1 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 2 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. 3 00:00:17,400 --> 00:00:24,160 Now I'm going to add concentrated sodium hydroxide, a strong base, to each solution. 4 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 5 00:00:30,880 --> 00:00:37,269 other solution hasn't changed. Let's reset. Here are the same two starting 6 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 7 00:00:41,870 --> 00:00:46,239 hydrochloric acid, a strong acid, to each solution. 8 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 9 00:00:52,350 --> 00:00:57,799 other solution hasn't changed. OK, let's look at our pairs of solutions. 10 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 11 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. 12 00:01:11,030 --> 00:01:17,000 Let's add more acid and more base to solution B and see what happens. 13 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! 14 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 15 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. 16 00:01:33,310 --> 00:01:38,270 This video is part of the Structure-Function-Properties video series. The structure, function, and 17 00:01:38,270 --> 00:01:43,370 properties of a system are related and depend on the processes that define or create the 18 00:01:43,370 --> 00:01:44,560 system. 19 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 20 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 21 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 22 00:02:04,530 --> 00:02:06,880 pKa. 23 00:02:06,880 --> 00:02:09,810 After watching this video, you will be able to: 24 00:02:09,810 --> 00:02:14,549 Describe how the structure, or composition, of a buffer functions to resist changes in 25 00:02:14,549 --> 00:02:19,530 pH Explain how the choices made in buffer design 26 00:02:19,530 --> 00:02:26,269 impact the properties of a buffer. 27 00:02:26,269 --> 00:02:31,310 In chemistry, solutions that resist changes to their pH when acids or bases are added 28 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 29 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 30 00:02:43,260 --> 00:02:48,609 experimental data and list the observations our model must satisfy: 31 00:02:48,609 --> 00:02:55,540 The starting pH was around 6 When we added one drop of strong base (enough 32 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 33 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), 34 00:03:07,139 --> 00:03:14,139 the pH of our buffer solution did not change Eventually, after addition of much more strong 35 00:03:14,819 --> 00:03:19,939 acid or strong base, the pH of our buffer solution did change 36 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 37 00:03:24,919 --> 00:03:31,579 acid in solution. But it's not. Relying on observation 3, explain why a solution comprised 38 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 39 00:03:38,150 --> 00:03:45,150 added. Pause the video. Your initial reaction to this question is 40 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 41 00:03:54,779 --> 00:04:01,629 therefore observation 3 could not be satisfied. This is correct, except for two cases: first, 42 00:04:01,629 --> 00:04:07,260 the added acid could be exactly the same strength and concentration as the acid already present. 43 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 44 00:04:11,499 --> 00:04:16,949 buffer solution. Second, the added acid could be much weaker or much less concentrated than 45 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 46 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 47 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 48 00:04:35,409 --> 00:04:40,350 buffered solution. But in both of these cases, even though it seems as though the solution 49 00:04:40,350 --> 00:04:45,280 is buffered, that "buffer-like" response depends on the relative strengths and concentrations 50 00:04:45,280 --> 00:04:50,770 of each acid, not on any intrinsic property of the solution. 51 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 52 00:04:57,020 --> 00:05:02,750 2 and 3, taken together, suggest that there are both acidic and basic species present 53 00:05:02,750 --> 00:05:08,290 in our solution, since additional acid or base must be neutralized to keep pH relatively 54 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 55 00:05:14,790 --> 00:05:20,320 the pH of the solution is slightly acidic. To better understand what might be happening 56 00:05:20,320 --> 00:05:25,890 at the molecular level, let's use Legos to model a solution that meets these criteria, 57 00:05:25,890 --> 00:05:31,880 and see if that model correctly predicts all 4 observations. We'll start with pure water. 58 00:05:31,880 --> 00:05:36,900 We could model water molecules using Legos but that would quickly get overwhelming, so 59 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 60 00:05:43,700 --> 00:05:50,700 molecules of base. In real solutions, there are on the order of 1022 or more molecules 61 00:05:50,840 --> 00:05:55,530 dissolved. That would be a lot of Legos, so we're choosing smaller numbers for convenience. 62 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 63 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 64 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. 65 00:06:17,080 --> 00:06:23,860 This piece represents the Na+ ion and this piece the OH- ion. Remember that strong acids 66 00:06:23,860 --> 00:06:28,270 and bases dissociate completely in water, so I'm going to take apart all the pieces 67 00:06:28,270 --> 00:06:31,240 here. And this is our initial model! It contains 68 00:06:31,240 --> 00:06:37,580 both acidic and basic species, and it contains more acid than base. But this solution will 69 00:06:37,580 --> 00:06:44,580 not resist changes in pH. Pause the video and explain. 70 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 71 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. 72 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 73 00:07:07,490 --> 00:07:12,590 back to our criteria. Remember that to satisfy these criteria we had the option of selecting 74 00:07:12,590 --> 00:07:18,860 either weak or strong acids or bases, and last time we selected the strong/strong case. 75 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. 