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Course: AP®︎/College Chemistry > Unit 3
Lesson 9: Separation of solutions and mixtures chromatographyColumn chromatography
Column chromatography applies the same principles as TLC on a larger scale. In column chromatography, a glass column is filled with a stationary phase (typically silica gel), and the mixture of compounds to be separated is placed on top. Then, a mobile phase (solvent) is passed through the column from top to bottom. As with TLC, the rate at which each compound in the mixture moves down the column depends on its relative attractions to the stationary and mobile phases. Created by Sal Khan.
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But how does the solvent ascend the silica gel ? By capillarity ?(7 votes)
Yep that's exactly correct. If it's thin layer chromatography then it ascends via capillary action. And if it's column chromatography it descends via gravity.
Hope that helps.(12 votes)
I understand why a polar solute would be attracted to the silicon gel via a dipole-dipole interaction, but why would a non-polar (or less polar) solute be attracted to the non-polar (or less polar) solvant passing by? Is it just momentum?(2 votes)
Hydrophobic interactions can form spontaneously because of increase in entropy of the solvent.
In other words, if hydrophobic molecules cluster together in what we refer to as a "solvent cage", surrounding hydrophilic molecules, they allow hydrophilic (or polar, if you will) molecules to form more hydrogen bonds between each other.
Thus, it isn't the strength of hydrophobic interactions that clusters those molecules together, it's the strength of hydrogen bonds that takes the priority, and non-polar molecules gather together to allow polar molecules to establish their interactions.(2 votes)
Video transcript
- In our previous video we talked about Thin Layer Chromatography and it was this technique used to figure out how many things you have in a sample or maybe say the relative properties say the relative polarity of the things that you have in the sample and so what you do is you put a sample on typically a silica gel, which is the stationary phase and then you put a mobile phase down here, which in this case might be less polar and then it's going to move its way up the silica gel and as it does, you can imagine, it's going to interact
with your original sample and so parts of the original sample are going to move up
with your mobile phase and different parts are going to be attracted to that mobile phase and not attracted to the silica gel to different degrees, you can imagine. If there's a part of your original sample that is more polar then it's going to be harder to move cause it's going to be attracted to the stationary phase and less attracted to the mobile phase and so it's going to move less. So this might be the more polar part of your sample as it moves from its original location and the part that's less polar, well it's not going to be as attracted to the stationary phase and so it's going to be more dissolvable in the less polar mobile phase and so it will go further. So this will be the less polar part of your original sample and so you can see that separation and you could come up with other insights that we talk about that in that other video. In this video, I'm gonna introduce you to a very very very similar idea it's just things are moving in a different direction and that is the notion
of Column Chromatography. What you do in Column Chromatography, just like Thin Layer Chromatography is let's say you have some type of vile, some type of a column, I guess I could say you'll typically see it with a little tap drawn down here because you might wanna see what comes out through the bottom but what you do is you fill it with the stationary phase which once again is oftentimes silica gel. So this is stationary phase stationary phase and if it's silica gel in this case, silica gel it doesn't have to be silica gel but that's pretty common. This is going to be very polar so let me write this, very polar and then you put some of your sample at the top here. So that's your original sample you put it at the top and then you have some mobile phase, the solvent that you're going to put on top of that. So you're gonna do something like that and what do you think is going to happen? Well, your mobile phase is going to start moving its way down through your stationary phase and it's going to
interact with your sample. Now what's going to happen to the more polar
components of your sample? Well, the more polar
components of your sample are going to be more attracted to the stationary phase, less attracted to the mobile phase so they're gonna move less. So the more polar parts of your sample might only go, maybe that far while the less polar parts of your sample, they'll move, there'll be less attracted to your stationary phase and they'll move with
your mobile phase more and so you might have they might move down, down over there and so the big takeaway, the difference between the two is just the direction you're moving in. In Thin Layer Chromatography your more polar thing is the lower dot while in Column Chromatography, your more polar thing is the upper layer right over here, this purple area right over here and of course, all of that depends on the polarity of your stationary phase and the relative polarity
of your mobile phase but what I just showed you
is a typical situation. So keep in mind whether you're looking at Thin Layer Chromatography or Column Chromatography which to pay attention to the direction.