Notes for Teacher

MICROSCALE ANALYTICAL EXPERIMENT

Thin Layer Chromatography Analysis and Purification of Aspirin by

A. Materials

Chemical

Acetic acid
Aspirin
Caffeine (as impurity in sample)
Dichloromethane
Deionized water
Ethyl acetate

Glassware

Beaker (plastic)
Boiling stick
50-mL Conical flask
Cotton glove
Developing bottle (for TLC)
Filter adapter
Forceps
Hirsh funnel
Hot plate stirrer
10-mL Measuring cylinder
Spatula
25-mL Suction flask
Stand and clamp
Vial

B. Preparation of materials

Caffeine is used as the impurity in this experiment, and the impure aspirin sample (S) for TLC
analysis and recrystallization is prepared by mixing aspirin and caffeine (2% w/w) in their powder
form.

Development solvent for TLC analysis is prepared by mixing 10 mL of ethyl acetate and 10 drops
of acetic acid.

1
Bromocresol green stain reagent is prepared by dissolving 0.04 g of bromocresol green in 100 mL of
absolute ethanol, then add 0.1 M solution of aqueous NaOH dropwise until a blue color just appears in
solution. Ideally, this stain is to be stored in a 100-mL wide mouth jar. The lifetime of such a solution
typically depends upon solvent evaporation. Thus, it would be advantageous to tightly seal such jars
in-between uses.

The melting point of the compound in this experiment is determined by Electrothermal IA9100
Melting point apparatus.

The IR spectrum in this experiment is recorded by PerkinElmer Fr-IR Spectrum RXI Express
system.

C. Suggested time for the experiment

Part A: Thin layer chromatography: 15 minutes

D. Instruction for students

Experiment focus:
- Theory and techniques in TLC analysis
- Theory and techniques in recrystallization, including making a supersaturated solution for
recrystallization, completely dissolve the impure sample before cooling, slow cooling of the
hot supersaturated solution for forming crystals of bigger size and higher purity.
- TLC as a rapid, effective and simple method in analyzing the purity of a compound.
- Recrystallization as powerful purification method for solid sample
- Melting point as another method for characterization and determination of purity
- IR spectrum interpretation.

E. Student's common errors and possible mistakes and Suggestion to avoid the errors and
mistakes

1. Error: Too much developing solvent is added to the developing bottle, so some of the samples
spotted on the TLC plate being dissolved in the solvent.
Suggestion: Add developing solvent slowly to the bottle to control the amount added.

2. Error: Students used to hold TLC plates with fingers, this can cause contamination.

3. Error: Not marking the spots with pencil immediately after developing may result in
disappearance of the spots.

4. Error: Recrystallization: Adding too much water for dissolution of impure sample cause no
crystal form after cooling down.

2
 Suggestion: 5 mL of water is enough to dissolve all the solid, students are advised to add a small
amount (~ 3 – 4 mL) of water first, then heat the solution to boil, add more water (1 – 2 mL) if
there is still solid remaining. Add more water before boiling can cause the above error easily.
If it is the case, one can concentrate the solution by evaporation in a warm water bath. The
solution should be supersaturated for crystals formation.

5. Error: The flask in step 18 is put to the ice bath before completely cooling down to room
temperature, smaller crystals with lower purity result.
Suggestion: It is important to note that slow cooling is the key criteria for crystals to be formed
in high purity.

F. Further suggestions to the experimental procedures

1. It may take 15 – 25 minutes in step 18 to cool the mixture to room temperature

2. If time allowed, students can be asked to determine the melting point of the sample before
recrystallization and compare with the data obtained from the crystal after recrystallization.

G. Suggested answer to “Specific Chemical Hazards”

Hazard Flammable Corrosive Toxic Irritant Harmful Oxidizing Explosive Others

Chemical

Acetic acid   

Aspirin  

Caffeine  
Asphyxiant,
Dichloromethane   Carcinogenic,
CNS depression

Ethyl acetate   

H. Reference

1. Saqib Ali, Abida Kalsoom, Journal of Chemical Education, 1991, 68, 877.

2. Irving Allan Kaye, Henry Yuska, Journal of Chemical Education, 1970, 47, 703.

3. Gene A. Hiegel, Journal of Chemical Education, 1986, 63, 273.