Source: Vy M. Dong and Jan Riedel, Department of Chemistry, University of California, Irvine, CA
One of the most important properties of a crystalline solid is its melting point. It can be used to determine the purity of a known compound and gives important information about the stability of the formed crystals.
Solids can mainly be found in an ordered structure (crystalline solid) or an unordered structure (amorphous solid). Crystalline solids will organize themselves in a crystal lattice. Removing atoms or molecules from this lattice requires energy. Therefore, transferring an atom or molecule from the crystalline solid phase to the liquid phase takes energy in the form of heat. The melting point is the amount of energy in the form of heat that is necessary to remove atoms or molecules from the crystal lattice.
Not all molecules will crystallize in a single crystal lattice. Some organic molecules are known to crystallize in different lattices that are similar in energy. Theses solids are called polymorph because they do not have a defined melting point but rather a range of temperature of where the solids melts.
Another factor that can influence the melting point are impurities. In most cases the melting point of a sample will decrease when containing an impurity. Measuring the melting point can therefore give a qualitative measure of the purity of a known sample.
Measuring the melting point requires a special apparatus, mostly a copper block that can be heated electronically. The temperature can be taken from an electronic sensor or a thermometer.
1. Sample Preparation
- Take a special glass capillary that fits the melting point apparatus in use.
- Dip the open end of the capillary into a sample of benzoic acid
- Gently tap the glass capillary on a hard surface, like a table, so that the benzoic acid moves down to the very bottom of the glass capillary
2. Measuring the Melting Point
- Insert the glass capillary into the melting point apparatus.
- Heat the apparatus with a rate of 10 °C/min until is close to the expected melting point.
- Decrease the rate than to 1 °C/min and observe the sample closely.
- Record the temperature when the sample starts to melt and when it is completely molten.
Melting Point, the temperature at which a solid melts, is a useful property for characterizing solid organic compounds.
This technique is performed by observing the temperature, or range of temperatures, at which a solid melts. Purity and identity can then be determined by comparing to published data.
This video will illustrate the principles of melting point, its measurement, and its applications.
The solid phase is a fundamental state of matter, where its components are held close together by large attractive forces. Solids can be further classified by the arrangement of it constituents: amorphous solids are unordered, while crystalline solids are organized into a lattice structure. To melt a solid, energy, usually in the form of heat, must be provided to overcome the intermolecular forces that hold its structure together.
When a solid is heated, the temperature will eventually increase to its melting point. For a pure compound, this is the thermodynamic condition where solid and liquid phases coexist. At this temperature adding more energy results in molecules leaving the solid phase and entering the liquid phase, otherwise known as melting.
Since the melting point is a thermodynamic property of a chemical, published chemical and thermodynamic data can be used to help identify an unknown compound. It should be noted that for some organic compounds, melting occurs over a range of approximately 1 to 2 degrees Celsius.
Another factor that affects melting points is the presence of dissolved impurities that do not fit into the crystal lattice of a pure compound and disrupts its ordering. This phenomenon results in the broadening of melting range and lowering of the melting point, making melting point determination a good method to qualitatively determine purity.
Melting points are measured using a heating block that can be electronically controlled, and a thermometer to measure temperature. The solid is placed in a capillary tube, heated and observed, the temperatures at which melting begins and completes are recorded.
Once the melting point is known, it can be used to elucidate information about the identity and purity of a sample. This method is a quick and convenient way to eliminate candidates before using more rigorous methods like IR spectroscopy.
Now that we've seen the theory, let's look at a procedure where we will demonstrate measuring the melting point of benzoic acid.
First, collect the sample and glass capillary tubes, and bring them to the melting point apparatus. Ensure the apparatus is powered on.
Using a melting point capillary tube, dip the open end of the tube into the sample. Then, tap the capillary tube on a hard surface. This is necessary to collect the sample at the closed end of the tube.
Next, insert the capillary tube into the melting point apparatus. Rapidly heat the sample until the expected melting point is approached.
Finally, reduce the heating rate to 1 degree Celsius per minute and observe the sample closely. Record the temperature when the sample starts to melt and when it's completely molten
Now that we have covered how to measure the melting point of a solid organic compound, let's look at some applications.
Melting points can also be used to identify liquids. An alcohol can be identified by first esterifying with 4-nitrobenzoic acid. The melting point of the ester can be used to identify the alcohol by comparing the measured melting point with published thermodynamic data for 4-nitrobenzoic acid ester derivatives.
Solid lipid nanoparticles are a drug delivery vehicle designed for targeted treatments. In this application is used to characterize the thermal behavior of nanoparticles including its melting point. This thermal analysis reveals crystallinity of the nanoparticles, which is an important factor that affects drug delivery and dose.
You've just watched JoVE's introduction to melting point. You should now understand melting point, how it's measured, and its applications. Thanks for watching!
A sample of pure benzoic acid will melt at 122 °C.
Applications and Summary
The melting point is an important property of a crystalline solid. It can be measured easily and gives information about the identity and purity, as well as the bonding strength within the crystal lattice.
Due to its simple determination, the melting point is still an important property that is commonly employed to identify compounds and qualify their purity. The melting point can not only be used to identify solids, but also liquids like isopropanol. Through derivatization of isopropanol with 4-nitrobenzoic acid to its corresponding ester, the derivative becomes a defined solid with a melting point, which now enables the identification.