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Chromatography is an all-inclusive term that encompasses a set of laboratory methods used to separate mixtures. In this lab the objective was to determine the active ingredients in common analgesic tablets through thin-layer chromatography. To do this a thin-layer chromatography (TLC) plate was utilized. This plate is essentially a polar solid. When a mixture passes over this polar solid the polar compounds in the mixture will come to an equilibrium with the compound. This equilibrium will slow the movement of the polar compounds up the solid. However, the less polar solids will flow up the solid. This technique allows for compounds such as hydrocarbons which are not very polar, to be separated from compounds containing functional groups like amines and alcohols. In this lab the thin-layer chromatography plate is formed by a section of silica being laid over a section of tin foil.
The other important aspect of this TLC plate is an eluting solvent (eluent). This is a polar solvent and depending on the amount of polarity will determine the rate the solution moves up the TLC plate. In this experiment an eluent of 95% ethyl acetate and 5% acetic acid will be used.
TLC is very useful in three functions, monitoring a reaction, determining the number of components of a mixture and comparison to a known compound. In this experiment a known compound is used to determine the compounds in an unknown analgesic tablet.
Analgesic tablets include, aspirin, ibuprofen and acetaminophen. They are used to reduce pain without other side effects. Many companies have taken certain amounts of each of these compounds to make over the counter drugs with a specific commercial name. In this experiment the component of “Excedrin” will be determined.
Originally the procedure required the pulling of capillary tubes from glass Pasteur pipettes, however these were prepared previous to the lab. First acquire two 30 cM sections of capillary tubes and a TLC plate. Then draw a sample baseline 1 cM from the bottom of the TLC plate. The mixture of studied compound must now be made. Put 5 mL of ethanol in a test tube and add one tablet of aspirin. Using a stirring rod crush up the tablet until it is thoroughly mixed. Next add 5 mL of eluent to a 250 mL beaker and cover. Finally before the TLC begins the standards of aspirin and ibuprofen must be obtained.
Using the capillary tube spot each mixture on the baseline drawn on the TLC plate. This is done by dipping the capillary tube in the mixture and then tapping it once on the baseline and finally breaking off a two-centimeter section of tubing. Make sure to label where and what each mixture is on the plate. Then place the plate in the 250 mL beaker. The eluent should not come over the baseline and should be seen climbing up the silica. Once it has reached 1-2 cM from the top of the plate remove it from the beaker and draw a line where the eluent was stopped. This is called the solvent front. Using the UV lamp visualize and circle where the dots are now on the plate. Measure the distance from the baseline to each of the dots and to the solvent front.
Repeat the procedure, instead, use Excedrin as the tablet of interest and include, caffeine, acetaminophen, aspirin, and ibuprofen standards.
The ratio of the sample height to solvent front (Rf) was then calculated using the equation: Rf = (distance to spot from baseline)/(distance to solvent front from baseline). The Rf values of each standard could then be compared to the tablet of interest to determine its content.
From the data presented above and the drawings of the TLC plates included, it can be determined that the first TLC plate done was accurate. The tablet dissolved contained aspirin and it lined up with the aspirin standard given. This leads to the conclusion that the standard is correct and that the TLC plating technique was accurately done.
The second TLC plate identifies acetaminophen as the main component of Excedrin. This is due to the fact that they lined up with one another on the plate and thus their calculated Rf values are the same.
The predicted placements of the compounds on the TLC plate were also affirmed. Caffeine based on its molecular structure was the most polar and thus should have been lowest on the plate. Ibuprofen is the least polar of the five compounds and thus was seen as the highest spot on both plates.
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