Unformatted text preview: METHODS AND M ANIPULATION 65 Distillation under reduced pressure. The student should first
read details of this operation on a macro-scale (p. 28). For micro-scale
work the apparatus shown in Fig. 40 is very convenient. A small pear- FIG. 41(D). shaped flask of 5 ml. capacity is fitted w ith a small side-tube through
which a piece of glass tubing can be passed. The latter is held in
position by a r ubber b ung,* and is drawn out into a fine capillary tube
to act as an air-leak. The upper end of the
tubing is fitted with pressure tubing and a
screw-clip to regulate the flow of air. Distilling flasks of alternative design are shown in
Figs. 4 i(A) and 41(6).
With a single receiver tube as shown in
Fig. 40 it is necessary to interrupt the distillation each time the receiver is changed. For
more precise work a multiple receiver as
shown in Fig. 42 should be used. The lower
part L of this apparatus contains 6 small
collecting tubes C, eachf supported in a wire
hoop attached to a fixed vertical glass rod.
The receptacle L is attached to the upper part
of the apparatus by a ground-in glass j oint, J,
carefully l ubricated by silicone grease. By
gently rotating the receptacle L about the
ground-glass joint J, it is possible to collect
the d ifferent fractions in the separate tubes
without stopping the distillation, and hence
without alteration of pressure.
* The type of a pparatus shown in Fig. 40 can be
obtained with ground-in glass joints thus obviating
the use of rubber bungs.
f Each t ube C s hould have an i dentification m ark, so t hat the b .p./pressure
of the c ontents of each can be recorded d uring the d istillation. ...
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