Page 1 Exp. 9, Separation by Simple and Fractional Distillation and Analysis by Gas Chromatography CHEM 221 Lab
Exp. 9, Separation by Simple and Fractional
Distillation And Analysis by Gas Chromatography
LEARNING OUTCOMES:
After performing this experiment the student will be able to:
1. Perform a simple distillation and a fractional distillation
2. Obtain a gas chromatogram on a sample and interpret it
3. Explain the difference between the separation of a simple distillation and of a fractional distillation and
the reasoning behind it
4. Describe how separation is achieved in gas chromatography
INTRODUCTION:
This experiment is a technique lab on separating and purifying
compounds by distillation and on using GC to analyze samples.
Distillation is a frequently-used technique involving boiling and
condensing and is used for mixtures of liquid compounds that have
different vapor pressures (different boiling points). When a liquid
mixture is boiled, the vapors above the liquid are richer in the lower-
boiling component. Condensing these vapors results in a purified
sample. Figure 1 shows a typical laboratory setup for a simple
distillation.
When the difference in boiling points of the components is large
(>40-60 °C), a fairly good separation often can be made with a
simple distillation (Figure 2b). When the difference in boiling
points of the components is small, then a simple distillation
cannot achieve a good separation (Figure 2a). If a better
separation is desired when the components have similar boiling
points, then a fractional distillation can be done. The simple
distillation setup in Figure 1 can be converted to a fractional
distillation setup by inserting a fractionating column at number
11 in the figure. The fractionating column can be thought of as a
place where the vapors condense and then boil again. Each time
a very small sample condenses and then boils again, the vapor
is even more enriched in the more volatile (lower boiling)
component. Thus, fractional distillation can produce samples
that are much purer. This increase in purity comes with a cost
of more complex equipment and more distillation time.
Gas chromatography (GC) is another separation technique.
Since the amount of sample that can be separated at one time is
very small, it rarely is used for purification of material. It is
used quite often in analyzing mixtures. In GC a sample in the
gas phase is passed through a column (stationary phase) with a
flow of a gas (mobile phase). Different compounds will be
retarded different amounts by different interactions with the
stationary phase, a coating on the inside of the column. With the column we are using many compounds pass
through the column in order of increasing boiling points, and is sometimes called a “boiling point column”.
However, the compounds are not separated by boiling points but are separated by a combination of the vapor
pressures of the components and how much each interacts with the stationary phase.
This experiment will be done in teams of two, with each team setting up and tending a simple distillation and a
fractional distillation. At specific volumes the temperature will be measured and a sample collected. Each team
Figure 1. Laboratory display of distillation:
1: Heating mantle 2: Still pot 3: Still head
4: Thermometer 5: Condenser 6: Cooling
water in 7: Cooling water out 8: Vacuum
adaptor 9: Receiving flask 10: Anti-bump
granules 11: Expansion place for inserting a
fractionating column. Adapted from
http://en.wikipedia.org/wiki/File:Simple_distillation_apparatus.svg
Figure 2. Distillation curves for a) poor
separation and b) good separation.
