3. Aspirin Analysis
In this experiment, you will analyze the purity of your crude and recrystallized aspirin
products using a method called thin layer chromatography (TLC). You will also determine the percent yield of your reaction.
Prelaboratory Assignment
Read this lab guide. Then, complete both the TLC tutorial and the prelab assignment in
Chem21. The prelab assignment is due at the beginning of lab on the day your lab meets.
3.1
Introduction
The purity of your aspirin samples will be tested using a technique called thin-layer chromatography or TLC for short. Chromatography separates compounds in a mixture from
one another based on their different polarities. The “thin-layer” part refers to a thin layer
of silica gel (it actually looks like a white powder) adhered to a plastic sheet called a plate.
An impure sample is imbedded in the gel near the bottom of the plate, and the bottom
edge of the plate is inserted into a small amount of solvent. The solvent “climbs” up the
plate and carries the sample with it. The silica gel is called the stationary phase because
it does not move, and the solvent is called the mobile phase because it moves up the
plate. Different compounds in the sample will be carried up the plate to varying degrees
depending on the affinity of the compound to the stationary and mobile phases.
Once the mobile phase has moved up the plate, the plate is removed from the solvent
and the distance traveled by the solvent traveled is marked with a pencil. This marking is
called the solvent front. In most cases, the compounds on the plate can not be seen by the
naked eye, but they do absorb ultra-violet (UV) light. If you hold the plate underneath a
UV light, the places where the compounds are on the plate will appear as dark spots on a
bright green (fluorescent) background. These spots are usually marked with a pencil and
the distance traveled by the compounds relative to the solvent front are reported as Rf
values.
Distance traveled by compound
(3.1)
Rf =
Distance traveled by solvent
Figure 3.1 shows a TLC plate for a mixture that contains 3 compounds. The darkness
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of the spot is a qualitative indicator of how much of a particular compound was in the
sample. In the example shown, the mixture contained mostly compound A with B and C
as impurities.
solvent front
(marked with pencil)
C
B
distance traveled
by the solvent
A
distance traveled
by compound A
initial position of sample
(marked with pencil)
Figure 3.1: A schematic diagram of a TLC analysis for a mixture containing three components: A, B, and C. The spot for A is significantly darker than the other spots indicating
that there is more A in the mixture than B and C.
3.2
Precautionary Measures
• Gloves and goggles should be worn at all times during this experiment.
• The TLC solvent is hazardous and volatile. Keep solvent in the hood at all times.
• No solvent containers should be in use when the Bünsen burner is in use.
3.3
Experimental
Continue your notes from the previous experiment, recording what you do in each step
and your observations. Be sure to record any measurements with units and the correct
number of significant figures!
Percent Yield
Weigh the filter paper and watch glass containing your recrystallized product. Subtract
the mass of the filter paper and watch glass to determine the mass of the product you
made. (This mass is referred to as the yield.) In your notebook, calculate the theoretical
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3. Aspirin Analysis
yield from the mass of salicylic acid you used to start with. Then, calculate the percent
yield using the formula below.
Percent Yield =
Actual Yield
× 100%
Theoretical Yield
(3.2)
Thin-Layer Chromatography
Preparing spotting capillaries
You will test four samples, so you will need to prepare four spotting capillaries as follows.
(Your instructor will demonstrate this procedure for you.) Since the capillary preparation requires the use of an open flame, NO SOLVENTS SHOULD BE USED WHEN
CAPILLARIES ARE BEING MADE.
1. Light a Bunsen burner.
2. Obtain a few open-ended capillary tubes.
3. Using both hands, hold the center of the tube in the flame (keeping your hands clear
of the flame) until you feel the glass of the tube start to melt and get “wobbly.”
4. Remove the capillary from the flame, and immediately pull your hands apart. The
glass will stretch into a very thin tube.
5. Snap the tube into two pieces near the center. You know have two capillaries.
6. If you have any broken glass, be sure to dispose of it in the broken glass container.
DO NOT put any glass in the trash can.
