In a TLC experiment, why should the spot not be immersed in the solvent in, the
ID: 769603 • Letter: I
Question
In a TLC experiment, why should the spot not be immersed in the solvent in, the developing chamber? Explain why the solvent must not be allowed to evaporate from the plate during development. Explain why the diameter of the spot should be as small as possible. Which of the two diastereomers of azobenzene would you expect to be more thermodynamically stable? Why? From the results of the TLC experiment with the azobenzenes, describe the role of sunlight. A student obtained the silica gel TLC plate shown in Figure 6.6 by spotting Samples of Midol , caffeine, and acetaminophen on the plate and eluting with petroleum etherrchloroform (9:1 v:v). What are the Revalues of acetaminophen and of caffeine, respectively? Based on this TLC analysis, what are the ingredients in a tablet of Midol ? What are the mobile and stationary phases, respectively, in this TLC experiment? No spots were observed visually when the TLC plate was removed from the developing chamber. How might the student effect visualization of the spots? Another student accidentally used Midol PM in her experiment and observed only one spot. Speculate as to which spot was absent and offer a possible explanation for the difference in this student's result.Explanation / Answer
8. (a) You have to measure the distance from the origin to the middle of the spots that is your "compound distance traveled" value, divide the compound distance traveled value by the measurement you get when you measure from the origin to the eluent/solvent front. So it's : compound distance traveled / solvent distance traveled = Rf. (b) The components in Midol separated into two well defined spots which clearly match up exactly with caffeine and acetaminophen so caffeine and acetaminophen are present in Midol. (c) The mobile phase is where the solvent traveled up the TLC plate and the stationary phase is the thin layer of silica gel or alumina (in other words, the actual TLC plate). (d) The student could visualize the spots by putting the plate under a UV light or using some type of stain. (e) Well for Midol PM clearly the caffeine spot would be absent because someone taking Midol PM would presumably be going to sleep and caffeine before bed would be a good idea. Hope this helps.