<strong>Microscale Method.</strong> In microscale experiments, there is often too little product available to use the semimicroscale
method described above. However, the method can be scaled down in the following manner. The liquid is placed in a 1-mm melting-point capillary tube
to a depth of about 4-6 mm. Use a syringe or a Pasteur pipet that has had its tip drawn thinner to transfer the liquid into the capillary tube. It
may be necessary to use a centrifuge to transfer the liquid to the bottom of the tube. Next, prepare an appropriately-sized inverted capillary, or
<strong>bell.</strong>
The easiest way to prepare a bell is to use a commercial micropipet, such as a 10-μL
Drummond "microcap." These are available in vials of 50 or 100 microcaps and are very inexpensive. To prepare the bell, cut the microcap in half
with a file, or scorer and then seal one end by inserting it a small distance into a flame, turning it on its axis until the opening closes.
If microcaps are not available, a piece of 1-mm open-end capillary tubing (same size as a
melting-point capillary) can be rotated along its axis in a flame while being held horizontally. Use your index fingers and thumbs to rotate the
tube; do not change the distance between your two hands while rotating. When the tubing is soft, it is removed from the flame and pulled to a thinner
diameter. When pulling, keep the tube straight by <em>moving both your hands and your elbows outward</em> by about 4 inches. Hold the
pulled tube in place a few moments until it cools. Using the edge of a file or your fingernail, break out the thin center section. Seal one end of
the thin section in the flame; then break it to a length that is a bout one and one-half times the height of your sample liquid (6-9 mm). Be sure the
break is done squarely. Invert the bell (open end down), and place it in the capillary tube containing the sample liquid. Push the bell to the
bottom with a fine copper wire if it adheres to the side of the capillary tube. A centrifuge may be used if you prefer. Figure 13.4 [sorry, I don't
know where my scanner is right now...] shows the construction method for the bell and the final assembly.
Place the microscale assembly in a standard melting-point apparatus (or a Thiele tube if
an electrical apparatus is not available) to determine the boiling point. Heating is continued until a rapid and continuous stream of bubbles emerges
from the inverted capillary. At this point, stop heating. Soon, the stream of bubbles slows down and stops. When the bubbles stop, the liquid
enters the capillary tube. The moment at which the liquid enters the capillary tube corresponds to the boiling point of the liquid, and the
temperature is recorded.
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