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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 deter- mine 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 chro- matography 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.
Rf =
Distance traveled by compound Distance traveled by solvent
Figure 3.1 shows a TLC plate for a mixture that contains 3 compounds. The darkness
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.
A
B
C
solvent front (marked with pencil)
initial position of sample (marked with pencil)
distance traveled by compound A
distance traveled by the solvent
Figure 3.1: A schematic diagram of a TLC analysis for a mixture containing three compo- nents: 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 B ¨unsen 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
1 2 3 4
Figure 3.2: TLC plate preparation.
- salicylic acid (the starting material for the aspirin synthesis)
- your crude product
- 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
- 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.
