So we have just come off of several lessons in which we have talked about acid/base titrations. And no acid/base titration would be complete without a discussion of indicators. In this lesson we are going to define what an indicator is and describe how one works. And select an appropriate indicator describe how one works. And select an appropriate indicator for a titration if you are given K_a values. An indicator is simply a substance which changes colors with pH. They are usually weak organic acids. They have an H they are usually written with an COOH what that represents is a C with a double bond O and the OH group attached . So many of the organic, not all, but man of the organic acids have different colors whether they ionize or un-ionize, So if they have the H attached, they have one color. Once the H had been dissociated, or it has been donated it will have a different color. So HIn, I am going to use to represent my indicator. The H at the front sis showing that it is an acid. And as an indicator if its placed in an aqueous solution it will be undergoing this equilibria. This equilibria is such that, this acids is such that it has a certain color, and I will call it color A for the acid form, and color B for the ionized form. This indicator will undergo changes in This indicator will undergo changes in its equilibrium according to Le Chatelier's principle. I am going to guide you through this concept of understanding how it changes its color with several questions. So you will be asked to answer several of these as we go through this process. So you will be asked to answer several of these as we go through this process. So the first one is several of these as we go through this process. So the first one is which way is this equilibrium going to shift if you add and acid. Now Le Chatelier's principle is if you add and acid. Now Le Chatelier's principle is in play here so Now Le Chatelier's principle is in play here so if you adding an acid you are increasing the in play here so if you adding an acid you are increasing the concentration of that substance. if you adding an acid you are increasing the concentration of that substance. So determine which way it will shift. Well if you said to the left then you are correct. Any time you add an substance it will shift away from that substance in order to Any time you add an substance it will shift away from that substance in order to re-establish its equilibrium. away from that substance in order to re-establish its equilibrium. So if it is shift toward the left when you add and acid, what color are you going to see? If you said color A then you are correct. Lets keep working through this. If you add a base, now if you add a base you would look at that reaction and say there is no OH- listed in there but we remember from our acid/base chapter that when the H_3O+ goes up, the OH- that when the H_3O+ goes up, the OH- goes down and visa versa. So if you are adding a base you are adding OH- so the H_3O+ has to decrease. So now if you add a base which way will this equilibrium shift? so the H_3O+ has to decrease. So now if you add a base which way will this equilibrium shift? Well if you said to the right, then you are correct. Now, if it shifts to the right what color are you going to see? If you said color B then you are correct. So this is the process of how an indicator works. If you said color B then you are correct. So this is the process of how an indicator works. If added to this acid/base titration. So this is the process of how an indicator works. If added to this acid/base titration. Then you have got within your flask it maybe an acid and you are slowly adding a base. And as that amount of acid and the it maybe an acid and you are slowly adding a base. And as that amount of acid and the amount of base is changing within the course of that it is affecting the quantity of the H_3O+ in that solution. As you affecting the quantity of the H_3O+ in that solution you are affecting where this equilibrium lies and how much A, or how much color B there is. As you get more and more color A or this HIn in the solution the color A will become prominent. or this HIn in the solution the color A will become prominent. If you get much more of the ionized form in there, then the color B will become prominent. So you need to find an indicator that changes colors near the pH of the equivalence point. Anywhere near that upwards slope we know that each one of these had a steep climb. Anywhere in that upwards slope each one of these had a steep climb. Anywhere in that upwards slope it would change colors. Anywhere in a pH there Anywhere in that upwards slope it would change colors. Anywhere in a pH there then it would be a good indicator. Now how to find one that changes colors in that range? Well, first of all let me define endpoint. Now how to find one that changes colors in that range? Well, first of all let me define endpoint. Endpoint, is where the indicator changes color so that is its switch over. Endpoint, is where the indicator changes color so that is its switch over. There is some point where there is equal amount of HIn and In-. At that point you should probably see a color somewhere in between those two. So we have purple and red and it is going to be a purplish red. You are not going to see a distinct color change until those two. So we have purple and red and it is going to be a purplish red. You are not going to see a distinct color change until In- is predominant. going to be a purplish red. You are not going to see a distinct color change until In- is predominant. Where that color change occurs is called the endpoint. Our goal, is to make the end point match the