Basic Principles of Acid-Base Titrations

Was this helpful?
SPEAKER: So in this experiment, we're going to learn about the principles and procedures of a basic acid base titration. So in this experiment, we're going to titrate acetic acid with sodium hydroxide. And the products are gonna be acetate ion and water. So generally what we want is we know the concentration of our sodium hydroxide. But the concentration of our acetic acid solution is unknown. So we want to determine this concentration. And we do that with a titration. Here's the basic equipment that we're using in the titration. We have a volumetric burette. You notice all the little lines on here? These are used for measuring the volume. Generally we can measure the volume to about 0.02 milliliters. And in here, we have a beaker containing our acetic acid. We call the acetic acid, the analyte. And in the burette, we have the sodium hydroxide. We call that the titrant. We have added 25.00 milliliters of acetic acid using this pipette. Pipette is used for delivering accurate volumes of fixed volumes. In this case, this is a 25 milliliter pipette. So we're going to titrate, or we're going to deliver the sodium hydroxide into the acetic acid and consume the acetic acid. And when we're done, we will measure the volume of the sodium hydroxide required. And then from that, we can calculate the concentration. But the question is, how do we know when we're done? How do we know when we've added just enough sodium hydroxide to consume all of these acetic acid but we haven't added too much? So if we go back here to the board, I've drawn here a graph, a rough graph of what a pH will look like as a function of the volume of the titrant, or in this case, the volume of the sodium hydroxide. In other words, this is what the pH is going to do as we add sodium hydroxide. So as you can see, the pH is low because acetic acid is an acid. And then as we add sodium hydroxide, our pH will come down on to be a plateau region. We call this the buffer region. And then it's gonna rise quite steeply. Right here in the middle, that is when we have exactly consumed all of the acetic acid. But how do we know when we get there? Notice here that the sodium hydroxide is colorless. The acetic acid is colorless. And the products are colorless. So how do we know when we do that? Well, in this case, we're gonna use an indicator. This is phenolphthalein. And we're gonna add three drops. And what that's gonna do is that's gonna change color. So phenolphthalein is gonna be colorless when it's acidic. And as it gets more basic, at some point, it's gonna change color to pink. And when it changes color to pink, we're gonna stop. And we're going to assume that that point is when we've consumed all the acetic acid. We call that the endpoint. If my burette contains approximately 0.1 molar sodium hydroxide and my acetic acid is also approximately 0.1 molar in concentration, although we don't know exactly what it is, and we've added 25 milliliters of this acetic acid, what would you think would be the volume, the approximate volume of sodium hydroxide we will add to reach that endpoint? So if the sodium hydroxide is approximately 0.1 molar and the acetic acid is approximately 0.1 molar and we've added 25 milliliters of that, then the volume of the sodium hydroxide that we will need to add, it's also approximately 25 milliliters. But that's approximate, so we need to be careful. So we're going to do our titration now. We've added our phenolphthalein failing indicator. And we've zeroed out our volume. We have one meniscus here that measures-- that's zeroed out on our burette. And now we're gonna do our titration. We have here our stop cork that we can use to control the flow. And we're going to go quickly down to about 22 milliliters and then go slowly until we get that pink color. So here we'll start. And so we're at 2 milliliters. And it'll just take a little while to get down. So we are at 7 milliliters, 12 milliliters, down to about 19. And I'm gonna start slowing down a little bit. Down to 20. Notice that I'm adding dropwise. 21 milliliters. And now at 22 milliliters. Still colorless, but we're close. So we wanna be careful here. Each drop that we add is 0.07 milliliters in volume. But remember that we can measure 0.02 milliliters on the volume of a burette. So we need to add fractions of drops. And we do that by suspending drops and squirting a little bit of distilled water into it. So we're gonna go add a drop at a Be very careful here. So there is a drop. Notice I have on the end of it, a little drop suspended. And I'm gonna to squirt that off with my water bottle. And I'm gonna no longer worry about my volume and just look at my color. Another drop. And you're looking for a little-- when a drop enters, how long does it take for the pink to go away? So here's another 1/2 a drop I've suspended. I'm gonna squirt it off. And I'm noticing that I'm getting a little bit of pink when those sodium hydroxide goes in. There's a drop suspended. Squirt that one off. You can tell by how long the pink lasts before it disappears. Patience is what's important here. Most students make mistakes because they lose their patience. And pink disappeared. We probably have half a drop to go. And there we go. It's pink. It's pinks placid. So the volume of titrant added or the volume of the sodium hydroxide was 23.05 milliliters. Let's go put that on the board. And let's go do a calculation. So here, volume of the sodium hydroxide was 23.05 milliliters. Up here, remember concentration of the sodium hydroxide, 0.1002, this is known. Volume of the acetic acid we added was 25 milliliters, volume of the titrant, 23.05 milliliters. What would be the concentration of the unknown acetic acid? Let's go do the calculation. So we started with 23.05 milliliters of sodium hydroxide. Now, the concentration of that was 0.1002 millimole per milliliter of sodium hydroxide. Now, we're using millimoles. Might be unfamiliar. But it's millimoles per milliliters concentration and is often moles per liter. But millimoles per milliliter gives us the same thing. And this is more convenient because the milliliters of sodium hydroxide cancel the milliliters of sodium hydroxide here, giving us millimoles of sodium hydroxide. Now, from our reaction that we indicated previously, there was one millimole of acetic acid per one millimole of sodium hydroxide, canceling our millimoles of sodium hydroxide, giving us millimoles of acetic acid. But we want concentration. Concentration is moles per liter or millimoles per milliliter. So we're going to divide here by our volume of acetic acid, which is 25.00 milliliters of acetic acid, giving us millimoles per milliliter. And this will give us our answer, 0.0924 molar acetic acid.