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Question: A colorless unknown substance is spotted on a TLC plate and developed in the correct solvent. The spots do not appear when visualization with a UV lamp or iodine vapors is attempted. What could you do to visualize the spots if the compound is the following:

C. An amino acid
My answer: A ninhydrin solution can be sprayed on the silica gel plate and it will react with the amino acid to give a color when the plate is heated.

D. A sugar
My answer: Sugar and other substances that are easily oxidized can be visualized by using chromium trioxide, potassium dichromate, and potassium permanganate.


ninhydrin is ok for C. I don't know how much you intend to heat the plate but one must be careful not to mess up the gel and/or break the plate. The ninhydrin/amino acid work I am familiar with was all done with paper chromatography. In this case, the sprayed paper was placed in an oven to evaporate the solvent and dry the paper. I don't remember the temperature of the oven but the color was purple.
For D, do you mean you must use all three (that's the way the answer is stated) or will any of the three work ok. I'm sure you intended to write or instead of and. Have you omitted the color formed; i.e., how do you know where the spot is? How do you apply these oxidants? An an aside, I wonder if conc H2SO4 would work here? It will char table sugar easily. But wouldn't spraying conc H2SO4 be a nasty thing?


I wrote the 'and' as it was written in the book but I'm sure you're right that is should be 'or'. There is no more detail about this procedure in the book, which is typical of the book, so that'll have to do.

Lab 4 is finished and thanks for all the help.

Sheryl

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