What is Melting Point?

Definition, Determination Principle, Influences, Measurement Tips and Hints, Regulations & More

Melting point is a characteristic property of solid crystalline substances. It is the temperature at which the solid phase changes to the liquid phase. Melting point determination is the thermal analysis most frequently used to characterize solid crystalline materials. It is used in research and development as well as in quality control in various industry segments to identify solid crystalline substances and to check their purity.

On this page you will gain essential knowledge about the melting point technique. Furthermore, practical tips and hints for daily work are provided.

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What Is Melting Point?
Why Measure Melting Points?
Melting Point Determination Principle
The Capillary Method
Pharmacopeia's Requirements for Melting Point Determination
Good Sample Preparation
Instrument Setup
Calibration and Adjustment of a Melting Point Instrument
Influence of the Heating Rate on the Melting Point Measurement
Melting Point Measurement – Tips and Hints
Effect of Impurities on the Melting Point – Melting Point Depression
Mixed Melting Point Determination

1. What Is Melting Point?

Melting point is a characteristic property of solid crystalline substance. It is the temperature at which the solid phase changes to the liquid phase. This phenomenon occurs when the substance is heated. During the melting process, all of the energy added to the substance is consumed as heat of fusion, and the temperature remains constant (see diagram below). During the phase transition, the two physical phases of the material exist side-by-side.

Crystalline materials consist of fine particles that for a regular, 3-dimensional arrangement – a crystalline lattice. The particles within the lattice are held together by lattice forces. When the solid crystalline material is heated, the particles become more energetic and start to move more strongly, until finally the forces of attraction between them are no longer strong enough to hold them together. The crystalline structure is destroyed and the solid material melts.
The stronger the forces of attraction between the particles, the more energy is needed to overcome them. The more energy is needed, the higher the melting point. The melting temperature of a crystalline solid is thus an indicator for the stability of its lattice.

At the melting point not only the aggregate state changes; quite a lot of other physical characteristics also change significantly. Amongst these are the thermodynamic values, specific heat capacity, enthalpy, and rheological properties such as volume or viscosity. Last but not least, the optical properties birefringence reflection and light transmission change. Compared to other physical values the change in light transmission can easily be determined and can therefore be used for melting point detection.

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What is the melting point of a substance

2. Why Measure Melting Points?

Melting points are often used to characterize organic and inorganic crystalline compounds and to ascertain their purity. Pure substances melt at a sharp, highly-defined temperature (very small temperature range of 0.5 – 1 °C) whereas impure, contaminated substances generally exhibit a large melting interval. The temperature at which all material of a contaminated substance is molten is usually lower than that of a pure substance. This behavior is known as melting point depression and can be used to obtain qualitative information about the purity of a substance.

In general, melting point determination is used in the lab in research and development as well as in quality control in various industry segments to identify and check the purity of different substances.

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3. Melting Point Determination Principle

At the melting point, there is a change in light transmission. Compared to other physical values the change in light transmission can easily be determined and can therefore be used for melting point detection. Powdered crystalline materials are opaque in the crystalline state and transparent in the liquid state. This distinct difference in optical properties can be measured in order to determine the melting point by recording the percentage of light intensity shining through the substance in the capillary, the transmittance, in relation to the measured furnace temperature.

There are different stages of the melting point process of a solid crystalline substance: At the collapse point, the substance is mostly solid and comprises only a small amount of molten material. At the meniscus point, most of the substance has melted but some solid material is still present. At the clear point, the substance has completely melted.

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4. The Capillary Method

The melting point measurement is usually performed in thin glass capillary tubes with an internal diameter of 1 mm and a wall thickness of 0.1 – 0.2 mm. A finely-ground sample is placed in the capillary tube to a filling level of 2 – 3 mm and introduced in a heated stand (liquid bath or metal block) in close proximity to a high accuracy thermometer. The temperature in the heating stand is ramped at a user-programmable fixed rate. The melting process is visually inspected to determine the melting point of the sample. Modern instruments, like the Melting Point Excellence instruments by METTLER TOLEDO, enable automated detection of the melting point and melting range and visual inspection by a video camera. The capillary method is required in many local pharmacopeias as the standard technique for melting point determination.

With the Melting Point Excellence instruments by METTLER TOLEDO up to 6 capillaries can be measured at the same time.

Learn more about the benefits of digital melting point instruments

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5. Pharmacopeia's Requirements for Melting Point Determination

The pharmacopeia's requirements for melting point determination include both minimum requirements for the design of the melting point apparatus and for performing the measurement.

The pharmacopeia's requirements at a glance:

Use capillaries with outer diameters ranging from 1.3–1.8 mm and wall thicknesses from 0.1–0.2 mm.
Apply a constant heating rate of 1 °C/min.
If not otherwise stated, in most pharmacopeias temperature at the end of melting is recorded at point C (End of Melting = Clear Point) when no solid substance is left.
The recorded temperature represents the temperature of the heating stand, which can be an oil bath or a metal block, in which the thermocouple is positioned.

