Download Low pressure effusion of gases

Chem 367-2/ Low Pressure Effusion
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LOW PRESSURE EFFUSION OF GASES
(S&G 4th ed. Experiment 6)
The description of this experiment has disappeared from the more recent
editions of S&G. Ask the instructor for a copy of the 4th edition. A copy
can be downloaded from the course web site.
Before starting this experiment, read the description in S&G, together
with the section on vacuum technique in the text and the section “Use of
Vacuum Apparatus” in one of the Appendices of the present manual. Be
sure that you understand the principles of operation of McLeod and Pirani
gauges. The data for this experiment may be collected with the help of a
computer equipped with the proper hardware; check with the instructor for
the availability of such equipment. Alternatively, the data may be collected
with a storage DVM (digital voltmeter).
1. Safety
Wear eye protection at all times in the laboratory, but particularly when
working next to a glass vacuum line. Wear thick protecting gloves when
handling liquid nitrogen.
2. Vacuum Line
A schematic drawing of the vacuum line used for this experiment is shown
in Fig. 1.
The physical arrangement is somewhat different from the one depicted in
S&G; in particular, an oil diffusion pump is used in the lab rather than a
mercury diffusion pump. Also most of the valves in the present set-up are
O-ring greaseless valves; make sure you understand how these devices
function.
• Cold traps. Before starting the experiment, fill the cold traps with
liquid nitrogen.
• O-ring greaseless valves. Turn open (counter clockwise) a greaseless
valve only the necessary amount (if opened too much, the teflon
barrel will eventually come out, resulting in a massive vacuum leak!).
When closing the valves (clockwise), again tighten only the necessary
amount; over-tightening may result in breaking the glass sleeve.
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Chem 367-2/ Low Pressure Effusion
3. Calibration
The Pirani gauge voltage reading * should be calibrated against the McLeod
gauge at several pressures in the range of 0.001 to 0.05 torr (1 torr =
1 mm Hg = 133.32 Pa); do not forget to record the value of the McLeod
gauge constant (and its units). The gauge presently in use has a non-linear
response, and must be calibrated for each new gas which is to be used. The
Pirani gauge will not read zero at infinite time, due to the presence of Hg
vapour in the manifold. Make an appropriate correction for this when doing
your calculations.
Figure 1. Schematic drawing of the vacuum system used for the low pressure
effusion of gases; the valve labelling attempts to be consistent with the labelling
used in S&G. The straight arrows point to the pumping direction.
The calibration of the Pirani gauge consists in introducing some gas in the
vacuum line, measuring its pressure with the McLeod gauge † and recording
*
Make a note of the unit displayed by the particular DVM used, i.e. mV or V.
†
Documentation on the operation of the McLeod gauge can be downloaded from the
course web site.
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Chem 367-2/ Low Pressure Effusion
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with a digital voltmeter the corresponding output of the Pirani gauge driver.
This is repeated for a series of decreasing pressures in order to construct the
calibration curve pressure versus voltage output. For this calibration, six to
eight pressure points in the range ≈ 0 to 60 mTorr (≈ 0 to 70 mV on the
readout) are sufficient
4. Experiment
The actual effusion run consists in introducing the gas to be studied (N2 )
into the effusing bulb and recording the change in pressure as a function of
time; actually, what is being recorded is the voltage output of the Pirani
gauge, the which output can be converted to pressure using the calibration
curve constructed in the first part of the experiment.
Be sure that the manifold pressure, as indicated by the McLeod gauge, is
less than 10−4 torr before starting an effusion run. Carry out two effusion
runs with N2 gas only, one run at a starting pressure of ≈ 60 mT and the
second at a starting pressure of ≈ 30 mT.
Record the volume of the flask (plus gauge) tagged on the bulb, and
records the temperature next to the effusing bulb before leaving the
laboratory. Do NOT switch off anything at the end of the experiment
(vacuum lines are best when left pumping all the time).
5. Data processing
To get the calibration curve (pressure as measured by the McLeod gauge
versus Pirani gauge voltage reading), fit the experimental data to a
polynomial of appropriate degree. The data are most conveniently processed
with the help of a least squares fitting program and/or a spreadsheet *. If
your effusion curve (the log plot) shows non linear behaviour, use the initial
slope of your log plot (slope of the tangent at the origin) to obtain the
effusion rate. Using the known molecular weight of the effusing gas,
nitrogen, from your data, determine the diameter of the pinhole.
Answer the questions in the text book. In particular to calculate the
minimum pumping speed required to maintain a pressure Pf = 10 −4 Torr in
the manifold, using the perfect gas law, express the pumping speed (l s −1 ), at
this manifold pressure, S = dV/dt, as a function of the effusion rate (number
of mole s −1 dn/dt at the maximum effusion pressure Pbulb ). The final result
should be expressed in l s −1 at the relevant manifold pressure.
Hand in with your report an electronic copy of your results entered in the
spreadsheet EFFUSION.XLT.
*
An EXCEL template (EFFUSION.XLT) is available from the course web site to help you
with some of the data processing. However, it is strongly recommended that you try first
to perform the calculations yourself to understand how to use properly the spreadsheet.
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Chem 367-2/ Low Pressure Effusion
6. Discussion
Construct a theoretical pressure-time curve for an equimolar He/N2 mixture
in this system (remember that gases effuse independently of each other).
Determine whether this can be distinguished within experimental error from
the curve for pure CH4 .
7.
References
D. P. Shoemaker, C.W. Garland, J.I. Steinfeld and J.W. Nibler,
Experiments in Physical Chemistry, 4th ed. McGraw-Hill, New York (1981).
Keithley Model 175 data logger operation instructions
If the computer interfacing with the DVM is not available, the Keithley 175
can be operated as a data logger and can store up to 100 readings at a preselected sampling rate.
• To select the sampling rate, press and hold the STO/CLR pushbutton;
the code (r = 1 or 2 or ...) for the different sampling rates will keep
scrolling on the display as long as the STO/CLR button is held
pushed. To select, release the button. For the present experiment, a
sampling rate of 1 reading/min (r = 3 on the display) is sufficient.
• To start logging data, press STO/CLR once; the STO enunciator will
show on the display.
• To stop logging data, press the STO/CLR button; the STO enunciator
will disappear.
• To retrieve the data, press and hold the RCL button. The display will
scroll through the stored data (and the HI/LO values of the data set).
Release the button at the desired data point and record the
corresponding value. To increment the data pointer to the next point,
push the RCL button once.
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