- Separations and purifications questions
- Simple and fractional distillations
- Principles of chromatography
- Basics of chromatography
- Thin layer chromatography (TLC)
- Calculating retention factors for TLC
- Column chromatography
- Gas chromatography
- Gel electrophoresis
- Resolution of enantiomers
Thin layer chromatography (TLC)
Learn about how chemicals can be separated based on polarity through thin layer chromatography (TLC). By Angela Guerrero. . Created by Angela Guerrero.
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- what are some limitations for TLC? Any trouble shooting?(6 votes)
- If your compounds are of very similar structure or polarity, they won't separate very much with TLC. You need to be careful with your choice of solvent.(12 votes)
- (At01:25) Why do we need to saturate the chamber?(5 votes)
- *note: the paper you add has the same mobile phase solvent on it
Answer: you saturate the chamber so your TLC stationary phase doesn't dry out while you are waiting for the mobile phase to travel up.
How: if the chamber is not saturated there is a risk that the mobile phase would be dried out by the air in the chamber and stop moving up the TLC plate prematurely.(7 votes)
- why do aromatic compounds fluoresce?(4 votes)
- Some aromatic compounds fluoresce because their delocalized pi electrons are liable to being excited by ultraviolet light. When these electrons return to lower energy levels, they emit their lost energy as light that may be visible to the human eye.(8 votes)
- Hi! I need to cite this video and I am having trouble finding the publication date.(3 votes)
- The video's publication date is Sep 17, 2013
Thin layer chromatography (TLC) (video) | Khan Academy. (Sep 17, 2013). Retrieved June 24, 2021, from https://www.khanacademy.org/science/class-11-chemistry-india/xfbb6cb8fc2bd00c8:in-in-organic-chemistry-some-basic-principles-and-techniques/xfbb6cb8fc2bd00c8:in-in-methods-of-purification-of-organic-compounds/v/thin-layer-chromatography(1 vote)
- What does adsorbed mean and what is an Rf value?(2 votes)
- The Rf value (for TLC) is simply the (migration distance of substance)/(migration distance of solvent front). The Rf value is often used, along with other evidence, to help identify a compound when compared with a known standard.(2 votes)
- Why we make saturation to the jar ?(2 votes)
- Answer: you saturate the chamber so your TLC stationary phase doesn't dry out while you are waiting for the mobile phase to travel up.
How: if the chamber is not saturated there is a risk that the mobile phase would be dried out by the air in the chamber and stop moving up the TLC plate prematurely.(2 votes)
- What if the solvent is more polar than the silica gel?(1 vote)
- TLC uses two different phases, stationary and mobile, where the stationary phase is the very very polar silica gel and the less polar mobile phase. Therefore if you use something that is more polar than the silica gel it will still just be attracted to the slight close to the Polarity stationary phase and travel less distance compared to other compounds which will travel a larger distance.(4 votes)
- Is TLC limited to mixtures composed of only aromatic compounds considering the spots can only be visualized under UV light (she mentions in the video that aromatic compounds fluoresce under UV light)?(2 votes)
- Great question!
When trying to identify compounds that are not aromatic, one must use TLC stains. The process is a bit of trial and error because you don't know what compounds you have so you have to try a variety of stains that identify different chemical properties.
Here is a list just for information (no point in memorizing any of the details):
- I have a question. Why does a high concentration of polar compound produce a long strip on TLC after being developed?(2 votes)
- The long strip is usually produced because too much compound was added initially. It is known as sample streaking. High polarity will mean that the molecule will travel faster along the plate. A high concentration of the molecule means that when you look at the chromatogram, the graph for that molecule will have a peak area and no reading will be a able to be determined. To reduce sample streaking less solution should be added to the plate.(1 vote)
- at 40-55 sec:
Why do we need to " take a spotter and dip it into reaction flask; then put a little spot on spotter" ? I thought, we put a little spot first on spotter and put it into chamber. Can anybody explain it, please?(1 vote)
- The wording is slightly ambiguous, the spotter is a thin glass (Capillary) tube, and liquid from the reaction will be drawn up. Then you take the spotter with liquid in it, and touch the spotter to the stationary phase. The liquid will soak into the stationary phase, leaving a small spot. Then you place the stationary phase in the diffusion jar and allow the TLC plate to work as explained.
