Page 22 - Gas Adsorption Equilibria

Introduction 7

d) Information concerning the gas to be adsorbed including
concentrations of components and possible impurities (humidity).
e) Measurement method including information about calibration of
instruments, carrier gases used, duration of experiment, reproduci-
bility of data etc.
f) Data evaluation and correlation, consistency tests, uncertainties of
data, discussion of possible systematic uncertainties of
measurements [0.26]. Example: gravimetric measurements using
microbalances may be influenced by drifts of the base line, i. e.
changes in the zero position of the data recording system of about
Here is the fictitious change of
sorbent mass over the time of observation corresponding
to the drift of the balance.

Useful information on measurement methods of standard thermodynamic
parameters like temperature, pressure, density of gases etc. can be found in
the literature [0.27, 0.28].

Thermal equations of state (EOS) of pure gases and gas mixtures are
represented in [0.29-0.30].

The standard method to measure pure gas adsorption equilibria most often
used today is the volumetric or manometric method, Chap. 2. Basically it is
the mass balance of a certain amount of gas partly adsorbed on the sorbent
material. This method can be realized in either open or closed systems, the
former ones often using a carrier gas, the adsorption of which normally being
neglected. Complemented by a gas analyzer (chromatograph, mass
spectrometer) this method also can be used to measure multicomponent or
coadsorption equilibria.

Volumetric measurements also can be combined with caloric
measurements. A special instrument allowing measurements of this type is
presented in Chap. 2, Sect. 5. It does not use thermocouples for temperature
measurements but instead a sensor gas, the temperature caused pressure
changes of which leading to time dependent signals allowing one finally to
determine the (integral and differential) heat of adsorption of the system.

The volumetric method has specific disadvantages discussed in Chap. 2.
More accurate and reliable measurements can be performed by weighing the
sorbent mass exerted to the gas atmosphere using a very sensitive
microbalance, preferently a magnetic suspension balance. This so-called
gravimetric method is presented in Chap. 3.