Page 167 - Gas Adsorption Equilibria

3. Gravimetry 153

in the pores of the molecular sieve or, equivalently, a shift of the critical
parameters (pressure, temperature) of the pore fluid compared to its values of
the bulk fluid phase [3.26].

3. THERMOGRAVIMETRY

Thermogravimetry, i. e. weighing material samples at elevated and
possibly changing temperatures today is the standard method to investigate
several kinds of thermally induced processes, especially desorption processes
of gases or vapors, i. e. volatile compounds or chemical decomposition
processes of solid samples [3.1, 3.8, 3.9]. By use of modern magnetic
suspension balances equipped with electric heating systems, a range of
pressure up to 50 MPa and temperatures up to 1250 K becomes feasible for
experiments – at least on principle [3.6]. At elevated pressures measurement
of the density of the gas surrounding the sample fixed to the balance causes
some problems. These can be solved using a sinker also fixed to the balance
and performing buoyancy measurements between the weightings of the
material sample. Details of this procedure are given in Chap. 4, Sect. 3.1.

Also at temperatures above 700 K thermal decoupling of the sample and
the permanent magnet inside the (hot) reaction vessel is mandatory.
Convection flows initiated by temperature gradients have to be suppressed or
at least reduced by appropriate flow guide tubes.

A list of commercial suppliers of thermogravimetric instruments is given
at the end of this Section, cp. Tab. 3.3, [2.3].

In Figure 3.22 a schematic diagram of a magnetic suspension balance is
given including a high temperature electric furnace which thermally is
decoupled from the magnetic suspension by a spacing tube (height: ca. 30 cm)
including tubes to reduce gas convection.