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Why is it important to mark solvent level on chromatography paper when you remove it from the chamber?

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    $\begingroup$ The distance traveled by a compound in thin-layer chromatography is a characteristic property of that compound, but it is also dependent on the size of the paper. Compounds will travel farther if the solvent travels farther on longer paper. The retention factor accounts for this by dividing by the distance traveled by the solvent. $\endgroup$ – Ben Norris Jun 29 '15 at 1:21
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    $\begingroup$ This could be a good answer @BenNorris $\endgroup$ – user15489 Jun 29 '15 at 1:41
  • $\begingroup$ Related: chemistry.stackexchange.com/questions/10054/… $\endgroup$ – Klaus-Dieter Warzecha Jun 29 '15 at 3:57
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Building on my comment, marking the distance that the solvent traveled allows us to calculate the retention factor $R_f$ (or apparently retardation factor, according to IUPAC meddlers who need to rename things with perfectly good names that everyone else uses). The retardation factor is the ratio of the distance traveled by the spot to the distance traveled by the solvent:

$$R_f=\dfrac{d_{spot}}{d_{solvent}}$$

If you are only performing one thin-layer chromatography experiment (TLC) ever, then you may be okay not marking the solvent distance. Since everything is on one piece of paper or one plate, you can compare readily:

enter image description here

From the above TLC picture, we can easily tell that unknowns A and B are probably the same compound as the standard, but unknown C is something else.

Now, what if we have two TLC plates/papers?

enter image description here

We can draw the same conclusions about the relationships between standard 1 and unknowns A, B, and C. Likewise, we can make some conclusions about the relationship between standard 2 and unknowns D, E, and F. However, we cannot easily determine if unknown F is the same compound as unknown A.

Determining the $R_F$ value lets us do that. Using the same solvent system, the distance traveled by the solvent and the distance traveled by a compound are both proportionally related to the length/height of the plate/paper. The solvent can travel twice as far on a paper that is twice as long. It turns out that the compound being analyzed also travels twice as far on a paper that is twice as long. The the ratio, $R_f$ is constant.

Let's drop in some numbers:

enter image description here

Now we can calculate the $R_f$ values for each spot:

enter image description here

After calculating the $R_f$ values, we can make some conclusions (assuming there are only two compounds in the entire experiment):

  • Unknowns A, B, D, and E are the same compound as Standard 1.
  • Unknowns C and F are the same compound as Standard 2.
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