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When I stain TLC spots with $\ce{KMnO4}$, I found that some spots appear differently as time goes by. Here is the example:

KMnO4-stained TLC plate

The leftmost one is the plate right after staining, and the others are listed sequentially with time.

Question 1: As time goes by, some white stains appeared and widened. Does this mean some materials exist in that area, or is this just result of diffusion? Are the white stains due to original yellow spots, or indicate the existence of different materials?

Question 2: Some white stains (numbered as 1, 2, 3) appeared in the same manner, but disappeared. Then should I consider the existence of this white stain? And should I record staining with time-scale whenever I stain the TLC with $\ce{KMnO4}$?

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As you have seen, potassium permanganate is very reactive. It can react with almost anything organic.

Question 1: Yes diffusion of the single component is causing the broadening of the spots.

Question 2: The explanation of spots turning white is that either you have acid in your eluent or the compound themselves are relatively acidic. When manganese dioxide reduces in the presence of acids, it is converted to manganese ions (which are very light pink in color).

Record the stain immediately. That is its correct retention.

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  • $\begingroup$ In the rightmost plate, the second column has pale-yellow spot at the top but leftmost doesn't have. In fact this spot was observed under UV radiation. But this spot doesn't appear right after staining. I am somewhat nervous whether I use information of the rightmost plate. $\endgroup$
    – Krang Lee
    Mar 3 at 3:49
  • $\begingroup$ If you are concerned, you will have to try two or three different solvent eluent systems just to check how many spots does a mixture generate. By chance is an HPLC available? $\endgroup$
    – AChem
    Mar 3 at 3:58

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