sample is placed in the adsorption chamber and a certain amount of nearly
not adsorbing gas (He, Ar at has been prepared at pressure (p*) in the storage
vessel If after expansion to the adsorption chamber the gas is at pressure (p), the
volume can be calculated via Eq. (2.2) by