Melting Point of a Pure Substance
Introduction
To become familiar with procedures to measure melting points and to understand the value of using melting point as a tool for characterizing organic compounds.
Background
Melting point is defined as the temperature at which solid and liquid phases are in equilibrium. At the melting point, a solid will begin to melt into a liquid or, inversely, a liquid will begin to solidify. (This is also known as the freezing point.) The melting point of a material is one of the physical properties that can help identify a compound and is also a good indication of a compound’s purity. A compound which is pure not only has a characteristic melting point but the process of melting occurs over a very narrow temperature range. In situations where the compound in question is not pure, the melting point is not as distinct. Even though the change is quick, a melting point usually refers to a temperature range and not a single melting point number. If a material is very pure, its melting point range is usually two degrees Celsius or less. A melting point range of greater than two degrees will usually indicate some impurities in the sample.
Melting points are one of the most important diagnostic tools for identification and characterization of a solid compound and determination of its purity. To take a melting point, four items are required: a melting point capillary tube, a small rubber band, a heating bath, and a thermometer.
A melting point capillary tube is a very small diameter glass tube made out of Pyrex® glass. The tube usually has a 1.1–1.2 mm inside diameter and is 100 mm in length. It is closed at one end. By placing a small amount of a compound inside the capillary tube, the melting point can be determined without contamination from the surrounding water bath or water vapor. To load a sample in a capillary tube, first place a small amount of the dry solid material on a piece of filter paper or on a clean watch glass. Carefully tap or insert the open end of the capillary tube into the middle of the pile of material. Some of the solid should become trapped inside the tube. Turn the capillary tube over and gently tap the bottom (closed end) of the tube on the tabletop until all of the solid falls down to the closed end of the tube. (Another method commonly used is to drop the capillary tube (closed-end down!) through a length of glass tubing.) Make sure the solid is tightly packed in the bottom of the capillary tube. Loosely packed solids heat unevenly and give broad melting point ranges. Be careful tapping capillary tubes; they are very fragile and break easily.
Use the smallest amount of material that can be seen in the tube for a melting point determination. Only a very small amount of material is required; usually about 1–2 mm of solid on the bottom of the capillary tube will suffice. Too much material will heat unevenly and require more energy to fully melt. This leads to a broader melting point range.
A good-quality thermometer is very important for obtaining accurate melting points. A mercury or digital thermometer that reads to one degree Celsius is preferred.
The solids used in this experiment all melt at temperatures lower than 90 °C, so a water bath can be used as the heating bath. (An oil bath is most often used with higher-melting solids.) To prepare the water bath, fill a 250- or 400-mL beaker about two-thirds full with water and place it on top of a stirring hot plate. Add a stir bar.
To determine a melting point, attach a capillary tube containing the solid sample to a thermometer with a small rubber band as shown in Figure 1. The bottom of the capillary tube should be about even with the bottom of the thermometer. Place the thermometer and capillary tube into the heating bath so that the thermometer bulb is about an inch below the surface of the liquid. Clamp the thermometer in place using a three-pronged clamp. Begin heating the bath. The rate of heating should not exceed 10 °C per minute. As the melting point is approached, turn off the heat and let the temperature rise slowly through the melting point range. At the first sign of melting, note the temperature, but keep watching the solid to also note when the solid is completely melted. Record these numbers; this is the melting point range. Prior to the melting point, the solid sometimes changes in appearance, such as a slight color change or a loss of luster. This is not the melting point! Melting is usually signaled by a few crystals becoming transparent. Finally, always repeat the melting point determination to confirm your results.
{12248_Background_Figure_1_Experimental setup}
Note: A Flinn Direct-Connect Temperature Probe or Standard Temperature Probe connected to a computer or TI-CBL System may be used in place of the thermometer. Consult your current Flinn Scientific Catalog/Reference Manual .
Experiment Overview
Melting Point Determination Melting points of low temperature organic solids (<90 °C) are best performed using a water bath on top of a magnetic stirrer/hot plate.
Melting Point Procedure The following solids may be used for low temperature solid melting point experiments. Preferred selections are in bold type and are less hazardous. All melting points are given with a range of ±1 °C.
{12248_Data_Table_2}
*Included in kit.
Materials
Water Capillary tubes, closed-end, 2 Beaker, 250-mL Insulated gloves or beaker tongs Magnetic stirrer/hot plate Ring stand Rubber bands, small, 2 Stir bar Thermometer, –20 to 110 °C* Thermometer clamp *See Lab Hints.
Safety Precautions
Never handle the water baths with bare hands; always use insulated gloves or beaker tongs. Do not use broken or chipped capillary tubes; only use new tubes in good condition. Always wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron.
Procedure
Setup
- Place a small sample of the organic compound on a piece of fi lter paper, a glass plate, a microscope slide or on a clean watch glass. Gently place the open end of the capillary tube into the solid sample. (It is also possible to simply insert the open end of the capillary tube into the vial containing the sample.)
- When a small amount of solid is in the tube, invert the tube and gently tap the sample down toward the sealed end or drop the capillary tube (sealed end down) through a piece of glass tubing. Make sure the solid is firmly packed in the closed end of the tube. The tube should only have 1 to 2 mm of material in it.
- Repeat steps 2 and 3 to fill a second capillary tube.
- Secure the capillary tube, sample (closed) end down, with a rubber band to the side of a thermometer. Be sure to secure the tube near the bottom of the thermometer so that the closed end of the capillary tube is even with the tip of the thermometer bulb (see Figure 1 in the Background section).
- Fill a 250-mL beaker about two-thirds full with water and place on a stirring hot plate. Add a stir bar and begin to heat the water bath. Use the magnetic stirrer or a stirring rod to stir the water bath. Do not use the thermometer as a stirring rod. The rate of heating should not exceed 10 °C per minute.
- As the water begins to heat, place the thermometer and tube in the water bath so that the thermometer bulb is about an inch below the surface of the liquid. Secure the thermometer in place using a thermometer clamp on a ring stand. The open end of the capillary should not be near the surface of the water bath.
- When the first sign of melting occurs, write down the temperature. Keep watching the solid until the last crystal melts. Write this temperature down. These two temperatures are the melting point range of the sample.
- Remove the thermometer and sample from the bath. Turn off the hot plate. Use insulated gloves or beaker tongs to carefully remove the water bath from the hot plate.
- Discard about one-fourth of the water from the beaker. Add cool water to replace the discarded hot water.
- Remove the first capillary tube and attach the second sample to the thermometer. Never reuse a sample; always return the sample to the instructor after it has been melted.
- Repeat steps 5 through 8. Use a slower heating rate (1–2 °C per minute) to achieve a more precise melting point.
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