Melting Point
Source: Vy M. Dong and Jan Riedel, Department of Chemistry, University of California, Irvine, CA
One of the most important properties of a crystalline…
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.
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Q1: What is melting point and why is it useful in organic chemistry?
Melting point is the temperature at which a solid transitions to a liquid phase. It is a thermodynamic property useful for characterizing solid organic compounds and determining their identity and purity by comparing measured values to published data. For pure compounds, melting occurs at a specific temperature; impurities broaden this range and lower the melting point.
Q2: How do impurities affect the melting point of a compound?
Dissolved impurities disrupt the crystal lattice structure of a pure compound, preventing them from fitting into the ordered arrangement. This disruption broadens the melting range, typically by 1 to 2 degrees Celsius, and lowers the overall melting point. Measuring melting point changes provides a quick qualitative method to assess sample purity.
Q3: What equipment and procedure are used to measure melting point?
Melting point is measured using an electronically controlled heating block and a thermometer. A solid sample is placed in a capillary tube, inserted into the apparatus, and rapidly heated until approaching the expected melting point. The heating rate is then reduced to 1 degree Celsius per minute, and temperatures at which melting begins and completes are recorded.
Q4: How does melting point help identify unknown organic compounds?
Since melting point is a characteristic thermodynamic property of each chemical, comparing a measured melting point to published data allows researchers to identify unknown compounds. This method is quick and convenient for eliminating candidate compounds before employing more rigorous characterization techniques like infrared spectroscopy.
Q5: What is the difference between crystalline and amorphous solids?
Crystalline solids have constituents organized into a regular lattice structure, while amorphous solids lack this ordered arrangement. Both are held together by large attractive intermolecular forces. The ordered structure of crystalline solids makes them ideal for melting point determination, as they exhibit sharp, well-defined melting transitions.
Q6: Can melting point be used to identify liquids?
Yes, liquids can be identified indirectly by converting them into solid derivatives. For example, an alcohol can be esterified with 4-nitrobenzoic acid, and the resulting ester's melting point is compared to published data for known 4-nitrobenzoic acid ester derivatives to identify the original alcohol.
Q7: How is melting point used in drug delivery applications?
Melting point is used to characterize solid lipid nanoparticles, a drug delivery vehicle designed for targeted treatments. Thermal analysis of these nanoparticles, including melting point determination, reveals their crystallinity, which is critical for controlling drug delivery rates and ensuring proper dosing.