Page 78 - Adsorption by Powders and Porous Solids

C~~pTER METHODOLOGY OF ADSORPTION
3.

- controlled
temperature
surroundings

- sample cell

Figure 3.13. Schematic representation of a calorimeter.
~iathermaf-compensation calorimeters: here again, sample temperature follows
surroundings temperature (usually constant in adsorption experiments), but now
by means of a power compensation within the sample cell (i.e. Joule or Peltier
effect). This reduces the response time.
Isoperibol calorimeters: no special connection between sample temperature and
surroundings temperature and, generally, the latter is kept constant. This is the
conventional 'temperature rise', 'Thomsen', or 'Berthelot' calorimeter. The word
'isoperibol' was coined by Kubachewski and Hultgren (1962), from the Greek, to
mean 'with isothermal surroundings'.

Adiabatic adsorption calorimetry
Here, the heat evolved on adsorption increases the temperature of the sample and its
container (usually a copper cylinder). The heat is prevented from flowing to the
peripheral shield (the 'surroundings') by an appropriate control of the shield temper-
ature. Thus, the shield is usually maintained at the same temperature as the sample
container by the use of a differential thermocouple and a heat coil - as indicated in
Figure 3.14. The temperature rise is measured by means of a resistance thermometer
attached to the sample container.
Adiabatic calorimetry is particularly useful for the study of closed adsorption
systems at low temperatures (where radiation losses are small) and for temperature
scanning experiments. It is the preferred type of measurement for the determination
of the heat capacity of adsorption systems, especially in the temperature range
4-300 K (Momson et al., 1952; Dash, 1975). The temperature scan is obtained by
means of the Joule effect applied to the sample container: the sample heating coil
shown in Figure 3.14 is used for this purpose.
In some respects, adiabatic calorimetry provides information which is comple-
mentary to that provided by heat-flow calorimetry. The latter allows a study to be
made of the full composition range at constant temperature, whereas the adiabatic
calorimetry study is canied out over the prescribed range of temperature with a con-
stant amount of adsorptive in the adsorption cell (of course, this does not mean that a
constant amount is adsorbed). Adiabatic calorimetry allows direct measurements of
the heat capacities of adsorbed films, although they are difficult to make accurately