Common techniques: Melting point

Laboratory techniques manual
Common techniques

Melting point

Taking the melting point of a sample and contrasting it with its literature value is a straightforward and effective method of characterising it and determining its purity; as a general rule of thumb, a 1% impurity will lower the melting point of a sample by 0.5 °C. The electrically-heated-block apparatus that are typically used for melting point determinations in the lab have three sample holes and the only real problem ever encountered with this type of equipment is broken capillary tubes in the heating block.

Capillary tubes
Capillary tubes are supplied either with one sealed end, two sealed ends or two open ends. To seal an end just touch the outer layer of flame from a microburner.

Sealing capillary tube
Figure B.4 Sealing a melting point capillary tube

Melting point determination
Put about 2-3mm of sample in the bottom of the capillary tube and put the tube into the heating block. With the 'boost' on raise the temperature to about 20 °C below the actual melting point (if an approximate value is known). Turn off the 'boost' and continue to raise the temperature. The 'melting point' is the range from the appearance of the first liquid droplet until complete melting of the crystals. If you do not know what melting point to expect you will have to carry out a rough determination before determining the real melting point.

Have a go at determining the melting points of aspirin and caffeine using the following simulation:

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