Melting Point Apparatus
Introduction to Melting point range determination
The melting point range for a compound provides one aspect of characterisation data. For a given polymorph (crystal form) of a pure compound, the melting point should be consistent, as this relates to the energy take to break the ordered arrangement of molecules in the solid form into the less ordered form of a liquid. The melting point range is reduced if the ordering of the solid form is disrupted in any way, for example by the presence of impurities, as the compound melting would then take less energy than for a perfectly ordered crystal structure.
Information from a melting point range
Melting points are always reported as a range, from the temperature at which melting was first observed to begin until the temperature at which melting was complete. The melting point range can be compared to previously reported data for a compound (unless the compound is novel), for both the absolute value and also the range over which the compound melted.
For a compound of regaular purity, such as a solid which has been recrystallised or purified by flash column chormatography, the melting point range is typically a couple of degrees Celcius, and with the upper value very close (or the same as) those reported in the literature. Unpurified compounds, would usually have a suppressed melting point, and a wider range (maybe 5 °C would be typical).
Reporting a range
Melting point ranges should be reported with appropriate precision and units. Whilst many melting point apparatus may report to a 0.1 °C, it is unlikely that this is a realistic level of precision unless a melting point has been recorded very slowly. Typically ranges are rounded to 1 °C. The literature melting point range is usually reported alongside, along with a reference to the source. Often the recrystallisation solvent is reported in brackets alongside the melting point, unless this information is provided in a procedure directly accompanying the data.
For example, a melting point range for benzoic acid (recrystallised from water) might be reported as:
Benzoic acid: MP 120-122 °C (water) lit: 121-125 °C (Sigma Aldrich).
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
Common issues
Solvent effects
As well as inpurities in the crystal matrix causing the melting point range to be depressed (or widened), the presence of solvents, and especially water, can result in the melting point range being less than ideal. It is a good idea to ensure that compounds have had sufficient opportunity to dry (eg spreading out on a watch glass, or allowing to dry for a period of time over a vacuum a sinter, or using a dessicator or vacuum dessicator) prior to recording the melting point range. Care should however be taken with compounds which are deliquescent, as these compounds have the reverse issue, and melting point values will become worse over time as the compounds absorb water (or other solvents) over time.
Decomposition range
Some compounds don't turn into liquids when heated, but decompose instead. For these a decomposition range would be recorded instead of a melting range.
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.
