On this page
Measuring a melting point is one of the quickest, cheapest and most informative tests in the organic chemistry lab. In about fifteen minutes and with a few milligrams of solid, it tells you two things: how pure your product is, and whether it’s likely to be the compound you think it is. This guide covers the method, the reasoning behind it and how to interpret the result.
Why melting points are useful
A pure crystalline solid has a regular, repeating arrangement of particles. All the forces holding that lattice together are the same, so the whole crystal melts at one sharp temperature, usually over a range of 1–2 °C or less.
Impurities disrupt the lattice. They weaken it in places, so melting starts at a lower temperature, and different regions melt at different temperatures, so melting happens over a broader range. This effect is called melting point depression, and it’s the same principle that lets salt melt ice on roads.
So:
| Observation | Interpretation |
|---|---|
| sharp range (1–2 °C) close to the literature value | probably pure and correctly identified |
| sharp range but far from the literature value | pure, but probably a different compound |
| broad range (several °C), below the literature value | impure (or still wet with solvent) |
Equipment
- electric melting point apparatus (heated metal block with a magnifier and thermometer or digital readout), or a Thiele tube with oil for older setups
- glass capillary tubes, sealed at one end
- the dry, finely powdered sample
- a watch glass or clean tile
- a spatula, and a length of glass tubing (about 50 cm) for packing
Safety
- Wear eye protection. The block gets hot enough to burn (often above 200 °C).
- Handle capillary tubes with care; broken glass is sharp.
- Dispose of used capillaries in the glass waste container, not the general bin.
- Know the hazards of your sample from its safety data sheet. See lab safety rules.
Method
1. Prepare the sample
The sample must be completely dry and finely powdered. Solvent trapped in the crystals acts as an impurity and lowers the melting point. Crush a little sample on a watch glass with a spatula.
2. Fill the capillary
- Press the open end of the capillary into the powder so a small amount enters.
- Turn it the right way up and tap the sealed end gently on the bench, or drop it down a long glass tube onto the bench so it bounces and packs the solid.
- Aim for a tightly packed column 2–3 mm high. Too much sample melts unevenly and gives a falsely broad range.
3. Do a rapid first run (if the melting point is unknown)
Heat quickly (around 10 °C per minute) to find the approximate melting point. This saves time. Then let the block cool to at least 15–20 °C below that value.
4. Do an accurate run
- Insert a fresh capillary. (Never re-melt a sample: it may have decomposed or changed crystal form.)
- Heat quickly to about 15 °C below the expected melting point.
- Then slow the heating to 1–2 °C per minute. This is the most important step: if the block heats faster than heat can pass into the sample, the thermometer runs ahead of the sample and the reading comes out too high.
5. Record the range
Watch the sample through the magnifier and record two temperatures:
- Start: the first drop of liquid appears (the solid may shrink or “sweat” slightly before this; that isn’t melting).
- End: the last crystal disappears and the sample is completely clear liquid.
Report the result as a range, for example “m.p. 121–122 °C”. Repeat for a concordant result.
Worked interpretation
A student recrystallises crude benzoic acid (literature melting point 122 °C).
| Sample | Measured range | Conclusion |
|---|---|---|
| crude product | 112–118 °C | impure: low and broad |
| after one recrystallisation | 119–121 °C | purer, but still slightly low |
| after drying overnight | 121–122 °C | pure benzoic acid |
The improvement after drying shows that the second sample was still slightly damp: water acted as an impurity.
Mixed melting point: confirming identity
Two different compounds can have the same melting point. To check whether your product really is compound X, use a mixed melting point:
- Mix your sample thoroughly with an authentic, pure sample of X (roughly 1 : 1).
- Measure the melting point of the mixture.
- If the mixture melts sharply at the same temperature, the two are almost certainly the same compound.
- If the mixture melts lower and over a broader range, they’re different compounds, because each acts as an impurity in the other.
It’s a neat, low-tech identity test.
Calibrating the apparatus
Thermometers and sensors can be inaccurate. To check, measure the melting points of pure reference standards with well-known values spread across the working range (compounds such as benzoic acid, around 122 °C, and others at lower and higher temperatures, supplied as certified standards). Plot measured against true values to make a correction graph. See calibration curves.
Special behaviours to watch for
- Decomposition: some compounds darken, bubble or char instead of melting cleanly. Record this as “decomposes at …” (often written “dec.”).
- Sublimation: some solids turn directly to vapour and disappear from the capillary. A sealed capillary can help.
- Polymorphism: some compounds can crystallise in more than one form, each with its own melting point. This matters a lot in the pharmaceutical industry, where different forms can dissolve at different rates.
- Liquid crystals: a few compounds form a cloudy liquid first, then a clear liquid at a higher temperature.
Common errors and fixes
| Error | Effect | Fix |
|---|---|---|
| heating too fast near the melting point | reading too high, range too broad | 1–2 °C per minute near the melting point |
| wet sample | low, broad range | dry thoroughly before testing |
| too much sample | broad range | 2–3 mm column only |
| loosely packed sample | uneven melting | tap down firmly |
| reusing a melted sample | unreliable result | always use a fresh capillary |
| uncalibrated thermometer | systematic error | calibrate with standards |
See experimental errors for the difference between random and systematic errors.
Link to other purity tests
Melting point is quick, but it’s not the only purity check. Thin-layer chromatography can show how many components are present, and spectroscopy confirms structure. Together they form the standard toolkit for checking a synthesised product. See how chemists identify an unknown compound. For liquids, the equivalent test is a boiling point determination.
Key takeaways
- Pure crystalline solids melt sharply (1–2 °C range); impurities lower and broaden the range.
- Use a dry, finely powdered sample packed 2–3 mm deep in a capillary.
- Heat slowly (1–2 °C per minute) near the melting point, and always report a range.
- A mixed melting point with an authentic sample confirms identity.
- Calibrate the apparatus with reference standards and watch for decomposition or sublimation.
Advertisement