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Capillary Tube
Measuring melting point in the lab generally starts with a thin fine glass tube called a capillary tube, which is closed at one end and open at the other. By dabbing the capillary tube into the sample of the compound, you force some of it into the open end; tapping the closed end gently on the surface of your bench forces the compound down through the tube. You do have to be careful to avoid breaking it, because capillary tubes are fragile.
Mel-Temp
Once you've loaded the capillary tube with a sample of your compound, you're ready to test the melting point. One common approach employs a Mel-Temp apparatus, a small, simple device with several slots where capillary tubes can be inserted. By turning a dial, you can choose the rate at which the temperature rises while watching the end of the capillary tube through an eyepiece in the side of the device. As soon as you see the sample is beginning to melt, you turn off the heat and record the temperature.
Fischer-Johns
Another common device that measures melting point is the Fischer-Johns apparatus, which holds a piece of crushed sample between two thin microscope coverslips instead of a capillary tube. Just as with the Mel-Temp, you can supply heat at a chosen rate, monitor the temperature and watch the sample through an eyepiece to see when it starts to melt. You should always turn the heat off after the sample melts, because the Fischer-Johns apparatus typically relies on a mercury thermometer, and the high heat could alter the thermometer's calibration.
Mixed Melting Point
Impurities reduce the melting point of a sample. In some cases, this can actually be an advantage. If you're pretty sure your compound is one of several that have the same or relatively close melting points, you may be able to rule out some of your options with a mixed-melting point test. First, you combine the unknown with a little of one of the known compounds, then you test the melting point of this mixture. If the melting point of the mixture is significantly less than the melting point of either the known or the unknown by itself, you know the known and unknown are different compounds. If you mixed your unknown with acetanilide, for example, and the mixture melted at 95 degrees C., you would know your unknown is not acetanilide, because acetanilide by itself melts at 114 degrees C.