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 aremeasured 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.” 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.