Think about if you took a straw and dipped it into a soda, some would remain in the straw, and then you could dab the straw onto a paper towel, depositing the soda in a little circle. Its the same idea.(1 vote)
When running reactions in the organic chemistry lab, you want to have a way to monitor them. One of those methods is known as thin-layer chromatography, or TLC for short. This works like all other kinds of chromatography in that you have a mobile phase and a stationary phase, and the compounds that you're trying to separate interact with these two phases. First, let's talk about the stationary phase. In TLC, you usually have a plate that's completely coated with silica gel. The silica gel is the stationary phase. There are other materials you could use, but silica gel is the most common. What you would do first is take a spotter and dip into your reaction flask, then put a little spot here. You want to find out whether is there one compound in there or are there multiple compounds. Next, what you'll need is to prepare the mobile phase. This is done in something known as the developing chamber shown here in white. First, what you would do is put in a tiny bit of your mobile phase, meaning it can be any solvent or any mixture of solvents, and you want to keep this to be a relatively small amount. If the level of this is too high, your spot will be completely submerged in this. You'll want to put in a piece of paper that will indicate whether or not this chamber is saturated. When setting this up, you want to check that this piece of paper is wet. This lets you know that the vapors from the mobile phase are completely everywhere in the developing chamber. Next, what you do is, you take your TLC plate and put it inside here. Finally, you would close the top. You want to do this because if you didn't and you left the top open, organic solvents are very volatile, meaning that they readily evaporate from liquid into gas. Next, what would you observe? Well, let's think about what could happen. You have your pink mobile phase, and what will happen next is that it'll travel up the TLC plate through capillary action. Capillary action is just a term for when you have some kind of solid thing like silica gel that sucks up a liquid. And when you see that it's gotten pretty close to the edge, say around here, you'll want to pull this out of the developing chamber. Make sure to use a pencil to mark where your mobile phase got up to. How come we can't see anything? That's because usually you need something like a UV lamp shining on this, so that you can visualize something. So let's say we take our UV lamp and shine it on here. Compounds that are aromatic will usually show up and fluoresce. And let's say that we had these two dots. So what does that tell us? That tells us that whatever was in our reaction flask was a two-component mixture and that there's at least two compounds in there. However, you would have seen more dots if there were three compounds or four compounds or even more. TLC plates can get really messy when you're doing research. But what can we tell about these two compounds? So TLC is a pretty qualitative method. It'll tell us whether things are more polar or less polar, so what we have to keep in mind is that the stationary phase, where the silica gel-- silica gel is very, very polar. So you can see this one didn't move to far-- it means it must have been really attracted to the silica gel-- but this one moved a lot more, so this is less polar and more attracted to the mobile phase. So we still don't really know what compounds these are exactly. But let's say that you knew that inside the flask you had naphthalene and benzoic acid. How can we tell which spots these correspond to? Well, you'd want to look at them and figure out which one is more polar and which one is less polar. As you can see, naphthalene is just made out of carbons and hydrogens, so it's pretty nonpolar. So we can indicate that this corresponds with this. And because of the carboxyl group in the benzoic acid, this group right here, you can tell it's a pretty polar molecule. So this would correspond to this. And that's how you do a TLC in a lab. Again, let's review. You first have your plate that you put a little spot on; you put it into the developing chamber, which contains the mobile phase; wait a little bit for the mobile phase to travel upwards; pull it out of the chamber; and then use a UV lamp to see what spots are there; and then try to compare those two spots in terms of polarity.