Determining Melting Points in Pharmacies, Industry, and Laboratories
Determining the melting point is one of the oldest and most reliable analytical methods. It accurately indicates the temperature at which a substance changes from a solid to a liquid state. Because this transformation can alter fundamental properties such as density, viscosity, and optical behaviour, its impact must be carefully considered.
As a substance‑specific parameter, the melting point serves as a central indicator of material quality. Refined traditional water‑bath methods use temperature‑controlled glass vessels, yet they remain prone to considerable variability and can produce temperature deviations of more than 1 °C between the bath and the sample material.
Modern systems, such as KRÜSS Optronic measurement devices, therefore rely on the open capillary method and high-precision metal heating blocks. The M5000 model also utilizes changes in light transmittance and delivers fully automated, reproducible results with a resolution of 0.1°C in the range of 25–400°C.
To ensure reproducibility in measurements, it is important to understand the definitions of melting temperature, the type of measurement, and the method used.
Contents
Melting point vs. melting range
Ideally, pure substances have a stable melting point. In practice, even the purest substances have a melting range. Other factors include contamination, the type of sample, various methods of preparation and analysis, and instrument-specific influences. All this has an impact on the measured value. Mixtures of several substances usually show a reduction of the melting point.
Semi-automatic and fully automatic measurement
Using our M3000 and M5000 devices, powdery substances may be analysed semi-automatically or fully automatically and with a precisely regulated – yet variable – heating rate. With the M3000 device, the measurement is carried out as a visual measurement using observation optics. With the M5000 device, a transmission measurement is performed and the melting process is detected fully automatically.
Measurement convention method.
Different measurement methods and evaluation approaches yield different melting temperatures. For the comparability of measurement results, it is important to specify the measurement method. The determination of the melting point, better known as the determination of the melting temperature, is a conventional method. This means it is subject to regulations and standardised rules.
Determination of melting temperature
Measurement method: The sample is placed in fine capillaries. To ensure reliable results, the starting temperature should be set around 5 °C below the expected melting temperature,, as the thermal preload has an influence on the result (convention method). By default, the analysis is carried out at a standard heating rate of 1°C/minute.
If a crystalline sample is heated, the applied energy leads to modifications in the crystalline lattice structures. The bonds between the particles in the grid separate and the previously solid substance melts.
Powdery crystalline substances are crystalline in the opaque condition and transparent in the liquid condition. This clear difference in optical characteristics is used to determine the melting point.
The phases of a melting process
Depending on the temperature applied, there are generally three phases in a melting process:
- Decay/collapse – start of liquefaction
- Miniscus point – The liquid phase predominates
- Transparent point – the crystals are fully melted, the substance is entirely present in the liquid phase
When does the melting temperature occur?
The USP (US Pharmacopeia) defines the melting temperature as the final point of the melt when all crystals have completely melted. However, this is only true if the temperature measurement can be performed within the sample. With modern analytical methods, the melting temperature can be determined indirectly via the heating block with an accuracy of a few tenths of a degree.
With our devices, the result can be read with an accuracy of 0.3 – 0.5 °C (depending on measurement range). There is a temperature gradient* between the heating block and the sample, therefore the miniscus point is used as the melting temperature in accordance with convention.
A temperature gradient describes the difference between the heating block and the sample temperature. If the sample begins to melt, the sample temperature (not measurable) remains constant despite further energy being supplied by the heating block.
The graphic shows: The heating block temperature continues to rise in the process at 1°C/min. This offset must of course always be considered / corrected in the determination of the translucent melting point. The M5000 model does this automatically. In the case of the M3000 device with observation optics, the miniscus point is generally defined as the end point.
Measurement tips
Use original products for capillaries if possible. Our KSPS1010 capillaries have an outer diameter of 1.4 mm and a wall thickness of 0.1 to 0.2 mm, and thus fully comply with standard specifications.
Inappropriate capillaries may possibly break faster. If the diameters are selected too thin, the melting temperatures are also assumed to remain low.
- Fill the capillaries to approx. 4 mm. Tapping can be used to achieve compaction. Depending on the type of sample, we also recommend compaction using a tamping wire.
