TLC is thin layer chromatography, chromatography in which compounds are separated on a thin layer of adsorbent material, typically a coating of silica gel on a glass plate or plastic sheet.
Lab coats and safety glasses must be worn as Ethanol which is flammable, Ethyl Ethanoate which is volatile, highly flammable and the vapor may irritate the eyes and Iodine crystals will be used in this experiment.
Hold the plate by the edges only, draw a very light line in pencil near the bottom of the plate, mark three evenly spaced X’s on this line, these will be used to place a small amount of solute aspirin on, two-class made aspirins, and one shop-bought aspirin.
Take two watch glasses and place a small amount of each of the class samples in each watch glass and using a spatula crush to a very fine powder (this makes it easier to make into a solution) add a few drops of Ethanol and mix to a solution (aspirin is soluble in Ethanol). Place a finger on the top of a capillary tube and touch the open end into the aspirin solution and a small amount will be drawn up, place over X 1 and remove the finger, this will spot the sample, record which sample went to X 1, repeat for X 2, do the same for the shop-bought aspirin and place on X 3.
Place the plate in an appropriately sized beaker, at the fume cupboard add just enough Ethyl Ethanoate so as it won’t evaporate, use a watch glass as a lid and keep it in a fume cupboard. Keep an eye on the chromatogram so as not to allow the solution to go over the top, this would make it difficult to measure. When the solution has reached a short distance from the top remove the chromatogram and put it in a beaker with a few Iodine crystals, the iodine vapors will react with aspirin as brown/orange spots.
After about 15 minutes the Iodine has reacted with the aspirin and the results are very visible. All three samples seem similar which suggests our class samples have the same purity as the manufactured aspirin.
If a permanent record is needed, take a photo as the spots of colour will fade.
Rf is retention factor, it is defined as the distance travelled by the compound divided by the distance travelled by the solvent.
X1 was Dylan’s sample, X2 was Jacks sample and X3 was shop bought sample. Distance moved by all three samples = 39mm, distance of solvent = 42.
| sample | Mobile phase, mm | Band travelled, mm | Rf |
| X1 | 58 | 42 | 0.928 |
| X2 | 58 | 42 | 0.928 |
| X3 | 58 | 42 | 0.928 |
39 / 42 = Rf 0.928, once again this comparison shows the purity of class and shop-bought samples is very close, this would suggest our recrystallization technique was good at purifying the crude product.
To achieve accurate results use a clean spatula and watch glass for crushing the samples to stop and cross-contamination. Measure the distances on the chromatogram several times and average, make sure the ethyl ethanoate does not rise above the spot line.
Our class samples purity according to titration is, % Purity = Mass / 0.1g x 100 = 0.0954 / 0.1 x100 = 95.4%, if we copied the same procedure for shop bought aspirin we could compare the purity more accurately.
Compare against other class results to get an overview of purity of aspirin.
