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
You're welcome, Sheryl! I'm glad I could help you with your lab questions. If you have any more questions in the future, feel free to ask. Good luck with your studies!
It seems like you have made some correct observations regarding the visualization of the spots on the TLC plate for both an amino acid (compound C) and a sugar (compound D).
For compound C (an amino acid), you correctly identified that using a ninhydrin solution can be sprayed on the silica gel plate to react with the amino acid and produce a color when the plate is heated. However, you mentioned that the ninhydrin/amino acid work you are familiar with was done with paper chromatography rather than TLC. This is a valid point, as paper chromatography is often used for amino acid analysis.
For compound D (a sugar), you suggested using chromium trioxide, potassium dichromate, and potassium permanganate as oxidants to visualize the spots. While it is possible that any of these oxidants could work individually, it is unclear from the information provided which one would be most effective. Additionally, the book's mention of using all three oxidants together could be a mistake, as it seems unnecessary.
You also mentioned the possibility of using concentrated sulfuric acid (H2SO4) as an alternative for visualizing sugar spots, as it can easily char table sugar. While this may be true, spraying concentrated sulfuric acid can be hazardous and may not be the best approach.
Overall, you have provided some good suggestions based on the information given, but additional details or experimentation may be needed to determine the most appropriate visualization method for these compounds on a TLC plate.
For visualizing the spots of a sugar on a TLC plate, using chromium trioxide or potassium dichromate can be effective. These oxidizing agents can react with sugars and produce a color change. It is not necessary to use both oxidizing agents; either one can be used individually. However, the specific color formed will depend on the type of sugar present. You can apply these oxidizing agents by dissolving them in a suitable solvent (such as water or ethanol) and then spraying this solution onto the TLC plate.
On the other hand, using concentrated sulfuric acid (H2SO4) to visualize sugars can indeed be effective. Concentrated sulfuric acid can dehydrate sugars, leading to the formation of carbon (char). This char formation can result in a visible black/brown color. However, it is important to note that using concentrated sulfuric acid can be hazardous and should be handled with care. It is also important to consider the potential damage that the acid may cause to the TLC plate.
In conclusion, for visualizing sugars on a TLC plate, using oxidizing agents like chromium trioxide or potassium dichromate, or concentrated sulfuric acid can be options. However, it is important to follow proper safety precautions and consider the potential risks and limitations of each method.