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www.thinkSRS.com/products/MPA.htm
Melting Point Determination
Application Note #1
Introduction
A few basic guidelines must be carefully followed to avoid errors during melting
point determinations with OptiMelt. The way in which the sample is prepared and
the instrument is programmed have the greatest influence on the accuracy and
reproducibility of a melting point measurement. Subjective interpretation of the
changes observed in the sample (visually and/or automatically) during the
analysis can also lead to unreliable results.
Background
The melting point of a substance is the temperature at which the material
changes from a solid to a liquid state. Pure crystalline substances have a clear,
sharply defined melting point. During the melting process, all of the energy added
to a substance is consumed as heat of fusion, and the temperature remains
constant.
A pure substance melts at a precisely defined temperature, characteristic of
every crystalline substance and dependent only on pressure (though the
pressure dependency is generally considered insignificant).
Determining the MP is a simple and fast method used in many diverse areas of
chemistry to obtain a first impression of the purity of a substance. This is
because even small quantities of impurities change the melting point, or at least
clearly enlarge its melting range. Melting point determinations are more than just
a classroom exercise in the organic chemistry lab. The test is still an important
technique for gauging purity of organic and pharmaceutical compounds.
The determination of melting points is one of the oldest identification and test
methods for organic substances. The melting point is easy to measure, tabulate
and classify. Extensive collections of tables give the exact values of many pure,
inorganic and organic compounds. The MP determination is a fast and cost-
effective technique which remains a strong link to the vast pre-instrumental
chemistry literature.
Capillary Method
The procedural rules for melting point determinations are defined in the
pharmacopeias. The medical handbooks include minimum requirements for the
Stanford Research Systems Phone: (408) 744-9040
Melting Point Determination www.thinkSRS.com/products/MPA.htm
design of the melting point apparatus and for performing the measurements.
Automated melting point determination procedures are generally included. Very
often, the pharmacopeias also list special methods for difficult or unusual cases
of melting point determination.
The pharmacopeias regard the capillary method as the standard technique for
melting point determination. In this methodology, a thin glass capillary tube
containing a compact column of the substance to be determined is introduced
into a heated stand (liquid bath or metal block) in close proximity to a high
accuracy thermometer. The temperature in the heating stand is ramped at a
user-programmable fixed rate until the sample in the tube transitions into the
liquid state. While determining a melting point, several observations and the
temperatures are recorded.
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