 So, the first one we want to have a look at is the titration involving a strong acid and a strong base. This is by far the most common type of acid-base titration that you will carry out, and what you can notice is that we have sodium hydroxide here, which is our strong base, and it's going to be added to a strong acid, such as hydrochloric acid. As the volume of added base increases, so we can monitor changes in the pH. Now, of course, if you're carrying out a titration and you don't have a pH meter or a pH probe, you're not going to know what's going on here, so you have to use an indicator in order to try and identify what's going on. Notice for the reaction between a strong acid and a strong base, we produce a neutral salt and water. The neutral salt is the key here, because the neutral salt is going to have a pH of 7. Our equivalence point is the point where we have the correct ratio of acid to base. So, if this ratio, for example, is 1 to 1, then the equivalence point is the point where that occurs. But what you notice happening with a graph like this is that there is a minimal change in the actual pH as we're adding our base until we have reacted a fairly large amount of that acid, and then the change is very rapid. So you can see here from, say, around about 20 mils to 30 mils, so only 10 mils of our base solution, we have a massive rise in pH going from about 2 at this point to 12 Now that's a massive rise, trying to figure out exactly what the point where the solution reached the pH of 7 is virtually impossible. So what we want to try and do is we want to try and instead find what we call an end point. And the end point is where we end the titration. That's what we're going to stop. And that point, we want to be as close as possible to that vertical section of the graph. Now in reality it's going to be probably somewhere in this region and certainly we hope it is not somewhere up here because that's an overshoot. You've massively gone past that end point and you've added too much of your solution and so it's going to affect the accuracy of your final calculations. This is the point when we're looking at different types of acid based titrations as well because that tells us something about the type of indicator that we need. So if we start with our acid and then we're coming out at a pH of around 11, then phenol phthalene is a particularly good indicator to use. Down in the acid region it is colorless. Please don't write clear, please write colorless and it changes to a pink color as it reaches that higher pH. In fact a pH above about 8.5, something like that. So it's going to go through that point and you're going to start to get that very pink tinge. And that's what you want when you're doing your titration with these two. You want that very pink tinge to tell you that you've just hit that end point. You've gone past the equivalent point which is very hard to stop at but you've got that end point and you must stop there.