W. H. Chan Hong Kong Baptist College, 224 Waterloo Road, Kowloon, Hong Kong Vacuum distillation is one of the fundamental techniques for advanced organic chemistry. When more than one fraction has to be collected from a vacuum distillation, i t becomes necessary to use either a fraction cutter or a cow receiver. However, the high cost of a fraction cutter (usually in the range of US$400-500) severely limits its use in undergraduate laboratorv teachine. On the other hand. a cow ieceiver, which is cheaper, can accommodate only three to four fractions and is quite troublesome to set up. We describe here a homemade fraction cutter which can he easily assembled with standard laboratory glassware as shown in . . the figure. At the beginning of vacuum distillation stopcock A of the addition funnel is open. (To cut down the dead volume of the system, we suggest using a 10-mL addition funnel.) The system is evacuated through stopcocks B and C by a vacuum pump while the distillation mixtures are heated up slowly. The oressure of the svstem can be monitored bv a manometer rinnerted to the sidearm of stoprock R.7'0-change fraction, atupcock A is first closed, then stopcock C is switched to the atmosphere to release the vacuum inside the rereiving flask before it is removed. Since the rest of the svstem isstill connected to the vacuum pump throughstopcoik B,distillation will not be disturhed with distillate temporarily collected in the addition funnel. T o evacuate thehewlyinstalled receiving flask, stopcock B is shut off momentary and stopcockC is adjusted in line with the vacuum pump again. Until the vacuum is restored in the receiving flask, stopcock B is switched back to the vacuum line, then stopcock A is open for the collection of new fraction. A stepwise operation pro-
cedure of this vacuum distillation system is summarized in the table. The procedures can be repeated for the collection of different boiling point fractions. T o our experience, a smooth distillation can be achieved by using the set up with minimal pressure disturbance during the change of receiving flask.
Vacuum Dlstlllatlon Operatlon Procedure Stopwck B
Stopcock A system
Collecting distillate
open
Changlng receivingflask (I) to remove collected fraction
(2) to evacuate new receivingflask
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Journal of Chemlcal Education
close
-00
Stopcock C
vacuum
recslving flask
c@+ I
vacuum
