Boiling Point Determination


A compound's boiling point is the temperature at which the vapor pressure of the liquid is equal to the external pressure.  There are several methods used to determine the boiling point and the ultimate choice of method depends on the amount of material available and the accuracy required.   The methods we will be working with are:  regular scale distillation, mill scale distillation and micro scale distillation.  Each has its strengths and weaknesses.

Note:    At the completion of this exercise,  pour all left over sample into the container labeled 'waste solvent' in the hood.  Be sure to close the container after transfering your material.

Regular Scale Distillation Procedure:

This method is reasonably accurate (+/- 0.5oC), but requires a large amount of sample (> 5 mL), and is time consuming. The advantage of regular scale distillation is that you will also automatically purify your sample.

The procedure just boils down (no pun intended) to recording the temperature at which the distillation of the sample occurs. The distillation takes a reasonably amount of time so that reading the temperature can be done easily.  A large amount of sample is required because of the volume of the distillation flask and still head that have to be filled with sample vapor before distillation occurs.  A typical setup is shown below:

It is important to make sure that you whole setup is firmly supported, that all of the connections are leak free (use a VERY SMALL amount of silicon grease), that the water lines are firmly attached, and that you put in a boiling chip before you start the heating mantle.  A specific procedure follows:

  1. Measure ~10mL  of the sample to be distilled in a graduated cylinder.  Record the volume used to 0.1mL.
  2. Transfer the material to a 50mL, single neck, round bottom flask.  Support the flask in a suitably sized cork ring while transferring.  Add a boiling chip.
  3. Assemble the glassware as described in the figure above. Make sure to firmly clamp the boiling flask to a support rod.  Use a portable ring stand to support the receiving flask.  Ensure that the water inlet is toward the bottom of the condenser and the outlet is toward the top.  Make sure the set up provides a vent for any pressure buildup.  Have your set up checked by your instructor before heating.
  4. Turn on the water to the condenser so that a slow stream of water exits the condenser into the sink.  Be careful not to turn the water flow up too much or a hose connection may fail.
  5. Connect the heating mantle via the supplied electrical cord to the variable transformer.  (Never plug a heating mantel directly into an outlet.)
  6. Set the voltage on the transformer to approximately 50% of full scale.  Allow the liquid to come to a boil.  The vapors will rise past the thermometer bulb and condense in the condenser.  The distillate will collect in the receiving flask.
  7. The reading on the thermometer should begin to rise as the vapors contact the bulb.  Begin recording the temperature and time when you see the temperature begin to rise.  Continue recording for several minutes until the temperature remains within a 1-2 degree range.  This is the boiling range of your compound.
  8. After 5 minutes in the boiling range, shut off the transformer and allow the system to cool.  Combine the collected distillate with any material left in the boiling flask (a.k.a. 'pot').  You will use this material in the milli distillation procedure.

Milli Scale Boiling Point Procedure:

This procedure uses 1-2 mL of sample and gives reasonable accurate results in a fairly short time.  The procedure requires a reaction tube (or 13 x 100 mm test tube, thermometer, bead bath, and a boiling chip.  A typical set up is shown below using a reaction tube.  A 13 x 100 mm test tube can be substituted.

Clamp a 13-100mm test tube to a support and add ~1 mL of the sample.  Add a single boiling chip.  Clamp a thermometer so that the bulb is just above the level of the liquid and does not touch the sides of the test tube.  Place the test tube in a bead bath and adjust the heat so that the liquid refluxes about 3 cm up the thermometer but does not boil out of the apparatus.  Droplets of liquid must drip from the thermometer bulb in order to heat the mercury adequately.  The boiling point is the highest temperature recorded by the thermometer and maintained for a 1 minute interval.


Micro Scale Boiling Point Procedure:

This method is more accurate than regular distillation (+/- 0.2oC), can be accomplished very quickly, and requires the smallest amount of sample (approximately 0.2 mL).  However, it does require the fabrication of a Bell capillary tube and delivery pipet.  A typical setup is shown below:

The procedure consists of determining the temperature at which the external pressure on the boiling liquid is large enough to overcome the vapor pressure in a capillary tube inserted closed side up in the liquid. When this happens, the vapor will condense and liquid will rise into the capillary. Before you can determine a micro scale boiling point, you must construct several Bell capillary tubes.  First, heat a Pasteur pipet with the inner blue flame of the Bunsen burner about 1 cm from constricted part as shown below:


When the glass is soft, remove it from the heat and immediately stretch until you have a thin capillary tube between 9 and 12 inches long. It should resemble the drawing below:

Carefully break off and discard the bulb part at the position shown above.  Then carefully break off five or six 6 mm lengths from the end of the pipet.  Use a pair of forceps to grasp one of the 6 mm sections and hold it in the Bunsen burner flame to seal one end.  This should only take a second or two.  Repeat for the other segments.  These are your Bell capillaries, the remaining portion of the original pipet is your micro capillary delivery pipet.

Now that you have your Bell capillary tubes, you can determine the boiling point using the following procedure:
  1. Using a micro pipet fill about 1/2 inch of a melting point tube with sample
  2. Insert a Bell capillary tube, open end down
  3. Push the Bell capillary down into the liquid with a section of a long capillary tube. The long capillary tube resting on top will keep the Bell capillary submersed. Make sure that the sample does not fill the Bell capillary.
  4. Insert the sample loaded tube into the melting point apparatus
  5. Slowly heat the sample (about 2 oC per minute); bubbles will start to come out of the inverted Bell capillary (expansion of gas)
  6. Keep heating until a steady stream of bubbles comes out of the Bell capillary.  In actually, the bubbles are forming on the rim of the capillary (the rough surface serves as a site of nucleation) and are not really coming out of the capillary.
  7. Turn the heater off.
  8. Note the temperature at which the liquid just starts to rise into the inverted Bell capillary.  As soon as the temperature drops below the boiling point, the vapor in the capillary condenses and a partial vacuum thus created pulls the liquid into the capillary).
  9. Record temperature.

(Posted 9/02/03, C.R. Snelling)
(Updated 8/28/09, J. Neilan)