Page 199 - Gas Adsorption Equilibria

4. Volumetric – Gravimetric Measurements 185

likewise step down) procedure more often than 3 – 4 times, cp. remarks in
Chaps. 2 and 3.

The instrument Figs. 4.1, 4.2 is subject to several severe restrictions which
we would like to discuss in brief. First, all two beam balances commercially
available today [2.3, 4.5], are designed such that the sorptive gas contacts the
inner region of the balance including parts like electromagnetic coils which
are highly sensitive to corrosion. Hence these balances only can be used for
non-corrosive sorptive gas mixtures including components like (He, Ar,
(CO)). Of course a permanent flow of inert gas could be used to cover
and thus to protect the microbalance region. But this technique, though in
former days having often been used in traditional thermogravimetric
experiments, has not proved to lead to reproducible and accurate results as it
seems to be difficult to attain stable equilibrium conditions in systems
including a permanent gas flow contacting a microbalance [2.3, 2.4, 4.5]. Also
traditional microbalances normally can operate in fairly narrow ranges of
temperature and pressure – usually about ambient conditions – only. Hence it
would be desirable to extend these ranges of temperature and pressure and
also to allow corrosive gases like An instrument
which can do this is provided by the magnetic suspension microbalance
*)
already mentioned in Chap. 3 (2.2). We will give more details about this in
Sect. 2.4 and Sect. 3 of this chapter.

2.2 Theory

The masses of a two component adsorbate can be determined
from the results of a volumetric-gravimetric experiment, namely the gas
pressure (p) in the system, the temperature (T), and the microbalance
recording as follows: We first denote the mass balance equations for each
component

the thermal equation of state (EOS) of the sorptive gas mixture with masses
i = l,2)

* ) Manufacturer: RUBOTHERM Praezisionsmesstechnik GmbH, Universitätsstr. 142,
D-44799 Bochum, Germany, www.rubotherm.de