We have done titrations before in this class, but I want to take time to look at what happens to the pH of a solution during a titration. Here is a pretty typical titration curve. This is a titration between a strong acid and a strong base. You are starting with the strong acid in your flask and you are adding a strong base from your burette. I know this for several reasons. 1. I am starting with a really low pH initially. That is the pH of what is in the flask. That indicates to me that I am starting with a strong acid. I am ending with a really, really high pH, which tells me that at the end I am going to have an excess of a strong base. The key point is this equivalence point. That is the stoichiometric point where you have matched the exact amount of base and acid. If you take a look here, the equivalence point happens kind of midway between your two plateaus, where you start out with a low pH and where you end up with a high pH. You will see that in this diagram the equivalence point is at exactly 7. We know when you mix a strong acid with a strong base, you are going to get salt and water. The salt that you are going to produce must be neutral. This is the pH curve of adding a strong base to a strong acid. We could do just the opposite. We could add a strong acid to a strong base and we have just flipped the curve. You are starting with a strong base so you start with a really high pH and you are adding an acid and so the pH is going down. You will note however that you get these really strong inflection points. The reason is initially you have an excess of base and at the end you have an excess of acid or in the first curve. At the beginning you have an excess of acid and at the end you have an excess of base. The equivalence point happens when you have a transition between the excess of one material and the excess of the other material. If you have an excess of an acid you will have a very low pH. If you have an excess of base you will have a very high pH and that transition happens very quickly through the equivalence point. Around the equivalence point you have a very sharp and sudden change in pH. That is why you have had that frustrating experience doing a titration where your solution is clear, clear, clear and then all of a sudden was a single drop. Your indicator changes color very rapidly. Just in the span of one or two drops you can have a very sudden change in pH. This is a titration curve of a strong base being added to a weak acid. I know that for several reasons. One my initial pH is starting a little bit higher than before which means my acid wasn't as strong as in the initial graph that we looked at. My pH is still ending up relatively high so that is telling me that I am going to have an excess of a strong base at the end. What the key here is that if you look at your equivalence point, at that midpoint between the two plateaus, my equivalence point is now at a pH higher than 7 and we know if we mix a weak acid with a strong base, the resulting salt will be basic and at the equivalence point again you just have the salt and water. So you are going to have a basic equivalence point. It should be no surprise then if I start with a weak base and add a strong acid, you will get a pH curve that looks a little bit like this. Here you will see that my initial pH wasn't as high as the last chart I looked at but my final pH is really low. So that is telling me I am starting with a weak base ending with a strong acid. The key point here is that if you look at the equivalence point, instead of pH lower than 7 which means that the resulting salt that I am making is acidic. Again, between a weak base and a strong acid we would expect an acidic salt as a product. I have this diagram of doing a titration between a strong base and a polyprotic acid and theoretically this is what it looks like. You should be getting an equivalence point after the first protonization and equivalence point after the second protonization so this would be a diprotic acid. For a triprotic acid we would expect a third hump like that. Now I have a master's degree. I studied chemistry throughout college and I took two years of chemistry in high school. I have never been able to replicate this curve in the lab but theoretically this is what it should look like. When doing a titration we have a choice of indicators that you can use to find out when you hit that equivalence point. And you can see that from this range these indicators all change colors at different pH ranges. When choosing an indicator you actually want to choose one that changes color near the pH you expect of the resulting salt. If a strong acid and a strong base you want an indicator change somewhere around 7. A strong base and a weak acid you would want something in a higher pH range and a strong acid and a weak base you want a lower pH range. It's not essential that you hit it perfectly however because you have such a sudden change in pH around the equivalence point. As long as your indicator is in the ballpark one or two drops will get you from the equivalence point through the indicator. As long as you pick an indicator around the ballpark that you want you will be in good shape. You will note here for example that phenylthylene starts changing color around a pH of 8 and when we used that for a sodium hydroxide hydrochloric acid titration earlier we would expect that pH to be 7. Yet we were still happy to use phenylthylene. The reason we use phenylthylene so commonly as an indicator is because a lot of these other indicators are fairly chrysanogenic and phenylthylene though not pristine itself is a safer option than a lot of these other indicators. The last thing I want to talk about is what happens when you do a weak acid with a weak base. Here is a curve of a weak base being added to a weak acid and you will see that the curve is not nearly as distinct as we saw when you dealt with a strong acid or a strong base. That's because you have very subtle changes in pH throughout the titration. So picking an indicator and seeing a sudden change in the indicator happening is actually pretty difficult to do here. So for our intensive purposes we are going to avoid weak acid weak base titrations. We are always going to have at least one strong substance in here so that you can have a sudden change in pH around the equivalence point and that you can have a definitive change in color if you are an indicator.