The METTLER TOLEDO melting point instruments fully comply with Pharmacopeia's requirements.

For detailed information on international norms and standards, visit

www.mt.com/MPDP-norms

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6. Good Sample Preparation

Good sample preparation is crucial to achieve highly accurate melting point measurements.

For sample preparation, a dry powdery substance is ground in a mortar and filled into the capillaries, which are then inserted into the furnace. The melting point accessories box contains sets of 150 melting point capillaries, an agate pestle and mortar, tweezers, a spatula (b), and 5 capillary filling tools (a). Furthermore, the accessory box contains three melting point standards – either the METTLER TOLEDO melting point reference substances (Benzophenone, Benzoic acid, Saccharin) or the USP melting point reference standards (Caffeine, Vanillin, Acetanilide).

Sample preparation process using METTLER TOLEDO melting point tools:

Step 1: First, the sample needs to be dried in a desiccator. Then a small portion of sample is finely ground in a mortar.

Step 2: Several capillaries are prepared simultaneously for measurement with a METTLER TOLEDO instrument. The capillary filling tool perfectly assists the filling as the empty capillaries are securely held in a peg-like grip. Collecting a small sample portion from a mortar is easily done with the assistance of the tool.

Step 3: The small amount of sample at the top of the capillaries is then moved down the capillary by releasing the grip and gently bouncing the capillaries on the table several times. This action packs the sample tightly down into the bottom of the capillary. The 'bouncing effect' causes tight packing of the substance and avoids the inclusion of air pockets.

Step 4: The correct filling height can be checked with the engraved ruler on the capillary filling tool. Generally the filled height should not exceed 3 mm.

Step 1 of the sample preparation for a melting point analysis

Step 2a of the sample preparation for a melting point analysis

Step 2b of the sample preparation for a melting point analysis

Step 3 of the sample preparation for a melting point analysis

Step 4 of the sample preparation for a melting point analysis

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7. Instrument Setup

Along with proper sample preparation, the settings on the instrument are as well essential for the exact determination of the melting point. Correct selection of the start temperature, the end stop temperature and the heating ramp rate are necessary to prevent inaccuracies due to a heat increase in the sample that is incorrect of too fast:

a) Start Temperature

Melting point determination starts at a predefined temperature close to the expected melting point. Up to the start temperature, the heating stand is rapidly preheated. At the start temperature the capillaries are introduced into the furnace, and the temperature starts to rise at the defined heating ramp rate.
Common formula to calculate the start temperature:
Start Temperature = expected MP – (5 min * heating rate)

b) Heating Ramp Rate

The heating ramp rate is the fixed rate of temperature rise between the start and stop temperatures for the heating ramp.
Results depend strongly on the heating rate - the higher the heating rate the higher the observed melting point temperature.
Pharmacopeias apply a constant heating rate of 1 °C/min. For highest accuracy and non-decomposing samples use 0.2 °C/min. With substances that decompose, a heating rate of 5 °C/min should be applied. For exploratory measurements a heating rate of 10 °C/min may be used.

c) Stop Temperature

The maximum temperature to be reached in the determination.
Common formula to calculate the stop temperature:
Stop Temperature = expected MP + (3 min * heating rate)

d) Thermodynamic / Pharmacopeia Mode

There are two modes for melting point evaluation: Pharmacopeia melting point and thermodynamic melting point. The pharmacopeia mode neglects that the furnace temperature is different (higher) during the heating process than the sample temperature, meaning that the furnace temperature is measured rather than the sample temperature. As a consequence, the pharmacopeia melting point depends strongly on the heating rate. Therefore, measurements are only comparable if the same heating rate is applied.
The thermodynamic melting point on the other hand, is obtained by subtracting the mathematical product of a thermodynamic factor ‘f’ and the square root of the heating rate from the pharmacopeia melting point. The thermodynamic factor is an empirically determined instrument-specific factor. The thermodynamic melting point is the physically correct melting point. This value does not depend on heating rate or other parameters. This is a very useful value as it allows melting points of different substances to be compared independently of experimental setup.

Melting Point and Dropping Point – Automated Analysis

This melting point and dropping point guide explains the measurement principle of automated melting & dropping point analysis, and gives tips & hints for better measurements and performance verification.

Visit this page to download the Automated Melting & Dropping Point Analysis guide

Automated melting point and dropping point analysis

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8. Calibration and Adjustment of a Melting Point Instrument

Before the unit is put into operation, it is recommended to verify its measurement accuracy. In order to check the temperature accuracy, the instrument is calibrated using melting point standards with exact certified melting points. Thus, the nominal values including tolerances can be compared with actual measured values.

If calibration fails, which means if the measured temperature values do not match the range of the certified nominal values of the respective reference substances, the instrument needs to be adjusted.

In order to ensure measurement accuracy it is recommended that the furnace is calibrated with certified reference substances on a regular basis, for example once a month.

