THEORY
Simple distillation was a method of separating two or more components that had large difference in their boiling points. However if the two components have a boiling point difference of about 40 degrees or less, simple distillation will not be a successful technique for separating the two liquids. In this case, fractional distillation is used.
Fractional distillation can be considered a series of simple distillations; however, rather than having many condensers and receivers, the evaporation-condensation cycles occur in a long tube, the distillation column.
Distillation columns come in many types, from a low efficiency unpacked column to the advanced "spinning band" column. Usually a column comes with some kind of packing, such as glass beads, stainless steel rings or coils, and so on; the packing increases the surface area on which condensation can occur. Increased surface area lowers the Height Equivalent to a Theoretical Plate (HETP), and a better separation of components is possible. Consider that one evaporation-condensation cycle happens over one theoretical plate. Keep in mind that Column Height / HETP = Number of Plates.
In our CHM 245 course, you will use a "Vigreux" column. This column has no separate packing, but numerous glass protrusions inside the column increase surface area. You will not be determining the HETP of the column; that is a more advanced, and more time consuming, lab.
If
we extrapolate the idea of increasing surface area to improve column
efficiency,
we reach an absurd conclusion. Consider what a very high surface
area packing would be .... glass wool, cotton, or sand, for
example.
These would clog the column and become saturated with distillate.
No liquid could be collected.
This
example is an extreme illustration of the concept of "hold up".
Any
column packing will end up being coated by the distilling liquid.
Liquid that sticks to the packing will not be distilled. There
will
always be some liquid "held up" in the column. Typically, the
more
surface area there is, the more hold up there will be.
The
fractional distillation process works because a temperature gradient is
set up over the length of the column. Ideally the applied pot
temperature
is equal to (not higher than) the boiling point of the liquid
mixture.
The head temperature should be equal to (not higher) than the more
volatile
component's boiling point. (We're assuming a perfect thermometer
and one atmosphere pressure.) These conditions require the
gradual
application of heat and a slow distillation rate. Most liquid in
the column should be refluxing (i.e., condensing back into the
pot).
The reflux ratio is a measure of material allowed to reflux compared to
material being collected.
The finest kind of
fractional
still is the spinning band column.
The
"packing" is a rotating spiral made of Teflon. Reflux is enhanced
because the spiral tends to take liquid back down the column. We
have one in the back of the lab.
EXPERIMENTAL The
liquid mixture that you will separating will contain ethyl acetate and
n-butyl
acetate.Be sure to look up in
advance the relevant information about these two compounds:formulas,
structures, boiling points, and toxicities. Also
look up their refractive indices.
You will
start
with a 50/50 mixture of ethyl and butyl acetate. Place 10 mL of
ethyl
acetate and 10 mL of n-butyl acetate and 2-3 boiling stones in a 50 mL
RBflask (still pot). Set up a fractional
distillation
apparatus. Use a 10 or 25 mL glass graduated cylinder as a
receiver,
and a Vigreux column. You can clamp the receiver as show in the
picture,
or you may use a lab jack.
In
the next laboratory meeting, you will analyze your three fractions by refractive
index, and by gas chromatography.
Start with
moderate heating (~6 on the heating mantle.) The material in the
pot will begin to boil. Do not read the thermometer at this
point!
The thermometer does not register the boiling point of the more
volatile
component until vapors read the stillhead and liquid condenses on the
themometer
bulb. In the photo, the thermometer is a placed little high - the
thermometer is in the correct location when it reads the closest to the
correct boiling point.
Once the
temperature
levels off, collect about 2 mL of distillate (Fraction I) in a
screw-top
vial. You'll find these in the white cabinet.
Continue distilling until about half of the liquid is distilled.
Either the head temperature will begin to rise - or, the distillation
stops.
If the latter occurs, increase the setting on the heating mantle.
(Not enough heat is being supplied to boil the less volatile
component).
If necessary, "tent" the still with aluminum
foil.
As the head temperature rises about the boiling point of ethyl acetate,
collect another 2 mL (Fraction II). Eventually the head
temperature
will stabilize again. Be sure the pot
does
not go dry!! Turn off the
distillation,
let it cool thoroughly (an ice bath may be used). The liquid
remaining
in the pot we'll refer to as Fraction III.