7. Put all your capillaries into a small beaker until you’re ready to use them.
Preparing the plate
Obtain a TLC plate and, with the plate oriented with the long sides on the left and right,
use a ruler to gently draw a pencil line approximately 1 cm from the bottom on the grainy
side. The line should be absolutely parallel to the bottom of the plate. (Try not to scratch
the silica gel with your pencil, and DO NOT use pen.) Then, gently draw 4 hash marks
across your line spaced as evenly apart as possible and number them 1-4. Be sure to stay
at least 0.5 cm away from the edges. (See Figure 3.2.)
Preparing the samples
Place very small potions (about the size of a match head) of the following 4 samples into
separate wells of a spot plate. (Wipe off your spatula between samples!)
1. commercial aspirin (grind the aspirin tablets with a mortar and pestle if this has not
been done for you)
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1
2
3
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Figure 3.2: TLC plate preparation.
2. salicylic acid (the starting material for the aspirin synthesis)
3. your crude product
4. some of your recrystallized aspirin product
Add a couple of drops of acetone to the first sample and use the small end of one of
your spotting capillaries to gently mix the sample. Notice some of the solution has been
“sucked” into the tube. Holding the capillary perfectly up and down, touch the small
end of the capillary to the first hash mark on your TLC plate, and a very small drop of the
solution will be added to the plate. (If you don’t see the solution run out of the capillary,
you are probably not holding the capillary perfectly perpendicular to the plate.) After the
sample has been loaded onto the plate, rinse the capillary by inserting the tip into a small
amount of acetone. Then, blot the tip onto a paper towel to draw the acetone back out.
Repeat the rinse a few times.
Repeat this process with each of your samples, adding the acetone just before you
load the sample onto the TLC plate, and rinsing the capillary before each use. Each new
sample is added to the next hash mark. Once you’re finished, set your plate aside.
Running the TLC
Place 10 mL of the TLC solvent in a 400 mL beaker and cover it with a watch glass. This
beaker is your solvent chamber. Use tweezers to carefully place your spotted TLC plate
with the sample end down in the beaker, and immediately cover the beaker. The bottom
of the plate should be completely submerged in the solvent, but the solvent level should
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3. Aspirin Analysis
not be above the pencil line. (If the solvent level is above the pencil line, you must start
over from the beginning because your samples were rinsed into the solvent instead of
being allowed to run up the plate.) Watch your plate carefully. Allow the solvent to move
up the plate until it is about 1 cm from the top edge. (Don’t let the solvent reach the top of
the plate!) Remove the TLC plate from the beaker and immediately use a pencil to gently
draw a line that marks the solvent front (the distance the solvent traveled up the plate).
The solvent will begin to evaporate immediately, so you must work fast.
Developing the plate
Once the solvent has evaporated completely, place the plate underneath the UV lamp.
Gently outline each visible spot with a pencil. Take note of the darkness of each spot and
any other characteristics you observe.
Recording your observations
In your notebook, you should sketch your TLC plate as accurately as possible. Be sure
that the distances the spots and the solvent front have traveled on the actual plate are
accurately reflected in your sketch so that you can determine the Rf values for all the
spots.
3.4
Hazardous Waste and Clean-up
3.4.1
Hazardous Waste
Pour the used TLC solvent into the waste container in the hood at the front of the lab.
Solid aspirin waste can go in the trash can.
3.4.2
Clean-up
Dry your glassware and return it to your station. Then, wipe down your benchtop with a
wet paper towel and dry it. Have your instructor check your station before your leave.
BEFORE YOU LEAVE LAB: Tear out the carbon-copy pages of your notebook. Make sure
your name, your partner’s name and your section number are on each page. Staple these
pages together and turn them in to your instructor.
3.4.3
Lab Assignment
Enter all of your data, calculations and answers to questions in the Lab 3 Assignment
in Chem21. You will need to refer to your notebook for this. The Assignment is due
approximately 15 minutes before your next lab meeting.
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3.5
Calculations
The information in this section will help you complete the lab assignment in Chem21. You
are encouraged to stay and work on this part before you leave the lab. When you click
on the assignment in Chem21, you will be asked to enter all your data before proceeding
with the calculations. Once your data has been entered, you will be asked to calculate the
following:
• Mass of acetylsalicylic acid
• Theoretical yield of acetylsalicylic acid (See section 4.3 in your textbook.)
• Percent yield of acetylsalicylic acid (See equation 3.2.)
• Rf values for each spot on the TLC plate (See equation 3.1.)
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