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Chemistry: Demo: Extractions

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  • Type: Video Tutorial
  • Length: 8:15
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 89 MB
  • Posted: 07/14/2009

This lesson is part of the following series:

Chemistry: Full Course (303 lessons, $198.00)
Chemistry: Laboratory Techniques (10 lessons, $12.87)

This lesson was selected from a broader, comprehensive course, Chemistry, taught by Professor Harman, Professor Yee, and Professor Sammakia. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/chemistry. The full course covers atoms, molecules and ions, stoichiometry, reactions in aqueous solutions, gases, thermochemistry, Modern Atomic Theory, electron configurations, periodicity, chemical bonding, molecular geometry, bonding theory, oxidation-reduction reactions, condensed phases, solution properties, kinetics, acids and bases, organic reactions, thermodynamics, nuclear chemistry, metals, nonmetals, biochemistry, organic chemistry, and more.

Dean Harman is a professor of chemistry at the University of Virginia, where he has been honored with several teaching awards. He heads Harman Research Group, which specializes in the novel organic transformations made possible by electron-rich metal centers such as Os(II), RE(I), AND W(0). He holds a Ph.D. from Stanford University.

Gordon Yee is an associate professor of chemistry at Virginia Tech in Blacksburg, VA. He received his Ph.D. from Stanford University and completed postdoctoral work at DuPont. A widely published author, Professor Yee studies molecule-based magnetism.

Tarek Sammakia is a Professor of Chemistry at the University of Colorado at Boulder where he teaches organic chemistry to undergraduate and graduate students. He received his Ph.D. from Yale University and carried out postdoctoral research at Harvard University. He has received several national awards for his work in synthetic and mechanistic organic chemistry.

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Thinkwell
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Suppose you have a compound that is dissolved in a liquid and you want to extract the compound out, meaning that you want to separate the compound from the liquid that it's in. And, in this case, what I have is some iodine and it's in water. And the reason why I chose iodine is just because it's colored. Now, iodine is a nonpolar molecule and it only has a relatively small solubility in water. But if we took some hexane and we added some hexane to this, the iodine is going to partition between the water and the hexane. That's to say that some of it will probably remain in the water but some of it will go up into the hexane layer. Why? Because the iodine, being nonpolar, prefers the hexane layer, which is nonpolar, but it doesn't necessarily prefer it 100%. In other words, it will partition between the aqueous phase and the hexane phase. Now, what we're going to do is we're going to use a technique that involves a separatory funnel, or a sep funnel for short, and what I'll show you is that, even if the iodine partitions between the water phase and the hexane phase, it is still possible to extract all of the iodine from this, so if this technique works, this water layer should be almost entirely colorless at the end of this experiment.
Now the way a separatory funnel works is that it's shaped like a regular funnel and it has a stopcock, a Teflon stopcock, and again same as with the burette, when it's sideways, that means it's closed and when it's aligned, that means it's open so liquid can flow through. And again, what we're going to do is we're going to put both the water and some hexanes into the sep funnel and the iodine is going to partition between those two layers.
So, first thing you do is make sure that this is closed and then put the aqueous solution of iodine in. And then we're going to add some hexane. Hexane is going to float on top of the water because hexane is less dense and already you can see, or maybe you can see, that it's starting to partition, that the iodine is moving into the nonpolar layer, the hexane layer. So what we have is we have now two layers. We have hexanes on top and we have water on the bottom. And the iodine you can see, still there is a lot of it in the water, but there's also some of it in the hexane layer. Now, we've put a stopper on top and press in tight. You can put grease on this ground glass joint, but the reason why I haven't done that is we're using hexane and grease is soluble in hexane so that if we did that, we'd get all kinds of grease into our layer and that's no good. What's better is that more modern separatory funnels use a Teflon stopper instead of a glass stopper and that sort of solves the problem of both. But the way you use a sep funnel is you hold the stopper in with one hand, or with one finger, and you hold the body of it with one hand, and then you operate the stopcock from behind. So, the stopcock is here and I'm going to operate it from behind. You slowly invert it and then you vent it. Now you don't want to point it at your friends and you don't want to point it at yourself, stuff could come shooting out this way, and so what you want to do is point it away from everybody and it made a little whooshing sound and then give it a gentle shake and vent it, and give it a gentle shake, and vent it, and you should keep doing that until there's no more whooshing sound and that means it has come to equilibrium.
What happened? Well, there's still a little bit of color in the water layer, but for the most part, all of the iodine has now moved from the water layer up to the hexane layer, which is exactly what we wanted. Remember what we're try to do is separate the iodine from the water. Now, you take the stopper out and you have to take the stopper out. And now here is where the sep funnel is sort of slick. What you do is we're going to remove the water layer and actually let's remove it into this container, which was our original water layer container. So, we'll remove the water layer and if you remove a little too much water, it's okay. I mean if a little bit of the hexane ends up in the water, that's okay. So now this is hexane and that has our solute of interest in there. And now what you do, is you pour the top layer, and this is a clean Erlenmeyer flask, and you pour it into that. So that's a pure hexane layer now and it's got a lot of the iodine. And very often, when you're doing extractions, what you have to do is you have extract more than once. So what we're going to do is we're going to take this water layer now, which still looks a little bit colored, and we're going to extract it again. So, we'll transfer it back to the sep funnel and we'll add some more hexanes. Now a fresh batch of hexanes, not the same hexanes that we've already used once, but some more new hexanes and put the stopper in again. And you can see that, again, it's starting to partition between the hexanes, the iodine is partitioning between the hexanes and the water layer. And then invert it, and vent. It's a good idea if you get a little bit of liquid up in here, to let that drain back. And then, let's shake, and vent, and shake, and vent, and you can see that we're getting more of that violet color which is indicative of the iodine in the hexane layer and the water is becoming progressively more and more colorless. It's becoming closer to pure water with no iodine left in it because all of the iodine is now going up into the hexane layer. Now, sometimes you have to wait a little bit for the two layers to separate and so that just requires a little bit of patience. Sometimes, you'll get an emulsion. You'll get a foam at the interface between your organic layer, the hexane is an organic compound, and the water and what you can do in that case is you can add a little bit of sodium chloride, a little bit of table salt and it'll help to break up that emulsion.
So you can see now that the water is considerably more colorless than it was before. Before it was sort of a yellowish brown and now it's getting close to white and we've extracted out more of the iodine. And you can see that the hexane layer is again colored, because there was some more iodine to be extracted into the hexane layer from the water layer. So, again, drain off the water, and we can add this layer to our original fraction, because it's hexanes that has the iodine in it. And we could repeat this one more time to be really complete and there might be a little bit of color left in here, but not very much. Again, what we've been able to do is extract the iodine, using a separatory funnel, out of the aqueous phase and into the organic layer, pretty cleanly. Most of that iodine is now sitting in this phase. And then if we wanted to isolate the iodine, if we wanted to get the solid iodine out, remember iodine is a solid at room temperature, all we would have to do is evaporate off the hexane and then we would have the iodine separated cleanly from the water layer.
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CIA Demonstration: Extractions Page [1 of 2]

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