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

Place a small amount of crystals, less than one mg, between two coverslips. Place the coverslips on the hot plate of the melting point apparatus.

• details of filling and placing the coverslips

The rate of temperature increase in the vicinity of the melting point must be small, about 2°C per min. 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 linked in below.
3. Observe the crystals through the lens and record the temperatures at which melting begins and at which the last crystal disappears.

• graph for Fisher-Johns

When 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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