Melting temperature measurement
What is melting ?
The fusion of a chemical compound corresponds to a modification of its physical state: it passes from the slide state to the liquid state. The temperature at which this change occurs is called the melting temperature. This temperature almost does not depend on the ambient pressure. Conversely, the boiling temperature of a chemical compound significantly expends pressure.
How to measure a melting temperature?
There are several methods to determine the melting temperature of a sample.
One of the simplest is the visual observation of the sample during a temperature variation. The Kofler bench is an example of a device for visually measuring the melting temperature of compounds whose melting temperature is above room temperature.
Another method consists in placing a thermometer within the sample during the change of state. Indeed, the temperature of the sample is constant at the melting temperature throughout the process of change of state.
Nevertheless, the method most used experimentally is calorimetry. The technique uses the fact that the melting of a compound is accompanied by an absorption of heat by the sample which is measurable experimentally. Calorimetry makes it possible to measure the endothermic flow of heat which is absorbed by the sample at the time of its melting during a controlled heating ramp. The temperature at which this endothermic effect begins is the melting temperature.
Schematic example of a fusion thermogram. The melting temperature corresponds to the temperature at which the endothermic signal of melting begins. The Onset method is often used to determine the melting temperature. This is the temperature at which two tangents to the heat flow intersect: one before the start if a phenomenon, and the other during the phenomenon.