WEBVTT 00:00:04.759 --> 00:00:10.559 Here are two solutions, A and B, to which I've added universal indicator. Here's the 00:00:10.559 --> 00:00:17.400 color key. As you can see, both of these solutions are around pH 6, very slightly acidic. 00:00:17.400 --> 00:00:24.160 Now I'm going to add concentrated sodium hydroxide, a strong base, to each solution. 00:00:24.160 --> 00:00:30.880 It only took one drop of strong base to dramatically raise the pH of solution A. The pH of the 00:00:30.880 --> 00:00:37.269 other solution hasn't changed. Let's reset. Here are the same two starting 00:00:37.269 --> 00:00:41.870 solutions, A and B, with the same indicator. But this time, I'm going to add concentrated 00:00:41.870 --> 00:00:46.239 hydrochloric acid, a strong acid, to each solution. 00:00:46.239 --> 00:00:52.350 It only took one drop of strong acid to dramatically lower the pH of solution A. The pH of the 00:00:52.350 --> 00:00:57.799 other solution hasn't changed. OK, let's look at our pairs of solutions. 00:00:57.799 --> 00:01:03.269 When we added a strong acid or base to solution A, its pH changed dramatically after only 00:01:03.269 --> 00:01:10.269 one drop! When we added one drop of acid or base to solution B, its pH stayed the same. 00:01:11.030 --> 00:01:17.000 Let's add more acid and more base to solution B and see what happens. 00:01:17.000 --> 00:01:22.230 It takes much more acid or base to change the pH of this solution by the same amount! 00:01:22.230 --> 00:01:26.780 How is this solution able to resist changes to its pH when strong acids and bases are 00:01:26.780 --> 00:01:33.310 added? How could we make and use such a solution? In this video, you'll find out. 00:01:33.310 --> 00:01:38.270 This video is part of the Structure-Function-Properties video series. The structure, function, and 00:01:38.270 --> 00:01:43.370 properties of a system are related and depend on the processes that define or create the 00:01:43.370 --> 00:01:44.560 system. 00:01:44.560 --> 00:01:51.560 Hi, my name is George Zaidan and I am [attribution] Before watching this video, you should know 00:01:51.810 --> 00:01:57.140 what an acid is, what a base is, be familiar with the concept of chemical equilibria, understand 00:01:57.140 --> 00:02:04.140 what distinguishes strong acids or bases from weak ones, and be able to define pH, Ka and 00:02:04.530 --> 00:02:06.880 pKa. 00:02:06.880 --> 00:02:09.810 After watching this video, you will be able to: 00:02:09.810 --> 00:02:14.549 Describe how the structure, or composition, of a buffer functions to resist changes in 00:02:14.549 --> 00:02:19.530 pH Explain how the choices made in buffer design 00:02:19.530 --> 00:02:26.269 impact the properties of a buffer. 00:02:26.269 --> 00:02:31.310 In chemistry, solutions that resist changes to their pH when acids or bases are added 00:02:31.310 --> 00:02:38.159 are called "buffers." Solution B in our demo was a buffer solution. Let's develop a molecular-level 00:02:38.159 --> 00:02:43.260 model of solution B to try and figure out how buffers work. First, let's review our 00:02:43.260 --> 00:02:48.609 experimental data and list the observations our model must satisfy: 00:02:48.609 --> 00:02:55.540 The starting pH was around 6 When we added one drop of strong base (enough 00:02:55.540 --> 00:03:02.219 to change the pH of our control solution), the pH of our buffer solution did not change 00:03:02.219 --> 00:03:07.139 When we added one drop of strong acid (enough to change the pH of our control solution), 00:03:07.139 --> 00:03:14.139 the pH of our buffer solution did not change Eventually, after addition of much more strong 00:03:14.819 --> 00:03:19.939 acid or strong base, the pH of our buffer solution did change 00:03:19.939 --> 00:03:24.919 Based on observations 1, 2, and 4, you might think that our buffer solution is simply an 00:03:24.919 --> 00:03:31.579 acid in solution. But it's not. Relying on observation 3, explain why a solution comprised 00:03:31.579 --> 00:03:38.150 solely of an acid in water could not effectively resist changes to its pH when more acid is 00:03:38.150 --> 00:03:45.150 added. Pause the video. Your initial reaction to this question is 00:03:48.989 --> 00:03:54.779 probably that the pH of such a solution would always decrease when more acid is added, and 00:03:54.779 --> 00:04:01.629 therefore observation 3 could not be satisfied. This is correct, except for two cases: first, 00:04:01.629 --> 00:04:07.260 the added acid could be exactly the same strength and concentration as the acid already present. 00:04:07.260 --> 00:04:11.499 In that case, the pH wouldn't change at all, and we might think we were dealing with a 00:04:11.499 --> 00:04:16.949 buffer solution. Second, the added acid could be much weaker or much less concentrated than 00:04:16.949 --> 00:04:23.540 the acid already present. Think about it like this: a 1 liter of a 1 molar solution of hydrochloric 00:04:23.540 --> 00:04:30.540 acid can hold its pH if a few milliliters of 0.1 molar acid or base is added. In that 00:04:31.200 --> 00:04:35.409 case, the pH might only change a little, and we might also think we were dealing with a 00:04:35.409 --> 00:04:40.350 buffered solution. But in both of these cases, even though it seems as though the solution 00:04:40.350 --> 00:04:45.280 is buffered, that "buffer-like" response depends on the relative strengths and concentrations 00:04:45.280 --> 00:04:50.770 of each acid, not on any intrinsic property of the solution. 00:04:50.770 --> 00:04:57.020 So if our buffer isn't just an acid, what is it? Let's review the observations. Observations 00:04:57.020 --> 00:05:02.750 2 and 3, taken together, suggest that there are both acidic and basic species present 00:05:02.750 --> 00:05:08.290 in our solution, since additional acid or base must be neutralized to keep pH relatively 00:05:08.290 --> 00:05:14.790 stable. Given observation 1, we can also hypothesize that there would be more acid than base, since 00:05:14.790 --> 00:05:20.320 the pH of the solution is slightly acidic. To better understand what might be happening 00:05:20.320 --> 00:05:25.890 at the molecular level, let's use Legos to model a solution that meets these criteria, 00:05:25.890 --> 00:05:31.880 and see if that model correctly predicts all 4 observations. We'll start with pure water. 00:05:31.880 --> 00:05:36.900 We could model water molecules using Legos but that would quickly get overwhelming, so 00:05:36.900 --> 00:05:43.700 we'll use this blue posterboard instead. Now let's add, say 60 molecules of acid and 40 00:05:43.700 --> 00:05:50.700 molecules of base. In real solutions, there are on the order of 1022 or more molecules 00:05:50.840 --> 00:05:55.530 dissolved. That would be a lot of Legos, so we're choosing smaller numbers for convenience. 00:05:55.530 --> 00:06:01.440 Now, should the acid and base be strong or weak? Let's start simple and make them both 00:06:01.440 --> 00:06:08.440 strong. So our acid could be HCl and our base NaOH. Here's 60 HCl molecules. This piece 00:06:08.940 --> 00:06:15.940 represents the H+ ion; and this piece represents the Cl- ion. And here's 40 NaOH molecules. 00:06:17.080 --> 00:06:23.860 This piece represents the Na+ ion and this piece the OH- ion. Remember that strong acids 00:06:23.860 --> 00:06:28.270 and bases dissociate completely in water, so I'm going to take apart all the pieces 00:06:28.270 --> 00:06:31.240 here. And this is our initial model! It contains 00:06:31.240 --> 00:06:37.580 both acidic and basic species, and it contains more acid than base. But this solution will 00:06:37.580 --> 00:06:44.580 not resist changes in pH. Pause the video and explain. 