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Chemistry: Demo: Pipetting

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  • Type: Video Tutorial
  • Length: 8:14
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 88 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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A technique that you're going to use over and over again in a laboratory is pipetting. The idea is that what we want to do is get a known volume of a liquid and then transfer it to another container. So I have here a collection of very beautiful, but very innocuous chemicals. This is just water with food coloring in it. And this is a pipette. Let me show you what the pieces are. We have two pieces of straight glass tube, and then there's a bulge in the middle. This is where most the liquid is going to go. It has a tip, and I'll come back to that in a second, and then there's always a little mark that goes all the way around. There's a mark there, and that's the line that tells you when you've got the right volume. So, in other words, we're going to bring liquid all the way up to this mark, and then the meniscus - remember, when we're doing water, the meniscus is always this way. It's lower in the middle than it is on the edges, and we want the bottom of the meniscus even with the mark on the pipette. So this is a pipette bulb. One thing that I'd like to caution you against is down jam this guy on really tight on to the end of the pipette. Besides the fact that it really ruins the piece of plastic here, there's no reason to, because I'll show you in a second that you really don't want to have it jammed on really tight.
Now, the properties of these pipettes, the volumetric pipettes, is that you don't blow the liquid out of the tip. In other words, you fill them up to the line, and then you allow the liquid to drain out. But if there's a little bit left at the tip, you leave it there. You can touch the tip to the side of your flask or the beaker to get that last drop that's hanging off the end of the tip, but there's going to be liquid left in the tip as a result of the fact that it just hangs there and you leave that there. So this is sort of important. So this is a 20 milliliter volumetric pipette, and I'm going to draw liquid up and, again, you put the pipette into the beaker. You can rest it on the bottom or you can pull it up a little bit, and then you take the pipette bulb and you squeeze it, and then you rest it gently on top of the top of the pipette, and as you gently release the bulb, liquid will be drawn up into the pipette.
Now, if you have this jammed on really tight, have the bulb jammed on really tight, you won't be able to remove it very easily. So let's suppose now that it's not entirely filled yet. I don't know if you can see, but the liquid level is right here, and the bulb is off. So I can either let it all drain out, but in fact, all you need to do is cover the top of the pipette with your finger and then squeeze the bulb again, take your finger off, and continue to draw liquid up. You're going to have to practice that a few times to get exactly the right feel. So what you do is draw the liquid so that it's above the mark on the pipette, and then cover it with your index finger. Some people like to cover it with their thumb instead, but I like to cover it with my index finger of my left hand. And then what you have to do is get down so that you're parallel with the mark on the pipette. So you have to drop your eye level to the level of the mark on the pipette, and then what you do is basically wiggle your index finger to let a little bit of air in. As you let a little bit of air in, the liquid is going to go down. If you overshoot, in other words, if the liquid goes down below the line, it's no big deal.
So I'm going to do that intentionally. Oh, darn, I've missed the line and all the liquid is here and I have to pull it up to the line again, so again, with my finger on top of the bulb, squeeze the bulb, and since I only need a little bit, you don't even have to squeeze it very hard. Then put the bulb back on top of the pipette, and draw up above the line again. Then quickly pull it off. That's why you don't want it jammed on there, because you need to be able to take it on and off quickly. I get down to the right level, and what I'm doing is just sort of very gently bouncing my finger up and down, and trying to let little bits of air in, and the level of the pipette, the meniscus is making its way down the pipe - oops, went too far again. And sometimes you have to do it two or three times, but that's no big deal. Oh, went too far again. My finger's a little sticky this morning. Okay, that's perfect. Now we'll transfer it to a receiving flash.
Again, you drain it until the liquid stops coming out, and you don't blow out the liquid that's retained in the tip. If there's a drop hanging off the end of the pipette, you can go ahead and include that in your flask. So, for instance, if we're getting ready to do a titration, this would be the first step in the titration, get a known quantity of the thing that you're going to titrate. If you were going to do a dilution, the first step would be to get a known volume of the concentrated solution.
Okay, so you can see that there's some liquid left in the tip. That stays with the pipette, but you can go ahead and touch the tip to the sides. Okay, so that's a volumetric pipette, and we delivered 20 milliliters. You'll see that it says 20 milliliters on the pipette. So we delivered 20 milliliters to our Erlenmeyer flask. Now, this is a different kind of pipette. This is called a graduated pipette, and you can see that it has markings on it so that you can deliver different volumes, depending on what you want. These, in general, are less precise and less accurate, but if you're just trying to get a certain volume that's not exactly something convenient, like 20 or 50 - so they make these volumetric pipettes in 20 and 50 and 100 and whatever, but if you want something that's in between, then you would need something like a graduated pipette. They work exactly the same way. You see that there are markings for zero, one, two, three, so if I wanted to deliver three milliliters, you fill it up to the zero, and then allow it to drain down to the three, and you'd be in business. So let's go ahead and do that. Let's say we want to add exactly three milliliters. We fill the pipette in exactly the same way.
It doesn't have to be exactly on zero, because what you're trying to do, remember, is deliver a volume. So if it starts at 1/10^th, then all you have to do is drain it to 3-1/10, and the difference between those two volumes is going to be exactly zero. What I'm finding for these pipettes is rather than bounce my finger around, just sort of try to raise my index finger ever so slightly so that it allows some air to go in. So it's on zero. Now I'm going to drain out exactly three milliliters. You'll have to get your eyes down to the level of the line. That's exactly three milliliters. So I'll put this back. So this is our solution that has 20 milliliters of blue and three milliliters of red. I see we've made a nice purple. This technique, pipetting, is going to show up over and over again when we do dilutions, when we do titrations, and so it's a good idea to get some practice using this technique.
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CIA Demonstration: Pipetting Page [2 of 2]

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