All You Need to Know about Melting Point Determination

What is Melting Point?

All You Need to Know about Melting Point Determination

Melting point determination
What is melting point
What is the melting point of a substance
Melting point principle
Melting point capillary

5. Pharmacopeia's Requirements for Melting Point Determination

Pharmacopeia's requirements for melting point
Melting point accessory box

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 1 of the sample preparation for a melting point analysis
Step 2a 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 2b of the sample preparation for a melting point analysis
Step 3 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
Step 4 of the sample preparation for a melting point analysis

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.

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

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 TA and TC 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

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 (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
Temperature increase of sample and furnace
Pharmacopeia melting point vs. thermodynamic melting point
Pharmacopeia melting point vs. thermodynamic melting point
Melting point determination

 

 

Melting point depression

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.

Diagram 1: Sample and reference substance are identical
Diagram 1: Sample and reference substance are identical
Diagram 2: Sample and reference substance are different
Diagram 2: Sample and reference substance are different