Melting point determination

Melting point determination

Determining the melting point of a compound is one way to test if the substance is pure and is often used to test samples made from organic synthesis (eg of aspirin or paracetomol).

You can find out more about melting points below.

Pure samples usually have sharp melting points, for example 149.5-150°C or 189-190°C; impure samples of the same compounds melt at lower temperatures and over a wider range, for example 145-148°C or 186-189°C.

The general method is to heat a sample indirectly by placing the prepared sample (either packed in a glass capillary or on a glass cover slip) in or on a heated medium and observing it, and the temperature, closely until melting is complete.

Determining the melting point of a compound is one way to test if the substance is pure and is often used to test samples made from organic synthesis (eg of aspirin or paracetomol).

Pure samples usually have sharp melting points, for example 149.5-150°C or 189-190°C;  impure samples of the same compounds melt at lower temperatures and over a wider range, for example 145-148°C or 186-189°C.

The general method is to heat a sample indirectly by placing the prepared sample (either packed in a glass capillary or on a glass cover slip) in or on a heated medium and observing it, and the temperature, closely until melting is complete.

There are a few ways of doing this

Melting point apparatus

This is the most common piece of apparatus for determining melting point (as shown in the picture at the top of the page). The sample is loaded in a sealed capillary tube and the temperature gradually raised by means of an internal metal block. .  The most commonly a heated metal block such as a Mel-Temp apparatus

Thiele Tube

The one problem with the melting point apparatus described above is that of cost. They come in at around £600.

A cheaper alternative, that with care can still produce accurate results, is the Thiele tube.

The Thiele tube is basically a set tube with a side loop (see diagram). The sample in a capillary tube is held next to the bulb of a thermometer by eg a rubber band and placed in the ‘main’, straight part of the tube.

the tube is filled with a suitable liquid, most commonly an oil and the side arm is heated.

As the liquid is heated, convection causes the liquid to circulate around the system distributing the heat. Care needs to be taken to ensure that heating does not happen too fast

Method

1. Familiarise yourself with the requirements of your particular melting point determining method.
2. Place the sample in a capillary tube and seal the end.
3. Affix to a thermometer, if required.
4. Place the sample in your melting point apparatus or Thiele tube and start heating.
5. It is standard practice (in order to make the most effective use of time)to carry out a rapid melting point determination initially (by heating rapidly) to establish an approximate melting point.

If, however, you are looking for confirmation of the purity of a substance and therefore know roughly what temperature your sample will melt at, this rough estimation may not be needed.

6. Then carry out at least two further careful determinations (by heating more gently, i.e. temperature changing only about 2 °C/min) until you obtain two consistent values.

Note that unlike boiling point, the melting point is relatively insensitive to pressure and no pressure correction needs to be made.