In paper chromatography, as the mobile phase (I used 1:9 acetone/petroleum ether) climbs higher and higher, will the solubility of the solutes decrease as the solvent moves higher? I speak mostly about the beginning, when the solvent just begins to climb up the paper. Will the solutes travel right at the solvent front at the beginning, and then start to lag behind (and separate) as the solvent front moves higher?
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asked Apr 18, 2015 at 13:25
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1 Answer
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The solubility of the solutes is the same at all points in the chromatography process. The solutes are clustered together at the beginning (at the solvent front) because none of them has moved very far yet. The reason for the separation of the solutes isn't that they become less soluble, it's that the solutes are moving at different speeds, like cars in an auto race:
_______
/____/SF\___
|_,._____,._)
---`'-----`'--
_______
/____/S1\___
|_,._____,._)
---`'-----`'--
_______
/____/S2\___
|_,._____,._)
---`'-----`'--
At the beginning, both cars (solutes) S1 and S2 are even with the solute front car (SF) because the race (chromatography) has just begun. However, over time, the SF car outpaces S1 and S2 because it is moving faster.
_______
/____/SF\___
|_,._____,._)
---`'-----`'--
_______
/____/S1\___
|_,._____,._)
---`'-----`'--
_______
/____/S2\___
|_,._____,._)
---`'-----`'--
And at the end, you get great separation:
_______
/____/SF\___
|_,._____,._)
---`'-----`'--
_______
/____/S1\___
|_,._____,._)
---`'-----`'--
_______
/____/S2\___
|_,._____,._)
---`'-----`'--
(Thanks to retrojunkies.com for the ASCII car)
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$\begingroup$ Another helpful concept is RF, which is the distance traveled by the solute divided by the distance traveled by the solvent front. A very soluble compound may have an RF close to 1, so you'd only be able to see a difference between it and the solvent front after a significant distance. $\endgroup$– user137Apr 20, 2015 at 22:57