Melting Point Excellence instruments leave the factory having been adjusted using METTLER TOLEDO reference substances. A three-point calibration with benzophenone, benzoic acid and caffeine is performed, followed by an adjustment. The adjustment is then verified by calibration with vanillin and potassium nitrate.

Melting point calibration and adjustment

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9. Influence of the Heating Rate on the Melting Point Measurement

Results depend strongly on the heating rate - the higher the heating rate the higher the observed melting point temperature. The reason is that the melting point temperature is not measured directly within the substance, but outside the capillary at the heating block, due to technical reasons. Therefore, the temperature of the sample lags behind the furnace temperature. The higher the heating rate, the more rapid the rise in oven temperature, increasing the difference between the melting point measured and the true melting temperature.

Due to the dependence of the rate of heat increase, measurements taken for melting points are comparable with one another only if they are taken using the same rates.

Temperature Behavior of the Sample and the Furnace

Melting point determination starts at a predefined temperature close to the expected melting point. The red solid line represents the temperature of the sample (see figure below). At the beginning of the melting process, both sample and furnace temperatures are identical; the furnace and sample temperatures are thermally equilibrated beforehand. The sample temperature rises proportionally to the furnace temperature. We have to bear in mind that the sample temperature increases with a short delay which is caused by the time needed for heat transmission from the furnace to the sample. While heating up, the furnace temperature is always higher than the sample temperature. At a certain point the furnace heat melts the sample inside the capillary. The sample temperature remains constant until the whole sample is molten. We identify different furnace temperature values T_A and T_C which are defined by the respective melting process stages: collapse point and clear point. The sample temperature inside the capillary rises significantly once the sample is completely molten. It increases parallel to the furnace temperature showing a similar delay as in the beginning.

Pharmacopeia MP vs. Thermodynamic MP

The thermodynamic melting point on the other hand, is obtained by subtracting the mathematical product of a thermodynamic factor ‘f’ and the square root of the heating rate from the pharmacopeia melting point. The thermodynamic factor is an empirically determined instrument-specific factor. The thermodynamic melting point is the physically correct melting point (see figure below). This value does not depend on heating rate or other parameters. This is a very useful value as it allows melting points of different substances to be compared independently of experimental setup.

Temperature increase of sample and furnace

Pharmacopeia melting point vs. thermodynamic melting point

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10. Melting Point Measurement – Tips and Hints

Colored or decomposing samples (azo benzene, potassium dichromate, cadmium iodide) or samples that show a tendency to include air bubbles in the melt (urea) may require either the lowering of threshold value B or usage of the C value as the evaluation criteria because the transmission increase will not be so high during the melting.
Samples that decompose (sugar) or sublime (caffeine): Seal the capillary with a flame. The volatile components produce an overpressure inside the closed capillary that inhibit further decomposition or sublimation.
Hydroscopic samples: Seal the capillary with a flame.
Heating rate: Usually 1 °C/min. For highest accuracy and non-decomposing samples use 0.2 °C/min. With substances that decompose, 5 °C/min; for exploratory measurements 10 °C/min.
Start temperature: 3 - 5 min before, respectively 5 – 10 °C below, the expected melting point (3 - 5 times the heating rate).
End temperature: A successful measuring curve requires an end temperature that is approx. 5 °C above the expected event.
Use the thermodynamic melting point if SOP and instrument permit it. The thermodynamic melting point is the physically correct melting point and is not dependent on instrument parameters.
Incorrect sample preparation: The sample being investigated must be fully dry, homogeneous and in powdered form. Moist samples must be dried first. Coarse crystalline sample and non-homogeneous samples are finely ground in a mortar. To ensure comparable results, it is important to fill all capillary tubes to the same height and to compact the substance well in the capillaries.
Use of melting point capillaries that are very precisely manufactured ensuring highly accurate and repeatable results, e.g. capillaries from METTLER TOLEDO, is recommended. If other capillaries are used, the instruments should be calibrated and, if required, adjusted using these capillaries.

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11. Effect of Impurities on the Melting Point – Melting Point Depression

Melting point depression is the phenomenon of reduction of the melting point of a contaminated, impure material compared to the pure material. The reason is that contaminations weaken the lattice forces within a solid crystalline sample. In conclusion, less energy is needed to break the forces of attraction and to destroy the crystalline structure.

The melting point is therefore a useful indicator of purity as there is a general lowering and broadening of the melting range as impurities increase.

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12. Mixed Melting Point Determination

If two substances melt at the same temperature, a mixed melting point determination can reveal if they are one and the same substance. The fusion temperature of a mixture of two components is usually lower than that of either pure component. This behavior is known as melting point depression.

For mixed melting point determination, the sample is mixed with a reference substance in a 1:1 ration. Whenever the melting point of the sample is depressed by mixing with a reference substance, the two substances cannot be identical. If the melting point of the mixture does not drop, the sample is identical to the reference substance that was added.

Commonly, three melting points are determined: sample, reference and 1:1 mixing ratio of sample and reference. The mixed melting point technique is an important reason why all high-quality melting point machines accommodate at least three capillaries in their heating blocks.