Transcript Mac

Experiment 8. Physisorption of Gases — The BET Isotherm
The vacuum system is
shown to right. The
manifold is isolated from
the vacuum pump by the
stopcock at far right. Gas
(N2 or O2) is admitted
through the adjacent stopcock and is stored in the
two bulbs. The sample cell
is at far left; its volume is
defined by the stopcock
isolating it from the
manifold. Pressures are
measured by the MKS Baratron gauge and read by the voltmeter (1.00 V = 100
Torr).
2
The manifold is shown in
more detail here. Its volume is
delimited by the five stopcocks
and includes the volume within
the P gauge head. You will
determine this volume by
expanding gas of known P from
a bulb of gas having known V
which is attached in place of the sample cell at far left. The
volume of the sample cell will similarly be determined by
expansion. Note that it will also be necessary to determined
the volume between the stopcock on the calibration bulb and
that on the manifold. A correct determination of these
volumes is essential to success in the experiment, which
amounts to (1) admitting gas to the manifold (usually
incrementing the existing P), (2) opening the stopcock to the
sample cell to give this gas access to the cold silica gel, and
(3) allowing the system to re-equilibrate.
3
In this experiment, it is very important
to exercise care in degassing the silica gel.
Expose it to vacuum stepwise rather than all
at once. Do this by sequentially closing and
opening the sample cell stopcock while
pumping out the manifold each time. Then
heat the silica gel gently with the heat gun.
It will appear to “boil.” Don’t let it
“bump,” or you may spray silica gel around
the manifold — not good for the
experiment! In this regard, be sure to install
the protective plug below the sample
stopcock to help prevent sudden pressure
surges and catch errant silica gel.