Page 219 - Gas Adsorption Equilibria

4. Volumetric – Gravimetric Measurements 205

3. DENSIMETRIC – GRAVIMETRIC
MEASUREMENTS OF BINARY COADSORPTION
EQUILIBRIA

3.1 Experimental

The magnetic suspension balance in Figure 4.4 can be operated in two
different positions. In the first one – the so-called zero point position – only
the permanent magnet at top and the stem below of it are floating whereas the
basket including the sorbent material is mechanically decoupled from it so that
its weight is transmitted via a load decoupling system to a mechanical support
and from there to the instrument. This position is used to tare and calibrate the
microbalance outside the suspension. In the second position the permanent
magnet and the stem are lifted somewhat so that the basket including the
sorbent material is decoupled from its support and also is floating. This so-
called “measuring position (1)” of the suspension is used to determine the
weight of the sorbent material reduced by its buoyancy in a fluid i. e. gaseous
or liquid environment. Now, in 1995, it has been proposed by the authors to
add to these two positions of the suspension a third one by lifting the
permanent magnet, stem, and primary load somewhat further and by this to
make a second load coupled similarly as the first one to the floating stem to
float and thus allowing one to measure the weight of the sum of both loads in
this “measuring position (2)”. This would allow consecutive weighing of two
masses in the same gaseous environment. Choosing as masses two different
(or differently activated) sorbent materials, this could double the measuring
capacity of the balance or reduce the time needed for gravimetric
measurements by 50 %. Choosing as masses a sorbent material and a dense
sinker of exactly known volume this would allow simultaneous measurements
of the reduced mass adsorbed in the sorbent and the density of the sorptive gas
A simplified schematic diagram of such a suspension is sketched in
Figure 4.12. Working positions (0), (1), (2) are marked from left to right.

Following a suggestion of the authors a magnetic suspension allowing 3
working positions was built by RUBOTHERM GmbH during (1996-1997)
and installed in the volumetric-gravimetric coadsorption instrument Figs. 4.4,
4.5 in the lab PB-A 0126 of the Institute of Fluid- and Thermodynamics at the
University of Siegen early in 1998. First measurements of weights of
sorbent/sorbate samples and simultaneously of the density of the sorptive gas
were performed on 11 February 1998, cp. snapshot Fig. 4.13 below. Data are
presented in Sect. 3.3.