Tuesday, November 13, 2012

Identification and Characterization of an Unknown Solid Using Melting Point and Mixes MP

YOUR SAMPLE FROM THE RECRYSTALLYZATION EXPERIMENT will be analyzed by melting point and mixed melting point. A sample must be dry (free of solvent) before an accurate melting point can be established.

Melting point may be used to determine the purity of a compound based on its melting range or to determine the identity of an unknown given a source of known compounds. An impure compound melts lover than expected and has a larger than two degree range. A higher mp indicates a completely different compound may be present.

The melting point of a solid is really a melting range, the sample should be heated at 1-2 degrees of Celsius  / min. the first drop of liquid is the start if the range and the temperature at which total melting occurs is the upper limit of the range. It is best to take two melting points of an unknown; a quick melting point with heating 5-10 degrees of Celsius / min to get in the area of the real mp. The second, more accurate mp, should be done slowly at 1-2 degrees of Celsius / min per near the mp. Use a new sample each time and be sure the apparatus has cooled to below the mp between runs. The sample cannot be reused because decomposition may have taken place during the first heating. The sample is placed in a capillary tube with one end closed. Fill with about 2-3 mm of crystals. Place the open end of the capillary over the crystals and push onto the crystals. Then turn the capillary over and tap on the lab bench until the crystals fall to the closed end.

When you have an idea of the identity of your compound you should carry out a mixed melting point with the authentic material that is available in the laboratory. To do this, mix a small amount of your unidentified material with the same amount of known material. Determine the mp of the mixture. Of the two compounds are the same, there should be no change in melting point.

However, if the unknown is something else, you will see a depressed mp with a broader range. Why? Try using the wrong compound intentionally. Simply comparing the mp of an unknown with the literature value is incomplete evidence to identify an unknown. Many organic compounds have identical melting points.

You will determine many melting points throughout the semester. Be sure you understand the technique now. Always make sure youy compound is dry (free of water) and free of solvent before putting it in the mp capillary. It will not dry in the capillary and you will get erroneous results.


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