Experiment 8. Physisorption of Gases

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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 stop-cock 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).
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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.
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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. Dont 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.