Note: Only homogeneous, powdered, and sufficiently compacted sample material ensures reproducibility of measurements - Each capillary should be cleaned of externally adhering substances before it is inserted into the capillary receptacle. In the oven, they may burn and spoil the optics or pollute the capillary receptacle.
- If possible, start the measurement 4-5°C below the expected measurement value. The rapid heat-up option optimises the waiting time until the start value is reached.
- During the measurement, work with a constant heating rate of 1°C/min (standard heating rate).
If you want to speed up the process (which often comes at the expense of precision and accuracy), you can also select other Heating rates.
Melting point measurement interlaboratory Test
Sample preparation for melting point measurement
Storage of the sample
The sample should be stored in a dark, dry place, as moisture can lower the melting point; drying specifications vary depending on the substance. We recommend drying in a vacuum desiccator with silica gel or storing the sample in a drying oven for 24 hours.
Thermal preload
If the starting point is set much too low for unknown samples, thermal bias can increase the measurement uncertainty; in such cases, repeat measurements are necessary, with the starting point being narrowed down in small increments. This also applies in cases where the Heating rate deviates from the standard rate.
Age of the sample
The proper compaction and particle size of the sample are crucial for reproducibility of measurements. Samples can age, decompose, oxidize, or absorb humidity, which alters their melting point. It is advisable to use samples that come with certificates, as these specify the shelf life after opening.
Typical areas of application
The melting point is one of the most common thermal parameters for characterising crystalline solids. Melting point determination is used in various industries, as well as in research and quality control.
The melting point is often measured to characterize organic and inorganic crystalline compounds and to determine their purity. The melting point is also frequently the starting point and reference value for additional analyses.
Melting point measurement devices are used in laboratories in the chemical, oil, fat and food industries, in hospitals, pharmacies and examination centres and also in research and teaching.
Feel free to check it out: Our table in the download section lists the melting points of the substances described in the monographs of the Ph. Eur., DAB, and DAC. This table is intended as a general overview. We’d be happy to discuss which devices are suitable during a personal consultation.
Melting temperatures (DAC system)
Standards and guidelines
Section 6 of the German Pharmacy Operating Regulations (ApBetrO) specifies that pharmacies must verify the identity of every raw material they obtain. In this case, the determination of the melting temperature is of great importance.
The M5000 model uses the open capillary method, which fully complies with the specifications of the German Pharmacopoeia, the European Pharmacopoeia (Pharmacopoea Europaea, Ph. Eur.), and the United States Pharmacopeia (USP).
The European Pharmacopoeia describes the open capillary method as a standardized procedure for determining the melting point. In about one-quarter of the monographs in the European Pharmacopoeia (Ph. Eur. 4.00 and supplements), the German Pharmacopoeia (DAB 2002), and the German Drug Codex (DAC), it is used as part of the identity test or to determine the physical properties of a substance.
We provide customised standards and reference materials to meet the requirements for verification and traceability of results. The application of these standards provides reliable analysis results and reduces the workload of validation in the laboratory.
Ph. Eur. 9.1 Measurement of the Melting Point—Comment
Adjustment and calibration
Devices used to determine melting temperature must be calibrated regularly usingmelting-point reference substances, such as those from the WHO or other suitable substances.
Our devices have been adjusted and calibrated at the factory with WHO reference substances. We also recommend the standards (vanillin, phenacetin, sulfanilamide, caffeine) for routine testing.
Calibration should be performed at regular intervals. Depending on requirements, the frequency can range from once a month to once a year.
If discrepancies persist even after repeat measurements, an adjustment is necessary. This can be done by the user – menu-guided. Depending on the exact type of deviation, you can choose among three different adjustments.
We also offer adjustments and subsequent calibrations as a service, either at our facility or at the customer’s location.
Please feel free to contact us if you need further assistance.
WHO reference substance values
How to Use – Practical Videos by Experts
The M3000 is your efficiency booster: three samples at once, rapid cooling, and flexible Heating rates for maximum productivity.
The M5000 sets the benchmark—fully automatic, highly precise up to 400°C, and featuring intelligent optical detection. Transmission measurement ensures absolutely reliable, objective measurement results with minimal effort.
https://www.youtube.com/watch?v=Byi5QG4j15M
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