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11. Running a melting point determination
11.1 Sample preparation
This section provides guidelines for sample preparation before performing a melting point
determination.
1. For accurate melting point determinations, ensure the sample is dry. Dry the sample overnight in a
vacuum desiccator with desiccant under reduced pressure if necessary.
2. Place a small quantity of the dry sample on a watch glass or in a mortar and grind to a fine
powder with a glass rod or pestle. Coarse, crystalline samples will give uneven heat transfer
through the sample.
3. The glass capillary tube should be sealed at one end; if the capillary tube is open at both ends, seal
one of the ends using the flame of a Bunsen burner. If the capillary is sealed at both ends, use the
cutter provided in the capillary storage compartment to cut the capillary in half. Roll the capillary
across the cutter to score the glass then press down firmly until it snaps cleanly across the score
line.
Note: Capillary tubes must be a minimum of 5.0cm in length to ensure that they can be
retrieved from the sample block.
4. Gather the powdered sample into a small pile. Press the open end of the capillary into the sample
to a depth of around 2mm.
5. Invert the capillary and tap the tube to pack the sample down in the sealed end. Repeat as
necessary; the final height of the sample should be between 2-4mm.
6. If preparing multiple samples, prepare them in the same way with the same height of sample in
each tube. For accurate melting point determinations it is recommended that three samples of the
same material are prepared in the same way and analysed at the same time. The average of the
results from the three capillaries can then be reported as the melting point of the sample.
11.2 Tips on running a melting point determination
❖
Ensure there is no sample on the outside of the tube before inserting it into the melting point
apparatus.
❖
A sample should be used only once for melting point determination as some substances
chemically decompose on heating.
❖
Do not heat the sample too quickly; generally 1-2ºC/minute is recommended.
❖
If the melting temperature of the substance is unknown, first perform a rapid melt with a
ramp rate of up to 20ºC/minute. Once the approximate melting point is known, a more
appropriate plateau can be set and a slower ramp rate used. See section 12 for how to
perform a rapid ramp measurement.