76 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, 77 00:07:34,430 --> 00:07:38,130 HA. Remember, weak acids don't dissociate completely when dissolved in water. The extent 78 00:07:38,130 --> 00:07:44,000 to which a weak acid or base dissociates is related to the equilibrium constant, Ka for 79 00:07:44,000 --> 00:07:49,830 an acid or Kb for a base. These equilibrium constants depend on the chemical structure 80 00:07:49,830 --> 00:07:55,060 of the acid or base. Pause the video here and write the equilibrium expression for a 81 00:07:55,060 --> 00:08:02,060 weak acid. The equilibrium expression would be this. 82 00:08:05,210 --> 00:08:09,200 We always use concentrations in our equilibrium expression, even though in our model we're 83 00:08:09,200 --> 00:08:13,920 using number of molecules; but the principle is the same. 84 00:08:13,920 --> 00:08:20,160 When we dissolve 100 molecules of weak acid in water, some of them will dissociate, forming 85 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 86 00:08:28,820 --> 00:08:33,380 at any given point after the system has reached equilibrium, there will be a fixed number 87 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, 88 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 89 00:08:49,980 --> 00:08:54,770 conjugate base. So now we have a model of our weak acid. We 90 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 91 00:09:03,210 --> 00:09:10,210 NaOH would completely dissociate in water. Now what? 4 OH- ions react with H+ to form 92 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 93 00:09:23,930 --> 00:09:30,430 via a typical weak acid-strong base reaction, forming 36 molecules of A-, the conjugate 94 00:09:30,430 --> 00:09:37,010 base. And we're still not through. Remember that the dissociation of our weak acid HA 95 00:09:37,010 --> 00:09:42,840 was at equilibrium, and we've disturbed the equilibrium by adding NaOH. To reestablish 96 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 97 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 98 00:09:54,340 --> 00:10:01,290 base present in solution. Instead of 4 molecules dissociating, perhaps 1 will dissociate; the 99 00:10:01,290 --> 00:10:06,990 exact number could be calculated from the equilibrium constant of the acid. So now in 100 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 101 00:10:16,430 --> 00:10:21,440 from before, but does it explain our four observations from the initial experiment? 102 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 103 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. 104 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 105 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 106 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 107 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 108 00:11:16,990 --> 00:11:22,290 cases, equilibrium would be re-established after addition of acid or base by a slight 109 00:11:22,290 --> 00:11:29,150 adjustment in the dissociation or reformation of HA. So the number of H+ ions does change 110 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 111 00:11:34,130 --> 00:11:40,190 if this was pure water. Finally, can we exceed the buffering capacity by adding enough acid 112 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 113 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 114 00:11:52,420 --> 00:11:55,810 be a buffer. And there we have it! We've constructed a 115 00:11:55,810 --> 00:12:00,860 plausible model of our buffer solution: a solution of a weak acid and its conjugate 116 00:12:00,860 --> 00:12:05,720 base. You can go through an analogous modeling process for a weak base and its conjugate 117 00:12:05,720 --> 00:12:11,690 acid. That will form a buffer, too. The key to the buffering capability of any buffer 118 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 119 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 120 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 121 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 122 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 123 00:12:39,550 --> 00:12:45,920 H+ ion reserve. 124 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 125 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 126 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 127 00:13:01,390 --> 00:13:07,260 you study Helicobacter pylori, a bacterium which colonizes the human stomach. Your experimental 128 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 129 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 130 00:13:17,940 --> 00:13:23,339 to pH changes as possible. In designing a buffer solution, you have a lot of choices 131 00:13:23,339 --> 00:13:28,010 to make. Pause the video and suggest a few factors you should consider when designing 132 00:13:28,010 --> 00:13:35,010 a buffer solution. First, you have to choose your specific acid/conjugate 133 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 134 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 135 00:13:50,000 --> 00:13:55,440 want to have at equilibrium. Each of these decisions affects the pH and buffer capacity 136 00:13:55,440 --> 00:13:59,990 of your final buffer solution. Let's look at each in turn. 137 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 138 00:14:04,959 --> 00:14:09,800 "weak" encompasses a range of strengths. For example, acetic acid is much stronger than 139 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. 140 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, 141 00:14:21,600 --> 00:14:26,130 the stronger the weak acid, the lower the pH of the final buffer. 142 00:14:26,130 --> 00:14:30,709 But we also need sufficient conjugate base to make the solution function as a buffer. 143 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 144 00:14:34,709 --> 00:14:41,680 higher the pH of the final buffer. But again, that's not all. Remember the physical 145 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 146 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 147 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, 148 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 149 00:15:13,810 --> 00:15:18,720 than the conjugate base sink, this buffer would be very good at resisting pH if base 150 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 151 00:15:24,250 --> 00:15:29,470 one direction. Intuitively, you might expect that a buffer with an acid:conjugate base 152 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 153 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 154 00:15:41,209 --> 00:15:47,170 considerations (for example, target pH). And of course, it's not just the ratio between 155 00:15:47,170 --> 00:15:52,200 the acid and its conjugate base that influences buffer capacity. Can you imagine a situation 156 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 157 00:15:57,640 --> 00:16:04,640 an effective one? Pause the video. Suppose we have a buffer system in which the 158 00:16:09,339 --> 00:16:14,850 concentrations of weak acid and conjugate base are very low, in the micromolar range. 159 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 160 00:16:20,080 --> 00:16:26,250 that the system would get overwhelmed by the addition of even dilute acids or bases. 161 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 162 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 163 00:16:36,589 --> 00:16:43,589 there is enough of each species to provide adequate buffering capacity. 164 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 165 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 166 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 167 00:17:00,310 --> 00:17:05,470 designing a buffer and how those choices may impact the properties of the buffer. 168 00:17:05,470 --> 00:17:09,280 We hope that by better understanding the function of various buffer components, this video will 169 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 170 00:17:13,199 --> 00:17:18,880 buffer solutions.