Page 143 - Gas Adsorption Equilibria

3. Gravimetry 129

magnitude better than results of volumetric / manometric measurements, cp.
Chap. 2.

2.2 Single Beam Balances

2.2.1 Experimental

For measurement of adsorption equilibria of corrosive gases like
etc.) on porous materials like catalysts etc. decoupling of the
microbalance from the sorptive gas is mandatory. This can be achieved by
using a magnetic suspension, i. e. a magnetic field acting on a permanent
magnet which is coupled by a suspension rod to the sorbent load in order to
keep it in balance against gravity, i. e. to make it float freely in the sorptive
gas atmosphere. A schematics of such an instrument is given in Figure 3.4
followed by a photo taken in lab. PB-A0126/1 of IFT in 2002, Fig. 3.5. The
sorbent sample is placed in a basket (stainless steel wire cloth) provided with
a permanent magnet, coupled via an external magnetic field penetrating the
adsorption vessel’s walls to an electromagnet. The electric current in the
electromagnet is always chosen such that the permanent magnet, basket, and
sorbent sample (suspension) floats freely inside the adsorption vessel. In fact,
*)
in today’s magnetic balances the suspension is oscillating at high frequency
(ca. 10 kHz) with small amplitude around its mean position. Hence, in
practice it can be considered to be at rest. As the current necessary for stable
flotation of the suspension can be measured, its weight can be determined
and, after appropriate calibration and vacuum measurements, the mass of
sorptive gas adsorbed by the sorbent sample, cp. Sect. 2.2.2, [3.3, 3.4].

The procedure for pure gas adsorption measurement using the installation
of Figs. 3.4, 3.5 is basically the same as with two beam balances which
already has been described in Sect. 2.1.1. Nevertheless some additional
remarks reflecting more than 10 years of practical experience with magnetic
suspension balances (MSBs) seems to be appropriate:

1. MSBs offered today are fairly stable against small oscillations or motions
of the basement, floor, or laboratory’s table where they are mounted. The
same is true as far as sneezing of the experimenter is concerned. However,
one is well advised to avoid any unnecessary mechanical disturbance of a
MSB installation as for example pneumatic (compressed-air) hammer
activities in nearby laboratories etc.

* ) Rubotherm GmbH, Bochum Germany. System’s prize (2004) (subject to change):
US $ 40,000.-