Determination of Melting Point
Determining the melting point of a compound is one way to test if the substance is pure. A pure substance generally has a melting range (the difference between the temperature where the sample starts to melt and the temperature where melting is complete) of one or two degrees. Impurities tend to depress and broaden the melting range so the purified sample should have a higher and smaller melting range than the original, impure sample.
- Fill a capillary tube with crystals about 3 mm high. Put the capillary tube (open end down) into the crystals and tap it on the bottom of the crystallization dish to get the crystals into the tube. Force the crystals to slide to the bottom of the tube using one of the following methods: tap the tube (open end up) on the lab bench; drop the capillary tube through a 2-3 foot piece of glass tubing; or rub the capillary tube along a piece of wire gauze.
- Place the capillary tube in the MEL-TEMP melting point apparatus. Set the MEL-TEMP at a high enough level to make a rapid determination of melting point. Observe the melting process though the magnifying lens.
- Once a melting point range is determined, prepare another capillary tube (tubes should only be used once and then discarded) and set the MEL-TEMP to the appropriate power level, based on the power level/temperature chart. This time, make sure that the increase in temperature is no more than 2oC per minute. Again, observe through the lens.
Figure 1. A Fisher-Johns melting point apparatus.
- Place a lens cover in the circular well and scoop crystals onto the lens cover (see Figure 2).
- Place another lens cover on top of the crystals and move the magnifying glass over the well.
- Set the temperature by using the dial and turn on the apparatus by flipping the switch.
- Watch the compound through the magnifying glass and record the temperature at which it melts (see Figure 3).
Figure 2. Crystals on the lens cover
Figure 3. Compound after being melted
- Set up a ring stand with a bunsen burner (which should be attached to a gas valve using rubber tubing), a ring above it, and wire gauze on the ring (see Figure 4).
- Place a beaker of mineral oil on the wire gauze.
- Place a sample of the compound into a capillary tube and use a thin piece of rubber tubing as a rubber band to attach the capillary tube to a thermometer (see Figure 5).
- Insert the thermometer through a hole in a cork, and clamp the cork to the ring stand as shown.
Figure 4. The set-up for the procedure
Figure 5. The thermometer with a capillary tube attached using rubber tubing
- Use the bunsen burner to heat the mineral oil slowly.
- Record the temperature at which the solid in the capillary tube melts.
- When a satisfactory melting point range has been determined, choose a known substance that has a melting point within 5°C of the observed value. Make a homogeneous mixture of equal amounts of the unknown and the known substances. Grind them together using a mortar and pestle or a fire polished glass stirring rod and then fill a capillary tube with the mixture.
- Determine the melting point of the mixture. If the unknown sample is identical to the known sample, the melting point will remain unchanged. If the two samples are different, the melting point will be depressed.