00:06:48.530 --> 00:06:55.530 The OH- ions would just react with the H+ ions to form neutral water. Since there is 00:06:56.680 --> 00:07:02.140 an excess of H+ ions, we would be left with a hydrochloric acid solution after the reaction. 00:07:02.140 --> 00:07:07.490 And we've already shown that a solution of a strong acid is not a buffer. So let's go 00:07:07.490 --> 00:07:12.590 back to our criteria. Remember that to satisfy these criteria we had the option of selecting 00:07:12.590 --> 00:07:18.860 either weak or strong acids or bases, and last time we selected the strong/strong case. 00:07:18.860 --> 00:07:25.860 So this time let's choose a mixture of weak and strong; say, a weak acid and strong base. 00:07:26.330 --> 00:07:33.330 As before, we'll start with the acid. Here are 100 molecules of a generic weak acid, 00:07:34.430 --> 00:07:38.130 HA. Remember, weak acids don't dissociate completely when dissolved in water. The extent 00:07:38.130 --> 00:07:44.000 to which a weak acid or base dissociates is related to the equilibrium constant, Ka for 00:07:44.000 --> 00:07:49.830 an acid or Kb for a base. These equilibrium constants depend on the chemical structure 00:07:49.830 --> 00:07:55.060 of the acid or base. Pause the video here and write the equilibrium expression for a 00:07:55.060 --> 00:08:02.060 weak acid. The equilibrium expression would be this. 00:08:05.210 --> 00:08:09.200 We always use concentrations in our equilibrium expression, even though in our model we're 00:08:09.200 --> 00:08:13.920 using number of molecules; but the principle is the same. 00:08:13.920 --> 00:08:20.160 When we dissolve 100 molecules of weak acid in water, some of them will dissociate, forming 00:08:20.160 --> 00:08:27.160 H+ and A- ions. Some of those H+ and A- ions will react with each other to reform HA. But 00:08:28.820 --> 00:08:33.380 at any given point after the system has reached equilibrium, there will be a fixed number 00:08:33.380 --> 00:08:40.380 of HA, H+, and A- ions. Let's say that at equilibrium, there will be 96 HA molecules, 00:08:41.690 --> 00:08:48.690 4 H+ ions and 4 A- ions. The molecule formed when HA is deprotonated, A-, is called the 00:08:49.980 --> 00:08:54.770 conjugate base. So now we have a model of our weak acid. We 00:08:54.770 --> 00:09:01.770 still have to add our strong base. Let's add 40 molecules of sodium hydroxide. First, the 00:09:03.210 --> 00:09:10.210 NaOH would completely dissociate in water. Now what? 4 OH- ions react with H+ to form 00:09:14.190 --> 00:09:21.190 water. But things don't end there. The remaining 36 OH- ions react with 36 molecules of HA 00:09:23.930 --> 00:09:30.430 via a typical weak acid-strong base reaction, forming 36 molecules of A-, the conjugate 00:09:30.430 --> 00:09:37.010 base. And we're still not through. Remember that the dissociation of our weak acid HA 00:09:37.010 --> 00:09:42.840 was at equilibrium, and we've disturbed the equilibrium by adding NaOH. To reestablish 00:09:42.840 --> 00:09:49.440 equilibrium, our weak acid HA must re-dissociate. But it will do so to a lesser extent than 00:09:49.440 --> 00:09:54.340 if it was in pure water, since there are already a substantial number of molecules of conjugate 00:09:54.340 --> 00:10:01.290 base present in solution. Instead of 4 molecules dissociating, perhaps 1 will dissociate; the 00:10:01.290 --> 00:10:06.990 exact number could be calculated from the equilibrium constant of the acid. So now in 00:10:06.990 --> 00:10:13.990 solution we have 59 HA molecules, 1 H+ ion, and 41 A- ions. This model satisfies the criteria 00:10:16.430 --> 00:10:21.440 from before, but does it explain our four observations from the initial experiment? 