Lab Thin Layer Chromatography (TLC) – TLC of Analgesics

Objective The objective of this lab is to understand the principles of polarity of Organic Compounds and Thin Layer Chromatography (TLC)
Based on these principles, the purpose of this experiment is to determine the composition of a series of over-the-counter pain killers.
Chemical Equation N/A
Chemicals Needed • Ethyl Acetate with 0.5% Acetic Acid
• Methylene Chloride (Dichloromethane)
• Aspirin (Acetylsalicylic Acid)
• Acetaminophen
• Caffeine
Materials Needed • 2 – 50 mL beaker.
• 2 – 150 mL beaker.
• 2 – 250 mL beaker.
• Watch glass.
• TLC plates.
• Ruler.
Safety Consideration Dichloromethane is a carcinogen. Please use it in the hood.
Wear gloves at all times. If you leave the lab, take the gloves off, wash your hands, and use new gloves upon lab re-entry.
UV light is dangerous. Do not stare into the UV light.

Waste disposal information Dispose of all chemicals properly in labelled containers in the hood.

Dispose of all glass (pipettes) in the waste glass container.

Requirements before coming to lab Watch all videos posted on Blackboard for this experiment, and Read about the following topics in your Textbook or Online: (1 – 5)
1. Chromatography.
2. Separation.
3. Thin layer chromatography.
4. Functional groups.
5. Polarity and Dipole moments.
6. Prepare a pre-lab. This section, #6, can be omitted for fall 2020 only.
The pre-lab includes:
a. Title of the experiment
b. The aim or purpose
c. The chemical equation
d. The reaction mechanism
e. Safety considerations
f. A reagent table
g. Detailed procedure.

Procedure General procedure for TLC plate Use the following procedure for preparing and developing TLC plates.

1. Prepare a “chamber” by cutting the top and bottom off a circular piece of filter paper. This will go inside a 250 ml beaker, sized to fit a TLC plate, place the cut filter paper in the chamber as shown below. You will also need a watch glass to place on the top of the beaker.

2. Place the cut filter paper inside the beaker. Try to get it as close to the top as possible. This paper will help to “equilibrate” the internal atmosphere of the beaker to be saturated with solvent, which will help the plate develop.
3. Place less than 0.5 cm of solvent (ethyl acetate with 0.5% acetic acid) in the bottom, cover with the watch glass, and let the chamber “settle” while you do the next steps.
4. Prepare the TLC plate. LIGHTLY, mark a straight line 1 cm from the bottom with a pencil.
5. Spot three samples on the plate on the light line, and equally spaced from left to right. See the picture below. Use a capillary (a melting point tube which can draw the sample from an open end). The instructor will demonstrate this.
6. GENTLY, place the plate in the developing chamber. Don’t forget the watch glass.

7. Let the plate elute (develop upwards) until it has reached about 1.5 to 1 cm from the top of the plate.
8. Take the plate out, and IMMEDIATELY mark the solvent line with a pencil lightly.
9. Let the plate dry in the hood (about 5 minutes).
10. Look at the plate under a UV light in the hood. Circle any sample spots you see with a pencil.
11. Place the plate into the iodine chamber. Let the plate develop. Some of the spots may be spotted a brown-yellow color.
12. Measure the Rf. The Rf is the distance from the origin the spot (midpoint of spot) travelled over the distance from the origin the solvent travelled. The number is dimensionless and must ALWAYS be between 0 and 1.
The Rf is called the retention factor.

ALWAYS COMPARE Rfs. Do not compare distances travelled, except on the same plate.

TLC of known analgesics The solutions of know analgesics (aspirin, acetaminophen, and caffeine) are pre-made by the stockroom. There is also a solution of all three mixed together in a 1:1:1 ratio (Known mixture).

1. Run the first plate with the 3 knowns.
2. Use the general procedure to get Rf values.
TLC of unknown analgesics 1. Run another plate with your unknown, and with the solution of the known mixture.
2. Use the general procedure to get Rf values.
3. Using the Rf values, determine the components in your unknown. Using Appendix A, determine the commercial name of your unknown.

Check with Instructor (Omit this section for fall 2020 only) Check with the instructor to see that you have properly identified your unknown.

Properly dispose of the chemicals in the appropriate containers.
See waste disposal information.

 
Requirements to complete the lab Complete the worksheet for this lab and submit it on Blackboard by the due date
Last Updated August 10, 2020

Appendix A – Commercially available analgesics.
Commercial name Aspirin Caffeine Acetaminophen
Bayer Aspirin X
Generic Aspirin X
Excedrin X X X
Anacin X X
No-doz X
Tylenol X