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Fisher-Johns Melting Point Apparatus

The picture below shows a Fisher-Johns melting point apparatus. This type of melting point apparatus uses small round, glass coverslips.

Technique for Taking a Melting Point

Place a small amount of crystals on a coverslip.
Place a second coverslip on top.
Place the compound-coverslip sandwich on the round heating block.
Look through the lens above the heating block. Follow the directions given below to determine an appropriate heating rate.
When done, place the used coverslips in the used coverslip recycling jar behind the instrument.
This aluminum cylinder is used to quick-cool the heating block between uses. Simply move the lens out of the way and place it on the round heating block when you are finished.

The rate of temperature increase in the vicinity of the melting point must be small, about 2°C per minute. This insures that the temperature of the hot plate, thermometer, and sample will be in thermal equilibrium. Increase the temperature rapidly at first and then slowly as the melting point is approached in the following manner:

  1. Set the heating control at 100.
  2. When the temperature is about 15 degrees below the anticipated melting point, change the setting to that indicated on the graph below.
  3. Observe the crystals through the lens and record the temperatures at which melting begins and at which the last crystal disappears.
  4. If you do not know the melting point of a compound, first take a crude melting point by heating rapidly. Then cool the plate to 20° below the crude melting point by placing the aluminum cylinder on it, and proceed to take a more careful melting point on a second sample of the compound.

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