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6. Melting point determination
6.1 Terminology
The capillary technique is a standard method for determining the melting point of a substance. In this
method, thin glass capillary tubes containing prepared samples of a substance are introduced into the
heating block of a suitable device. The block is heated at a fixed rate over a temperature range that
brackets the expected melting temperature of the substance until the substance is completely melted
(clear point).
During the heating cycle the substance under test will often undergo a number of observable physical
changes:
1. Fine droplets adhere uniformly to the inside of the capillary tube.
2. A clearance between the sample and the capillary wall becomes visible.
3. The substance collapses and begins to liquefy.
4. A meniscus is visible at the sample surface but solid particles remains visible (meniscus point).
5. The sample is a clear liquid and no solid particles remain visible (clear point).
When determining the melting point of a substance it is necessary to select one stage in the melting
process which will be used to define the melting point. The meniscus point or clear point is commonly
used to define the melting point of a substance.
The
MP-800D
automatically reports the clear point, the point at which the sample is a clear
liquid and no solid particles remain, as the melting point of a substance.
Other changes in the physical appearance of the substance are sometimes observed, such as
sublimation (transition from the solid to gas phase with no intermediate liquid stage) or decomposition
(the separation of a substance into its elements or smaller compounds). These changes (if observed)
should be recorded manually.
Where a single melting point is difficult to define for a substance it is often useful to report a melting
range. The melting range of a substance is defined as the interval between the start of a melt and the
clear point.
6.2 Temperature Settings
The temperature settings used to determine a melting point are critical to the accuracy of the
determination.
Start or Plateau Temperature:
The start temperature is usually set between 5 and 10˚C below
the expected melting point of the substance.
End Temperature:
The end temperature is set at a temperature where all physical
changes in the substance are expected to be complete.
Ramp Rate:
The ramp rate is the rate at which the sample block is heated
during the melting point determination. For accurate
determinations a rate of between 0.5˚C/min and 2.0˚C/min is
recommended.
Ramp rate is the most important instrument parameter with regards to the accuracy of a
melting point determination. The use of fast ramp rates is the most common cause of
inaccurate results.
