ORGANIC LABORATORY TECHNIQUES 4 4.1
MELTING POINT
The physical properties of a compound, such as melting point and boiling point can provide useful
information which can help in the identification of a sample or to establish its purity. These pages
describe two common methods for determining melting point using i a Meltemp apparatus and ii a Thiele
tube set up.
The temperature at which a solid melts and becomes a liquid is the melting point. Since this
requires that the intermolecular forces that hold the solid together have to be overcome, the temperature
at which melting occurs will depend on the structure of the molecule involved - an example of the
relationship between structure and properties. Hence, different compounds tend to have different melting
points.
A pure, nonionic, crystalline organic compound usually has a sharp and characteristic melting
point (usually 0.5-1.0C range). A mixture of very small amounts of miscible impurities will produce a
depression of the melting point and an increase in the melting point range. Consequently, the melting
point of a compound is a criterion for purity as well as for identification.
The melting point of an organic solid can be determined by introducing a tiny amount into a small
capillary tube, attaching this to the stem of a thermometer centred in a heating bath, heating the bath
slowly, and observing the temperatures at which melting begins and is complete. Pure samples usually
have sharp melting points, for example 149.5-150C or 189-190C; impure samples of the same
compounds melt at lower temperatures and over a wider range, for example 145-148C or 186-189C. .
It is standard practice (in order to make the most effective use of time) to carry out a rapid melting
point determination initially (by heating rapidly) to establish an approximate melting point and then carry
out at least two further careful determinations (by heating more gently, i.e. temperature changing only
about 2oC/min) until you obtain two consistent values.
The general method is to the heat the sample indirectly by placing the prepared sample (either
packed in a glass capillary or on a glass cover slip) in or on a heated medium, these days this is most
commonly a heated metal block such as a Mel-Temp apparatus. There are other designs such as the
Fisher-Johns apparatus. A more basic, but just as effective method is the Thiele tube method where the
capillary is immersed in a heated oil bath. Note that the Thiele tube system is also used for boiling point
determination.
IMPORTANT : Note that unlike boiling point, the melting point is relatively insensitive to pressure and no
pressure correction needs to be made.
