A melting point has been defined as the temperature at which a solid changes state from solid to liquid. It is accepted as an index of purity because pure solids have well-defined melting points, whereas the presence of impurities can result in a spread of melting point temperature. Thus, melting point determination can further help in checking any substance’s purity.
Another interesting factor to know about the melting point is that during the melting stage, the temperature remains constant as all the heat energy input due to temperature increase is consumed as the heat of fusion.
Melting point determination is an established method which is easy and cost-effective. It can be resorted to before you decide to further establish compound identity and purity using sophisticated analytical techniques. The determinations are based on visual inspection of material heated in the capillary tube as well as automated absorbance of liquid melts which avoid personal observation errors.
Before we move on to the precautions that have to be taken during melting point measurement, let’s first see how is melting point determined. The process will help you see the significance of different steps so that you can follow proper tips and remain secure while doing the work.
How to Determine Melting Point
Melting point can serve as a trusted basis for substance identification and purity examination. But these tasks can only be successful if the melting point analysis is done correctly in the first place. Otherwise, the results will be faulty, and no proper judgment will be made. A few commonly practiced steps involved in melting point determination are:
The step’s name explains what exactly gets done here. In the first step, the substance’s sample is prepared for further analysis. Here, equipment like a capillary glass tube and a glass plate will be required. You need to start by depositing the substance sample on the open end of the capillary tube while ensuring that its other end is entirely closed.
Then invert the capillary tube and gently tap it. This way, the sample will end up at the bottom of the tube. Remember, the tap should be gentle, or it can cause trouble. However, it should be sufficient enough to pack the substance properly at the bottom. Else there will be a deflection in the actual melting point. Continue this step until the capillary tube is filled with the sample up to 2-3 mm. The level should not go beyond 3 mm; only then the sharp melting point can be determined.
Using Melting Point Apparatus
A melting point apparatus makes your work extremely convenient. You only need to follow these simple steps to use it, and the task will get done without any hassle.
- Put in the sample-filled capillary tube into the apparatus.
- Turn on the apparatus.
- Observe the given thermometer and the apparatus itself to adjust the heat settings accordingly.
- Use the “viewfinder” to watch the substance closely getting heated through the magnifying glass.
- The final step is to determine the melting point of the substance. Here, you will have to heat the compound on medium up until only 20-degrees are left to its known melting point. Then slow down the heat so that the temperature increases very gradually. In case you do not know the melting point already, you can continue providing the heat on medium. Then you will have to closely observe the substance and note down its approximate melting point.
Remember, you have to be very careful here. If you do not perform the experiment correctly, you will end up wasting the entire substance taken in the capillary tube. Then you will have to take another sample to conduct the experiment again.
You can also use a Thiele Tube Method for melting point determination. The capillary tube is inserted into the Thiele tube, and the experiment is conducted by heating one side of the tube. An external thermometer is used here. Due to this complicated procedure, most people prefer to go with the melting point apparatus method.
Now that you know how to determine melting point, let’s move on to the precautions that have to be taken during the process.
The accuracy of melting point determination is based on
- Sample preparation
- Sample handling
- Calibration of instrument temperature display
In the present article useful tips are provided on sample preparation and sample handling as both these factors contribute significantly to the accuracy of melting point determinations
- Sample homogeneity – samples should be finely powdered and any lumps or crystals should be finely pulverised. Samples should be ground using clean mortar and pestle.
- Samples should be dry and any moist samples should be dried first in a desiccator using a suitable desiccant
- Outside surface of capillary tube should be wiped clean with lint free tissue or cloth before inserting into the heating block
- Only gently tap the capillary. Tight compaction can result in bubble formation that interferes with meniscus and results in reading errors
- Do not force the capillary down the heating block
- Do not overfill the capillary tube. 2 or 3 mm sample height is sufficient. Excess sample will require extra heat to melt and result in a larger melting range
- Same set of sample capillary tubes should be used for calibration purpose
A wide melting range indicates that the sample is impure. Sometimes decomposition prior to liquefaction takes place. In extreme cases the material produces discolouration on decomposition which further complicates melting point observation.
Please share your experiences and offer your comments.