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Thin Layer Chromatography analysis of Analgesic Drugs

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  • Pages: 7
  • Word count: 1583
  • Category: Drugs

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The purpose of this experiment is to focus on the ingredients that the analgesic drugs may contain. That known analgesics will be used as well as an unknown, the determination of the experiment will be based of the exposure of the chemicals the analgesics may contain and will be seen under an UV light.

Chemical Equation:

Chemical Mechanism:

Safety Concerns:
• Not overheating ethanol dichloromethane
• Analgesics are hazardous if ingested
• Always wear gloves, goggles, and a lab coat
• Don’t look directly at the UV light
• Acetic acid can cause irritation if in contact with eyes and skin

Results and Observations:

In the lab, four analgesics where known and each student had to test it, there also was a mixture that was the control group that had to used for each plate. Three plates were used throughout the whole experiment, one had to contain two analgesics of your choice not the same as well as the second plate with the mixture included in the middle, the last plate had to contain the unknown of your choice, my unknown was unknown #68. For the first plate my choices were acetaminophen as #1 and caffeine as #2 and the mixture in the middle of the plate, for these two analgesics first the dots had to be added with capillaries only one small dot was to be made. After adding the dots, the plates had to be put in a solvent that had ethyl acetate and let the plate soak until the line marked at the top of the plate, after soaking and letting dry the plate was put under UV light and this method showed us the spots how the solvent made the dots run upwards.

For this first plate the mixture had 4 dots, acetaminophen 1, and caffeine 1 as well. The dots were to be circled so that after the UV light the spots could still be seen, acetaminophen spot was in the same level as the spot #2 of the mixture and caffeine was in the same level as spot #1 in the mixture. For the second plate my choice of analgesics was the last 2 left of the four analgesics, which were Ibuprofen as #3 and Aspirin as #4, the same procedure was done to the second plate as the first plate, the soaking and then the observation under UV light. The same results came out in the second plate, 4 spots on the mixture and 1 spot for ibuprofen and 1 spot for aspirin.

Ibuprofen was at the same level as spot #4 of the mixture and aspirin was at the same level as spot #3 of the mixture. As for the last plate the unknown had to be used in this plate and the mixture only the same procedure was done as plate 1 and 2, except that instead of ethyl acetate we had to use ethanol dichloromethane. The third plate contained 1 spot for the unknown and 4 spots for the mixture after visualizing under the UV light, the unknown was at the same level of spot #2 of the mixture and concluded that unknown #68 was like acetaminophen just by observing the plate. After getting approval by the professor the plates had to be placed in iodine chambers and shake them well to see the spots. Calculations had to be made by measuring the center of the spots to the initial line and measure distance from initial to final line and divide.

Rf = distance traveled by sample (cm)/distance traveled by solvent (cm)
Retention Factor Calculations
Rf Plate 1 Plate 2 Plate 3
Acetaminophen: 0.44cm – –
Caffeine: 0.15 cm – –
Mixture: Spot 1: 0.15 cm 0.13 cm –
Spot 2: 0.42 cm 0.32 cm –
Spot 3: 0.65 cm 0.54 cm –
Spot 4: 0.72 cm 0.64 cm –
Ibuprofen: – 0.64 cm –
Aspirin: – 0.55 cm –
Unknown#68 – – 0.35 cm

Discussion of Results:

For each plate that had a dot, calculations had to be done. Total of three plates, two plates with known analgesics and one with unknown analgesic. Total of four known analgesics and one mixture for all the plates. The Rf of each plate had to be determined for each analgesic spot and each spot for the mixture for the first two plates the third plate the mixture was not determined because the first two plates already had the calculations. After measuring and dividing the measurements it concluded that the one with lowest Rf was caffeine since it was the first dot with the same level as the first dot from the mixture concluding it was the lowest and the highest was Ibuprofen because it was at the same level as the fourth dot of the mixture, but all was concluded with the calculations that the spots were close or the same number as the mixture spots.

