Page 81 - Adsorption by Powders and Porous Solids

66 ADSORPTION BY POWDERS AND POROUS SOLIDS

taken into account:
The adsorptive must be carefully brought to the temperature of the microcalorime.
ter before entering the adsorption bulb.
It must also be introduced very slowly, so that the heat effect corresponding to the
gas compression in the calorimeter may be calculated accurately, as explained
by Rouquerol and Everett (Rouquerol et al., 1980). This also helps to meet the
previous requirement of efficient adsorptive pre-cooling or pre-heating.
The outgassing must be carried out carefully, bearing in mind that the adsorption
bulbs used in a microcalorimeter usually have much longer necks than standard
adsorption bulbs. Also, this arrangement may drastically change the actual resid.
ual pressure in the immediate vicinity of the sample during outgassing.

Diathermal-compensation adsorption calorimetry
At a time when the means for recording and integrating the signal from a heat
flowmeter were of limited accuracy, Tian (1923) proposed a way of compensating
the major part of the heat liberation by a steady Peltier or Joule effect taking place
just against the sample. This idea was refined and adapted to adsorption experiments
by Kiselev and his co-workers (Dzhigit et al., 1962). By means of a continuous Joule
effect in the vicinity of the adsorbent, they maintained a constant temperature differ-
ence AT through the heat flowmeter. As soon as adsorption produced an evolution of
heat, the Joule effect was interrupted discontinuously, just enough to keep AT
unchanged. The heat evolved on adsorption was simply derived from the sum of all
non-heating periods. This approach was no longer used when the quality of the
recordings allowed an accurate integration. Nevertheless, it would be worthwhile
updating it (with proportional, integral and differential - PID -control and with inte-
gration of the variable Joule power) with the objective of speeding up the experi-
ments, since this in situ power compensation shortens the time needed to reach the
desired thermal equilibrium. The same can be done using a Peltier effect (keeping
AT = 0).

Isoperibol adsorption calorimetry
The first experiments of gas adsorption calorimetry by Favre (1854) were made
with an isoperibol calorimeter. More recently, refrnements were introduced by
Beebe and his co-workers (1936) and by Kington and Smith (1964). Because of the
uncontrolled difference between the temperature of the sample and that of the sur-
roundings, Newton's law of cooling must be applied to correct the observed tem-
perature rise of the sample. In consequence, any slow release of heat (over more
than, say, 30 minutes), which would produce a large uncertainty in the corrective
term, cannot be registered. For this reason, isoperibol calorimetry cannot be used to
follow slow adsorption equilibria. However, its main drawback is that the experi-
ment is never isothermal: during each adsorption step, a temperature rise of a few
kelvins is common. The corresponding desorption (or lack of adsorption) must then
be taken into account and, after each step, the sample must be 'thermally earthed'
so as to start each step at the same temperature. In view of these drawbacks,