How to Determine the Melting Point of a New Compound
Chemists often use the melting point of a chemical as a means to discover either its identity or its purity. This is because the melting point of a pure compound is constant, so finding a chemical's melting point can help determine what it is. As well, the melting point of impure compounds will be different from that of pure ones. Labs often determine the melting point of newly synthesized chemicals as part of basic routine characterization of the substance's physical properties. You can find the melting point of a new chemical using an apparatus that slowly heats a small portion of that substance while you observe it.
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Instructions
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Place a few grams of the substance in a clean, dry mortar and grind it thoroughly with a pestle until it is a fine powder. Scrape some of the powder into a small mound in the center of the mortar.
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Push the open end of a glass capillary tube down onto the mound, so that some of the chemical is forced into the tube. Hold the tube upright again and gently knock the lower end on a counter top until the chemical falls down to the bottom of the tube. Repeat this process until the bottom part of the tube is filled to a height of about 1/8 inch to 1/4 inch with the chemical.
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Power on the melting point apparatus and use the keypad to set a start temperature, end temperature and temperature ramp rate. Set a start temperature which is about 5 or 10 degrees Celsius below where you anticipate your chemical's melting point might be and an end temperature 10 or 20 degrees above this anticipated temperature. Set a fairly fast heat-ramp rate of 5 degrees Celsius per minute. Allow the apparatus to warm up to the start temperature.
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Place the capillary tube containing the sample in the sample well of the melting point apparatus and press the start button on the keypad. The apparatus will begin to ramp up the temperature of its internal oven and simultaneously heat up the sample.
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Watch the sample through the apparatus magnifying window. When the sample begins to soften, note down the temperature displayed on the apparatus digital readout or thermometer. This is the start of the sample's melting point range.
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Continue to watch the sample until it becomes completely liquid. At this point, again note down the temperature. This is the end of its melting point range.
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Repeat the above test using a new sample and capillary tube, but this time use the coarse melting point range you just obtained to adjust the apparatus start and stop temperatures. Set the apparatus to a slower temperature ramp of 2 degrees Celsius per minute.
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Watch the sample through the apparatus window as before and note down the start and end temperatures over which it melts. You have now obtained the melting point range for the new chemical. For greater accuracy, you can repeat the test two or three more times on different samples. Note that even a pure sample will normally melt over a range of 1 or 2 degrees Celsius, so a melting point range is usually reported rather than a single melting point.
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Tips & Warnings
Some modern melting point instruments have a built-in capability to recognize the melting point and will calculate and display this value for you.
Some older melting point instruments will not have a keypad to enter temperature set points and ramp. Instead, you will set a voltage to the heater. Consult the manufacturer's instructions to determine how to set the required temperatures.
Use caution when grinding the chemical. Some chemicals may produce toxic dust during this procedure or could even catch fire or explode as a result of friction. Also, be careful when heating new substances, even in small amounts. Chemicals can potentially catch fire, explode or emit toxic fumes when heated. Work in a chemical fume hood and wear appropriate protective equipment such as gloves and goggles during this entire procedure.
Use caution when handling capillary tubes after they have been heated.
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