Melting Point Range Determination

Overview

Recording a melting point range

Common issues

Recording a melting point range

In order to determine a melting point range for a compound

Step by step instructions

Ensure you have a sutible sample of solid that can be loaded into a melting point tube. Ideally solids should be try and free-flowing, and larger crystals may require crushing into a powder before use.
Obtain a melting point tube that is suitable for the instrument you intend to use. Different instruments may require different tube lengths or diameters. Melting point tubes are thin glass tubes which are sealed at one end.
Push the open end of the tube onto the sample of compound. This may be done on a watch glass or in a sample vial. Hopefully some compound will be captured in the end of the tube.
Tap the closed end of the tube onto a firm surface (eg fume hood or lab bench) to cause the compound to fall to the closed end of the tube. Take care not to break the tube.
Repeat filling/tapping of the tube as required. You only require enough sample in the tube to be able to watch the sample in the instrument as it melts. Most melting point apparatus have a magnifing glass built in.
Clean the outside of the melting point tube with a tissue to ensure there is no compound left on the outside of the tube, otherwise this may melt and damage the instrument.
Insert the tube into an isstrument and ensure you can see the sample(s) in the viewing window. Most apparatus will allow multiple samples to be measured at a time.
Set up the instrument parameters (see below). Guides for individual melting point instruments can be found below.
Start the melting process and make a note of the temperatures at the start and end of the melting process.
Once the melting point range for samples have been recorded, set the instrument to cool back down.
Remove the tube and dispose into the appropriate waste container. Note these are glass and should be disposed of accordingly.

Parameters

Whilst there are many different apparatus for determing melting point ranges, most modern instruments require the setting of the parameters below, although the exact naming varies for different equipment. Detailed instructions for setting parameters on individual melting point instruments can be found on the .

Parameter

Meaning

Example

Plateau

The temperature the apparatus will rapidly heat up to (preheating). The instrument will then wait for the operator to start the melting point determination.

This would typically be set 15 - 25 °C lower than the expected melting point. So for the example with benzoic acid, somewhere around 100 °C would be a reasonable plateau.

Ramp rate

The speed at which the instrument increases the temperature whilst recording a melting point. Expressed in °C min⁻¹.

This is a compromise between accuracy of the melting point range and the time taken to record the result. A typical rate would be 5 °C min⁻¹. If starting ~20 °C below the expected MP, the instrument would take 4 minutes (20 °C ÷ 5 °C min⁻¹) to reach the expected melting point.

For compounds where the melting point is not known (novel compounds), it is common to do a rapid determination to find the rough melting range, then record a slower run to determine the melting range more precisely. For this purpose, 20 °C min⁻¹ is fairly typical.

Upper limit

The temperature at which the instrument will not heat above. Essentially a safety cutoff.

Typically set around 15-20 °C above the expected melting point, so that the instrument doesn't continue to heat excessively. Melting point apparatus also are limited by the maximum temperature they are able to reach, which is typically around 300 °C.

Filling a melting point tube

Mixed melting points

As the mixing of different compounds results in the suppressing of the melting point range, one method which used to be commonly used to confirm the identity of substances was the use of mixed melting points. Whilst modern spectroscopic and other analytical techniques have largely replaced its use, mixed melting points may still prove useful. Here, a mixture of known compound, and a sample suspected of being the same compound, are mixed together and the melting point range recorded. If no supression of the melting point range is noted for the mixture compared to the individual substances, this provides additional evidence that the identification of the compound is correct.

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