Learn about Senior Chemistry, Acids and Bases 15, in this comprehensive video by bannanaiscool.
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Acid-Base Titrations: The Apparatus Rob Lederer: I thought of begin to review a little bit of what the apparatus is called in an acid-based titration. So first of all, you probably want a pipette, some known quantity of a reactant inside of an Erlenmeyer flask and you would use maybe a ten milliliter pipette. Use a pipette pump that's attached to it. As the titration proceeds, you are going to be pouring from a burette into the flask, swirling continuously looking for a color change of an indicator. The reason we are doing a titration in the first place is to perhaps find an unknown concentration, an unknown concentration of something that is inside of the burette and/or titrate inside of the Erlenmeyer flask. Everything is held together here by the retort stand and the burette clan. Titrating Strong Acid into Strong Base Rob Lederer: Acid-based titrations: they are the most beautiful graphs in the world to be able to make. So let's take a very simple reaction like hydronium plus hydroxide makes 2H20, and let's say, we have equal concentrations of a base and the acid in solution, and we've got 10 milliliters of acid in that bottom Erlenmeyer flask that we were looking at, and we've got sodium hydroxide in the burette. Now what volume do you think we're going to need of that hydroxide to completely neutralize that acid? Equal concentrations of 1:1 ratio, 10 mills here, we're going to need 10 mills. How does this titration look, if we continually pour hydroxide and then exceed that 10 milliliters, pass that point of equivalence? Well, when we graph pH versus milliliters and NaOH added and always label your axis. We're going to start off with a high pH because of the presence of - we're going to start with a low pH because of the presence of the hydronium in the bottom flask. Then as we titrate, all of a sudden, boom, we are going to get a huge change, when we get to that point -- close to that point of equivalence. So the point of equivalence is always found, when you take the greatest change on the graph and go to the middle of it and right there ladies and gentlemen is where you find your equivalence point, that means that right here in terms of milliliters of NaOH added, there would be 10 mills right there, and what is the pH of that equivalence point? It's 7, because water is present. Yeah water and the ions that are swimming around in solution like sodium ions and chloride ions, so you'd have salt water at the equivalence point if you really did the titration well, but don't drink it, don't do that. Now, anyway I'll put on here for you. What's the indicator you would use, if you didn't want to actually have to do the titration in graph, but you just want to nail the equivalence point right away, maybe to calculate an unknown volume or concentration? Well, which indicator actually brackets that equivalence point very well? Bromothymol blue at 6.0, 7.6 because the pH here at the equivalence point is going to be 7. So it goes from -- if you put the indicator into the original Erlenmeyer flask where the acid is, it will go yellow, yellow, yellow, yellow, green, and then blue, it's very difficult to get that green color because the pH changes so very quickly given a little bit of volume. So getting the green here means that you've got a really great titration if use bromothymol blue indicator. By the way, when you get that green color or whatever color you end of in the titration, that's called getting the end point. The end point is the point at which the color changes in the titration. The equivalence point is that one point, where the exact mole to mole ratio is achieved. The end point could be anywhere along here that you stop, but there is only one equivalence point. Titrating Strong Base into Weak Acid Rob Lederer: Now, if you have that weak base being titrated into a weak acid, okay, titration curve looks a little bit different. Now we have a little shake at the beginning, but that's not that vital. We just have a little