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Initially, my column had been loaded with n-hexane and ethyl acetate, however I ended up having to use a more polar solvent to elute my product. It was my first time running chloroform through a column packed with silica and I noticed that as the chloroform made its way down the column, the silica turned from white to transparent.

My initial hypothesis was that chloroform and silica must have the same refractive index. I looked up the values, and found them both to be ca. 1.45 (see references), so I am inclined to believe this explanation.

However, if someone here could weigh in, and either confirm/deny this. (Or, alternatively, venture a new hypothesis).

I am guessing no chemical transformation takes place when silica looses its opalescence, because if that were the case then the whole procedure becomes completely useless. However, I could be wrong.

References:

i. http://refractiveindex.info/?shelf=main&book=SiO2&page=Malitson

ii. https://en.wikipedia.org/wiki/Chloroform_(data_page)

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    $\begingroup$ Sounds right to me. $\endgroup$ – Ivan Neretin Sep 14 '16 at 11:13
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    $\begingroup$ I even observe this using MC:MeOH as solvent system.. $\endgroup$ – Khan Sep 14 '16 at 11:38
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    $\begingroup$ I have done a lot of rock and mineral collecting. If you wet the rock with water you can often see much more detail of the rock since less light is scattered. The rock looks a lot like you have polished it. $\endgroup$ – MaxW Sep 14 '16 at 15:16
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You are correct, sir! (or madame) The silicic acid becomes translucent (not transparent) in chloroform because their refractive indexes are similar. This is a chance to check for air bubbles in the column, which reduces efficiency. Also, any material still on your column will probably not have the same RI, and will be visible as a white or colored band, probably at the top of the column.

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  • $\begingroup$ It would be more appropriate if you posted this as a comment. This doesn't add much to the discussion. Cheers! $\endgroup$ – getafix Sep 24 '16 at 2:35
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    $\begingroup$ He asked if it was the refractive index responsible, and I answered him. $\endgroup$ – Draco Freon Sep 28 '16 at 21:06

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