Measuring Melting Point
The determination of a melting point of a sample is a standard laboratory procedure and is relatively straightforward. It is used to identify a sample, establish its purity, and determine the thermal stability of the sample. When measuring a melting point, you will generally find that it is recorded as a melting range rather than the exact melting point. This is due to most samples appear to melt over a small temperature range. A melting range is the difference between the temperature at which the sample begins to melt and the temperature at which the sample has actually melted.
Methods of Measuring Melting Point
There are a variety of methods that you can use to determine the melting point of a sample. The most common and most basic method of determination is the capillary method. This method involves placing the sample in a capillary tube and running an experiment that will heat the sample until it reaches melting point. The melting point can then be recorded.
There are a few ways in which you can setup an experiment with the capillary method. The first is a manual apparatus setup with the aim of heating up a sample inside a capillary tube whilst visually looking for when the sample has melted. The concept involves placing a small amount of the substance in a capillary tube and joining this to a thermometer. The capillary tube is then placed into a heating bath and the experiment is conducted. If you don’t have a heating bath, you can use Mineral Oil in a Thiele Tube and heat the tube with a Bunsen Burner. It is important to heat the sample slowly using this method so that a thermal equilibrium can be established.
A more modern way of using the capillary method is to use a device called a Melting Point Apparatus. This device uses the same concept of heating a sample in a capillary tube but makes the process far more simpler and quicker. There are many different types of Melting Point Apparatus machines and they range in functionality and accuracy. At a basic level, the machine is designed so that a capillary tube with the sample can be inserted into the device and rapidly heated to a set temperature. Generally, you will heat the capillary tube to near melting point and then decrease the speed of the temperature increase so that you can observe when the sample melts. Observation is generally conducted through a viewing eyepiece. A Melting Point Apparatus will more than likely have the temperature displayed in digital format for easy recording of data.
Firstly, you must ensure that the sample is fully dry and in powdered form. Then with your capillary tube, use the open end of the tube to press down gently on the sample several times. You then need to get the powder all the way to the bottom of the capillary tube by tapping the closed end multiple times on a hard surface so that the sample is compacted down at the closed end of the tube (you could also use a method where you drop the tube multiple times through a glass tube approx. 1m in length). For further accuracy, you can use a packing wire to further compact the sample. Note that the sample height should be between 2 and 3mm to ensure best results.
Melting Point Tips
Here are a few helpful tips to assist with melting point experiments.
Never re-melt a sample! Always use a fresh sample and a new capillary tube.
Always use the same batch of capillary tubes for routine measurements. Not all capillary tubes are equal and using the same batch will ensure repeatable results.
Always clean the tube before running the melting point experiment. Failure to do so can result in a low melting point or a wide melting range.
- Don’t heat the sample too fast. It is recommended only 1-2°C per minute.