Page 303 - Gas Adsorption Equilibria

6. Impedance Spectroscopy 289

Today impedance measurements are used in many fields of science and
engineering, among them biosciences and medicine to analyze and
characterize complex material structures [6.5-6.7]. * )

The material presented in this chapter is organized as follows: In Sect. 2
the basic experiments necessary for dielectric permittivity measurements is
given, followed by an outline of the theory of dielectric polarization, which
considers the uncertainties of measurements and gives several examples
related to gas adsorption equilibria on microporous solids. In Sect. 3
combined dielectric-manometric and dielectric-gravimetric measurements of
adsorption equilibria are considered briefly. In Sect. 4 the pros and cons of
dielectric measurements are discussed. A list of Symbols used is given in
Sect. 5 followed by the references cited.

2. DIELECTRIC MEASUREMENTS OF GAS
ADSORPTION SYSTEMS

2.1 Experimental

An instrument to measure the dielectric permittivity of a gas adsorption
system basically consists of an electric capacitor (plates, cylinders, spheres)
placed within an adsorption vessel. The vessel should be placed within a
thermostat (water, oil etc.) and provided with tubes for gas supply and
evacuation. Also manometers and thermometers are needed to measure the
gas pressure (p) and temperature (T) inside the chamber. The capacitor is
filled with sorbent material (powder, pellets, continuous matter etc.) which
can be considered to be homogenous as long as its characteristic length – for
example the diameter of cylindrical pellets – is small compared to a
characteristic length of the capacitor.

The capacitor is connected to an impedance analyzer (Hewlett Packard HP
4192 A) which allows the application of an alternating voltage
(5 Hz - 13 MHz) to measure either the static or the complex impedance and /
or capacitance of the sorbent / sorbate system. A schematics of such an
instrument is sketched in Figure 6.1 for a plate capacitor. A photo of a
cylindrical capacitor which has been used for many years at IFT is presented
in Figure 6.2.

* ) Information on new developments in the field can be found in the “Dielectric Newsletter”
edited by G. Schaumburg, Novocontrol GmbH, 56414 Hundsangen, Germany,
www.novocontrol.com.