Measurement of enthalpy of fusion
What is fusion?
The fusion of a compound corresponds to a change in its physical state: it passes from the solid state to the liquid state. The reverse transformation is called solidification or sometimes crystallization. This fusion is accompanied by an absorption of energy called enthalpy of fusion. It is often expressed in J/g, or J/mol when dealing with pure compounds.
Why determine the enthalpy of fusion?
When a chemical compound or mixture is used in industry, it is important to know its physical state as a function of temperature and pressure. This determination consists most of the time in establishing the phase diagram of the sample in the temperature and pressure range where it will be used.
How to measure the enthalpy of fusion?
The integration of the heat flux during the melting peak makes it possible to obtain the enthalpy of fusion, expressed in J/g or J/mol in the case of pure substances.
The enthalpy of fusion measured by calorimetry, and most often by DSC (differential scanning calorimetry). For this, a few milligrams of sample are placed in a crucible placed in the device. Depending on the physical state of the sample at room temperature, a first cooling step may be necessary to solidify the sample. After that, the sample undergoes a controlled temperature rise.
Without a change of state, the heat flux measured during a temperature ramp corresponds only to the heat necessary for heating the compound (its heat capacity). When the melting point is reached, a large endothermic heat flux is measured. It corresponds to the enthalpy of fusion of the sample. The location of this enthalpy peak is determined by the onset method, and defines the melting point, or melting temperature (see figure opposite). This technique is much more precise and reproducible (0.5°C). The integration of the enthalpy peak obtained makes it possible to determine the enthalpy of fusion of the sample.