00:10:21.440 --> 00:10:28.440 Pause the video and discuss with a friend. First, is it acidic? Yes, because it has that 00:10:33.790 --> 00:10:40.790 one free H+ in solution. Second, how is it affected by addition of acid? Let's add HCl. 00:10:42.310 --> 00:10:49.310 The H+ ions react with the A- conjugate base, forming HA. The pH doesn't change, since all 00:10:51.290 --> 00:10:58.290 the added H+ ions are tied up here. So far, so good. Third, how is it affected by addition 00:11:01.029 --> 00:11:08.029 of base? Let's add NaOH. The OH- ions react with HA, forming water and A-. The overall 00:11:11.510 --> 00:11:16.990 H+ concentration doesn't change, so the pH stays the same. Note that in both of these 00:11:16.990 --> 00:11:22.290 cases, equilibrium would be re-established after addition of acid or base by a slight 00:11:22.290 --> 00:11:29.150 adjustment in the dissociation or reformation of HA. So the number of H+ ions does change 00:11:29.150 --> 00:11:34.130 upon addition of acid or base, but it doesn't change very much, certainly much less than 00:11:34.130 --> 00:11:40.190 if this was pure water. Finally, can we exceed the buffering capacity by adding enough acid 00:11:40.190 --> 00:11:46.760 or base? Definitely: if we add more than 59 molecules of strong base or more than 41 molecules 00:11:46.760 --> 00:11:52.420 of strong acid, we will use up all the HA or A-, and then our solution will no longer 00:11:52.420 --> 00:11:55.810 be a buffer. And there we have it! We've constructed a 00:11:55.810 --> 00:12:00.860 plausible model of our buffer solution: a solution of a weak acid and its conjugate 00:12:00.860 --> 00:12:05.720 base. You can go through an analogous modeling process for a weak base and its conjugate 00:12:05.720 --> 00:12:11.690 acid. That will form a buffer, too. The key to the buffering capability of any buffer 00:12:11.690 --> 00:12:17.440 is that there is a substantial amount of both acid and base present at equilibrium. In a 00:12:17.440 --> 00:12:22.670 buffer made with a weak acid and its conjugate base, the acid acts as a reserve of extra 00:12:22.670 --> 00:12:27.920 H+ ions that can react with added base, and the conjugate base acts as a sink, a place 00:12:27.920 --> 00:12:33.630 for the extra H+ ions from the added acid to go. In a buffer made with a weak base and 00:12:33.630 --> 00:12:39.550 its conjugate acid, the base acts as the H+ ion sink and the conjugate acid acts as the 00:12:39.550 --> 00:12:45.920 H+ ion reserve. 00:12:45.920 --> 00:12:49.880 Why would you want to make a buffer solution? Well, let's say you're modeling a reaction 00:12:49.880 --> 00:12:56.240 that occurs in human blood. Blood is a buffered solution with a pH of about 7.4, so you'd 00:12:56.240 --> 00:13:01.390 want to make sure that your experimental system is also buffered at this same pH. Or suppose 00:13:01.390 --> 00:13:07.260 you study Helicobacter pylori, a bacterium which colonizes the human stomach. Your experimental 00:13:07.260 --> 00:13:12.830 system would need to be buffered at around pH 2. And no matter what your target pH, you'd 00:13:12.830 --> 00:13:17.940 want your system to have a high buffer capacity: in other words, you want it to be as resistant 00:13:17.940 --> 00:13:23.339 to pH changes as possible. In designing a buffer solution, you have a lot of choices 00:13:23.339 --> 00:13:28.010 to make. Pause the video and suggest a few factors you should consider when designing 00:13:28.010 --> 00:13:35.010 a buffer solution. First, you have to choose your specific acid/conjugate 00:13:38.860 --> 00:13:45.020 base or base/conjugate acid pair. Then, you have to decide how much of the weak acid or 00:13:45.020 --> 00:13:50.000 base you want to use. Finally, you have to decide how much of the conjugate species you 00:13:50.000 --> 00:13:55.440 want to have at equilibrium. Each of these decisions affects the pH and buffer capacity 00:13:55.440 --> 00:13:59.990 of your final buffer solution. Let's look at each in turn. 00:13:59.990 --> 00:14:04.959 We know that a buffer solution has to have either a weak acid or weak base; but of course 00:14:04.959 --> 00:14:09.800 "weak" encompasses a range of strengths. For example, acetic acid is much stronger than 00:14:09.800 --> 00:14:16.680 boric acid, even though both of them are considered "weak" compared to a strong acid like HCl. 00:14:16.680 --> 00:14:21.600 The strength of the weak acid used will influence the final pH of the buffer: as you might guess, 00:14:21.600 --> 00:14:26.130 the stronger the weak acid, the lower the pH of the final buffer. 