equivalence point. as close as possible. So here is the upward slope of this one. If you follow this beginning point, and this ending point down to the graph, you would probably just have tiny range, just a few drops would switch through that range so if you could get an indicator changing color in this range the way you find it is you do the pK_a of that indicator and this would tell you it is you do the pK_a of that indicator and this would tell you where the color change would be. Lets think about why the pK_a would do that. where the color change would be. Lets think about why the pK_a would do that. If you have got the reaction of the indicator, and I will write it down here. In which you have got a significant amount of both the HIn In which you have got a significant amount of both the HIn over here and the In- well you have a weak acid and its conjugate base over here and the In- well you have a weak acid and its conjugate base and you have a buffer. So the Henderson-Hasselbalch equation would apply. and you have a buffer. So the Henderson-Hasselbalch equation would apply. And if the Henderson-Hasselbalch equation applies then we will write it So the Henderson-Hasselbalch equation would apply. And if the Henderson-Hasselbalch equation applies then we will write it pH = pK_a + log And if the Henderson-Hasselbalch equation applies then we will write it pH = pK_a + log of the base which is In- pH = pK_a + log of the base which is In- over the acid which is HIn. of the base which is In- over the acid which is HIn. When there is equal amounts of HIn and In this term is zero. So pH equals pK_a so if you know the pK_a of the indicator So pH equals pK_a so if you know the pK_a of the indicator you are going to know the point in which you have equal amounts of both of these. That is the switch over point for the color. In this demonstration, we are going to be looking at indicators. Indicators are use to help us know when the equivalence point is reached in a titration. We all it the end point, the color change. The indicator I have put in this flask is bromophenol blue. And bromophenol blue, does not look blue, it is yellow right now, because it is yellow when it is acidic. I have placed some hydrochloric acid in here and I will be titrating it with some sodium hydroxide. Now this video, is not about proper titration techniques and all of the work that goes into doing a good titration but I do want to show that change of color. So I will be adding show that change of color. So I will be adding the sodium hydroxide and it going to take a little over 10 ml, because I have 10 ml of the HCl in there. it going to take a little over 10 ml, because I have 10 ml of the HCl in there. As it starts getting close I can see a little bit of blue starting to form and I can swirl it away. Eventually, on drop will turn it from the yellow, and we will see the blue. There it is, there is the blue color, so in a good technique of titration you would slow down that process. There it is, there is the blue color, so in a good technique of titration you would slow down that process. But this lets us know that we have switched our pH technique of titration you would slow down that process. But this lets us know that we have switched our pH and you choice an indicator that changes colors after around the pH of the equivalence point. and you choice an indicator that changes colors after around the pH of the equivalence point. We are doing the same titration but with a different indicator. This is phenolphthalein, and phenolphthalein will change from We are doing the same titration but with a different indicator. This is phenolphthalein, and phenolphthalein will change from colorless to pink as it reaches its endpoint. As we put a little bit of the base in we will see a pool of pink forming. As we put a little bit of the base in we will see a pool of pink forming. But as I swirl it goes away. This is sort of like adding hot water to a cold bathtub. It is hot in that one spot This is sort of like adding hot water to a cold bathtub. It is hot in that one spot but then dissipates through out the cold bathtub. It is hot in that one spot but then dissipates through out the base is dissipating and reacting with the acid. But if I hit it just right I will go from one drop where swirls and goes colorless I will go from one drop where swirls and goes colorless to the next drop it will be pink. I am getting close, it takes a little longer for the pink to go away. And there we have the end point. Now this is just giving a whole array of various indicators and how they change color. And you see it is kind of a gradual thing maybe this is acid form of this one, and it is yellow. And this is HIn or the In the ionized form and it is blue. There are going to be a significant range in here in the middle where its green, because you have a little bit of both present. There are going to be a significant range in here in the middle where its green, because you have a little bit of both present. This indicator would not be a good indicator for a titration between a weak acid and a strong base because those are above 7. So it would be a terrible one for that. are above 7. So it would be a terrible one for that. So you have to choose and indicator that changes So it would be a terrible one for that. So you have to choose and indicator that changes color in that upwards slope. So this is the end of learning objective number 8. We now know what an indicator is we know how they work and why they change colors and how to select an indicator for the pH range you are looking for and how to select an indicator for the pH range you are looking for in you acid/base titration.
