compound's boiling point is the temperature at which the
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
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.
method is reasonably accurate (+/- 0.5oC),
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:
- Measure ~10mL of
the sample to be distilled in a graduated cylinder.
Record the volume used to 0.1mL.
- 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.
- 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.
- 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.
- Connect the heating mantle via the
supplied electrical cord to the variable transformer.
(Never plug a heating mantel directly into an outlet.)
- 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
- 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.
- 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.
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,
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.
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.
This method is more accurate than
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
Before you can determine a micro scale boiling point, you must
several Bell capillary tubes. First, heat
a Pasteur pipet with the inner blue flame of the Bunsen burner about 1
constricted part as shown below:
REMEMBER THAT HOT GLASS LOOKS THE
SAME AS COLD GLASS!)
the glass is soft, remove it from the heat and
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:
- Using a micro pipet fill about 1/2 inch of
a melting point tube with sample
- Insert a Bell capillary tube, open end down
- 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.
- Insert the sample loaded tube into the melting point apparatus
- Slowly heat the sample (about 2 oC per minute);
bubbles will start to come out of the inverted Bell capillary
(expansion of gas)
- 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.
- Turn the heater off.
- Note the temperature at which the liquid just starts to
rise into the inverted Bell capillary. As soon as the
drops below the boiling point, the vapor in the capillary condenses and
a partial vacuum thus created pulls the liquid into the capillary).
- Record temperature.
(Posted 9/02/03, C.R. Snelling)
(Updated 8/28/09, J. Neilan)