00:14:26.130 --> 00:14:30.709 But we also need sufficient conjugate base to make the solution function as a buffer. 00:14:30.709 --> 00:14:34.709 And so you might also correctly guess that the more of the conjugate base we add, the 00:14:34.709 --> 00:14:41.680 higher the pH of the final buffer. But again, that's not all. Remember the physical 00:14:41.680 --> 00:14:47.250 significance of our weak acid and its conjugate base: the acid is a reserve of extra H+ ions 00:14:47.250 --> 00:14:51.810 that could react with added base, and the conjugate base is a sink, a place for extra 00:14:51.810 --> 00:14:58.810 H+ ions from added acid to go. Would a system with an acid to conjugate base ratio of say, 00:14:59.190 --> 00:15:06.190 20:1 be an effective buffer? Pause the video. Since the acid reserve is 20 times larger 00:15:13.810 --> 00:15:18.720 than the conjugate base sink, this buffer would be very good at resisting pH if base 00:15:18.720 --> 00:15:24.250 were added, but not very good if acid were added. So, it would be a good buffer in only 00:15:24.250 --> 00:15:29.470 one direction. Intuitively, you might expect that a buffer with an acid:conjugate base 00:15:29.470 --> 00:15:34.850 ratio of 1:1 provides the widest range over which the pH is considered buffered, and you'd 00:15:34.850 --> 00:15:41.209 be right. Many real-life buffers don't necessarily have a 1:1 ratio, because of other design 00:15:41.209 --> 00:15:47.170 considerations (for example, target pH). And of course, it's not just the ratio between 00:15:47.170 --> 00:15:52.200 the acid and its conjugate base that influences buffer capacity. Can you imagine a situation 00:15:52.200 --> 00:15:57.640 in which the acid and conjugate base are present in a 1:1 ratio, but the buffer is still not 00:15:57.640 --> 00:16:04.640 an effective one? Pause the video. Suppose we have a buffer system in which the 00:16:09.339 --> 00:16:14.850 concentrations of weak acid and conjugate base are very low, in the micromolar range. 00:16:14.850 --> 00:16:20.080 Even though the acid to conjugate base ratio is 1:1, their absolute amounts are so small 00:16:20.080 --> 00:16:26.250 that the system would get overwhelmed by the addition of even dilute acids or bases. 00:16:26.250 --> 00:16:30.529 So designing a buffer system requires a delicate balance to make sure that the pH is where 00:16:30.529 --> 00:16:36.589 you want it to be, the ratio between the acid and conjugate base is close to 1:1, and that 00:16:36.589 --> 00:16:43.589 there is enough of each species to provide adequate buffering capacity. 00:16:46.300 --> 00:16:51.019 In this video, we created a conceptual model of a buffer. We saw that to effectively resist 00:16:51.019 --> 00:16:55.980 changes in pH, a buffer must contain a weak acid and its conjugate base or a weak base 00:16:55.980 --> 00:17:00.310 and its conjugate acid. We also discussed some of the choices that need to be made when 00:17:00.310 --> 00:17:05.470 designing a buffer and how those choices may impact the properties of the buffer. 00:17:05.470 --> 00:17:09.280 We hope that by better understanding the function of various buffer components, this video will 00:17:09.280 --> 00:17:13.199 give you some context for many of the calculations you'll need to carry out when dealing with 00:17:13.199 --> 00:17:18.880 buffer solutions.