Page 239 - Gas Adsorption Equilibria

4. Volumetric – Gravimetric Measurements 225

One may argue that for adsorption measurements of the type described
above, gas density measurements and hence a microbalance are not needed in
the device Fig. 4.20. This is true since measured values of the gas pressure
and the temperature (T) the density of the gas can be calculated from its
thermal equations of state (EOS). However, often neither pressure
measurements nor EOS available today in the open literature are accurate
enough to deliver precise and reliable numerical values of the gas density.
Hence we recommend – according to our experience – the use of a
microbalance to measure the gas density directly prior to and after the
experiment.

Preliminary measurements of the wall adsorption show that this
effect is becoming important at either very low pressures and in the vicinity of
saturation pressures for subcritical adsorptives. Results and respective data
will be published in a forthcoming paper.

4. PROS AND CONS OF VOLUMETRIC-
GRAVIMETRIC MEASUREMENTS OF BINARY
COADSORPTION EQUILIBRIA

In this section we will discuss the advantages and disadvantages of
combined volumetric-gravimetric and related densimetric-gravimetric and
densimetric-volumetric measurements of binary coadsorption equilibria
(N = 2). Basically, volumetric-gravimetric measurements (VGMs) combine all
advantages and disadvantages which occur in either volumetric or gravimetric
measurement procedures. As these already have been discussed in Chaps. 2
and 3 we can make only a few remarks reflecting specific experiences we
have had with VGMs on various binary coadsorption systems during the last
14 years.

4.1 Advantages

1. Automation

VGMs do not need adsorptive gas analysis by a gas chromatograph or mass
spectrometer but only include measurements of pressure and temperature of
the sorptive gas mixture and reading of a microbalance. Hence they easily can
be automated which already has been done by BEL Japan company, cp. Sect.
2.4, Figs. 4.11 a, b.