Kathleen Armstrong
Foothill College Chemistry

MELTING POINTS

The Mel-Temp apparatus

meltempTaking a melting point is very simple using a mel-temp apparatus. The solid is packed into the end of a small capillary tube, then inserted into the apparatus, where the melting can easily be viewed along with the temperature through a lighted, magnification lens.

The melting point is recorded as a range. The first temperature recorded is the temperature at which the first liquid droplet can be seen (not the temperature at which the first change is noticed). The second temperature recorded is the temperature at which all of the solid has melted. When comparing an experimental melting point to a literature melting point, always compare the top number, since literature values are typically obtained from highly purified crystalline compounds, and so their melting points will typically be somewhat higher than those obtained in an undergraduate laboratory.

It is very important that the rate of heating be kept slow when measuring a melting point. Once the approximate mp has been ascertained, aim for a rate of 1° C per minute. Any faster will lead to erroneous results, and point deductions.

For a video presentation on the melting point technique, click here.

 



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©Kathleen Armstrong March, 2009