Page 136 - Gas Adsorption Equilibria

122 Chapter 3

Here the main advantage of gravimetry compared to manometry can be used,
namely that it is possible to observe the approach to equilibrium during the
adsorption process at the balance’s data display. Therefore, one can choose
arbitrarily a certain small fraction of the pure sorbent sample’s mass
and a certain time interval and define technical adsorption
equilibrium of the system if the change of the balance recording is less than
within Actually, modern microbalances do have such a criterion
incorporated in their software system. But the experimenter is well advised to
check what the values of and in his balance system actually are,
not only to be aware of them but to change them appropriately if necessary.
After monitoring all relevant data like temperature, pressure, microbalance
recording and activation procedure, the gas pressure can be changed in order
to perform sequential pressure step experiments or the system can be
evacuated to check the mass of the sorbent sample again. If it has changed –
normally somewhat increased – the experiment should be repeated unless one
can give sound physical reasons for the remnant adsorption observed or rely
on experimental experience already gained with the sorbent material used.
Indeed remnant or permanent adsorption occurs in nearly all types of sorbent
materials if they are exposed to helium for the first time in order to determine
their so-called helium-volume. Examples are given in Sect. 2.3, cp. [3.21].

2.1.2 Theory

To elucidate the physics of gas adsorption measurements using a two
beam balance we consider the scheme sketched in Fig. 3.3 below. It shows the
two beams of the balance with an electromagnetic coil in its center and
equipped with two baskets containing a sample of the sorbent material
and a ballast or tare of approximately the same mass respectively.

The electric current in the electromagnetic coil is chosen such that the
resulting mechanical moment of the magnetic forces keeps the beam
exactly horizontal. Then the balance of moments is

Here and are the mechanical moments of the gravity forces of the
beam, wire, and basket of the balance on its sorbent sample site (s) and ballast
or tare site (k) respectively. Likewise and indicate the moments of the
buoyancy forces of beam, wire, and basket on the sorbent site (s) and ballast
or tare site (k) respectively. The other quantities and parameters in Eq. (3.1)