Acetaminophen was concluded to be the second since it was at the same level as the second spot of the mixture and aspirin the third. For the Unknown #68 it was concluded by observation that it was like acetaminophen since it was matching the second dot of the mixture. The calculations prove our observation to be correct since for acetaminophen the retention factor was 0.44 cm and the retention factor for unknown #68 was 0.35 cm, six digits away from being the exact number, but concluded that they had similarities since the retention factor for spot #2 of the mixture of the first plate was 0.42 cm.


On this lab experiment, TLC was conducted to know the identity of an unknown. The lab was based mainly on the results of what was obtained from the unknown that the student had. Known analgesics were used as well. On the analgesics that were known that were given to students to test, from those the unknown had to be figured out and to which one it had similarities with. Unknown #68 had similarities with acetaminophen.

Post Lab Questions:

1. Which compound will have the larger Rf value on a Si02 TLC plate using 10 % ethyl acetate/ hexane as eluent: 3-decanone or 3-decanol, toluene or benzoic acid, cyclooctane or cyclooctanone? Why?

3-decanone has higher Rf value than 3-decanol, Toluene has higher Rf value than benzoic acid, cyclooctane has higher Rf value than cyclooctenone. Non-polar compounds will have a higher Rf value since non-polar solvent sticks to non-polar making the compounds have a higher value than those with polar compounds.

2. What does TLC stand for in today’s experiment?

Thin Layer Chromatography

3. Arrange the following organic solvents in order of decreasing polarity and give their respective structures when doing so.

Acetone, Heptane, Acetic Acid, Toluene, Methanol
Acetic acid>methanol>acetone>toluene>methanol

4. Define Rf.

Retention factor, is basically to show the solubility of a substance by the distance it travels with the solvent.

5. Arrange the following compounds in order of increasing Rf value on silica gel and explain your reasoning.

Benzaldehyde, n-decane, benzoic acid. What effect would be observed if the TLC were run with a more polar solvent?
Due to silica gel’s polarity, non-polar components tend to be removed before more polar ones. Rf value is:
Benzoic acid < Benzaldehyde < n-decane.

6. Calculate the molecular weights of all analgesics used in this lab. Use the periodic table I gave you. Don’t Google these…

7. How many moles of acetaminophen does a 500 mg tablet of Tylenol contain?

Contains 3.31×10-3 mol

8. Why was a 50/50 mixture of dichloromethane and ethanol added to the crushed analgesic tablets?

It was used to help dissolve the tablets much easier since the analgesics wouldn’t be solved with just polar solvent since the tablets had buffering agents that would not let them dissolve in the polar solvent.

9. Why is it advisable to mark a TLC plate with pencil and not a pen or marker?

The ink of the pen would run up with the analgesics as well making the plates contaminated and analgesics mixed in with the ink of pen.

10. Why is a small concentrated spot desirable in running a TLC?

A small amount is better use since when the plate with the solutions is added to the solvent will start running upwards making the small amount spread while running up making them bigger.

11. Why was the mixture of knowns run on both TLC plates?

The mixture was kind of a control group for the analgesics being used to have a better explanation of the reagents being used, it basically was there to describe and make it easier to identify an unknown.

12. A folded piece of filter paper was introduced in the TLC developing chamber. Why?

It helps with the stabilization of the plate, speeds up the process of the solvent running, and it creates a saturated atmosphere for the TLC plate.

13. Why must the developing solvent level at the bottom of the developing chamber be kept below the sample spot applied to the TLC plate?

Since the dots are at 1 cm away from the bottom the perfect way to make the sample spots from being dissolved would be to start from the bottom making the plate absorb the solvent to make the dots go upward giving better access to see the spots instead of a mess if the solvent started from the middle since it wouldn’t have be able to touch the spots at all since its running upwards.

14. Why do the sample spots increase in diameter as the TLC plate develops?

When a small drop is added and then the plate is put into the solvent the plate absorbs the solvent running the solutions spreading them while running upwards making the spots wider and larger while running upwards.

15. Why must the solvent front be marked immediately after removal from the developing chamber?

The solvent dries up very quickly.

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