In order to identify or assess the quality of a substance or mixture of substances, both its chemical and physical properties, such as thermal values, can be studied. In this guide, we would like to share with you the know-how METTLER TOLEDO has gained over the last 50 years in the area of determination of thermal values. We will explain the relevant techniques, as well as standards that apply to the following physical properties, along with tips and hints for assay sample preparation:
- Melting point
- Boiling point
- Cloud point
- Slip melting point
- Dropping point
- Softening point
If you would like more information on specific applications of these techniques, please visit our Application library.
The melting point is the temperature at which a phase transition from solid (1) to liquid (3) takes place (see Fig. 1), and provides an indication of a substance's purity: pure substances normally melt over a range of only 1–2 °C, while impurities broaden the melting range. The melting point can therefore be used to evaluate substances' quality and is widely used for their characterization and identification.
Melting process of a crystalline substance
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The boiling point of a chemical compound is the temperature at which a phase transition from liquid to gas occurs under normal conditions. It is a substance-specific property that can provide useful information about the identity and purity of a substance, and is often used to select an optimal process temperature – such as determining ideal storage conditions. Knowledge of this property is also required for Material Safety Data Sheets (MSDS).
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The cloud point of a solution corresponds to the temperature above or below which a sample becomes turbid. It is most commonly used in quality control for surfactant production or in their further utilization in emulsions, dispersions, etc. Often, cloud point protocols ask for a 1% weight dilution of the corresponding surfactant in water. If we consider one of the most common non-ionic surfactants used in industrial processes – alkyl ethoxylates – water solubility of these substances tends to vary inversely with temperature: the higher the temperature, the lower the solubility, and the cloud point is the temperature at which the solution reaches saturation and becomes turbid.
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Slip Melting Point
The slip melting point is often used to characterize fats, oils, and waxes – or any other solids that do not have a defined or sharp melting point. Also known as open tube or open capillary melting point, this technique is described in detail in ISO 6321.
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Dropping and Softening Point
Ointments, synthetic and natural resins, edible fats, greases, waxes, fatty acid esters, polymers, asphalt and tars, as well as other synthetic or naturally occurring products that are important raw materials in a variety of industries, do not show a defined melting point. These materials gradually soften as the temperature rises and melt over a relatively large temperature interval. Generally, the dropping or softening point test is one of the few easily-achievable methods available to characterize such materials. To ensure comparable results, standardized test equipment and conditions, as well as appropriate sample preparation, are required.
Download the guide for more information on the principle of dropping and softening point measurements, and tips and hints for sample preparation.
Alternative Measurement Methods
As is the case for many other parameters, most thermal values can be tested manually by warming the sample in a liquid bath consisting of water, oil, or another medium. Manual methods require visual inspection of the process, which can be tedious as the full attention of an operator is required for continuous observation.
Download the guide for more information, tips and hints.