A buffer can be seen as a kind of "shock absorber" – for example, like here between two train cars. A chemical buffer, it acts as a shock absorber for acids and bases. We'll take a look at how a buffer works to explain. Here on the far left we draw a beaker of water. (We'll have room for three more beakers here.) Over here we also write that we add some BTB, which colors the water green, because it's neutral. Now I'll take a pipette here with a little hydrochloric acid in it. Here I'll then add a single drop of the hydrochloric acid. What happens then? Yes, as you can see, it turns yellow immediately, and the solution becomes acidic. Now we take the same beaker here with the acidified water. In this pipette, there is instead a little sodium hydroxide with the same concentration as the hydrochloric acid. Here I add a single drop again – and bang! – it immediately turns blue instead. Of course, if it had been an exactly the same size drop, it would have turned green again. But now it became a slightly, slightly larger drop, and if I just add this quickly from a dropper, it's almost impossible to get it perfect. What you should take with you in any case is that in pure water, very little acid or base is enough for the pH to immediately change to acidic or basic. Now let's see what happens instead if I mix in a small buffer. In this beaker here, I have mixed together sodium acetate and acetic acid so that both have a concentration of 0.05M. This solution then actually becomes slightly acidic, which is how BTB turns yellow. If I now add hydrochloric acid in the same way as before, it may not come as a surprise that the solution continues to be acidic. But we take the same buffer again with 0.05M sodium acetate and 0.05M acetic acid, and then we add some sodium hydroxide solution instead. In this beaker, a single drop was enough to make it basic, and turn blue. But what happens now when I drop a few drops of sodium hydroxide into the solution? Yes, it continues to be slightly acidic this way, and the color doesn't change at all. This is what is typical of chemical buffers: Even if I add quite significant amounts of acid or base, the pH doesn't change much at all. Unlike pure water, where the pH changes quickly and sharply. It's best that we take that and write it down too, how pure water differs from buffers. In pure water, the pH changes significantly even when a small amount of acid or base is added. In a buffer, however, the pH changes almost not at all when a small amount of acid or base is added. We will also take and draw some pictures to explain what happens when we add a little acid to a buffer. Here we draw a beaker with an acetate buffer like before. In this buffer I draw two acetic acid molecules. In this buffer, I now also let one of the two acetic acid molecules be protolyzed, so that we get a dissolved hydrogen ion here as well. What happens now if we add a strong acid, such as hydrochloric acid? Then I'll draw what's happening here, but don't draw this really yet! Here are some animations first! When hydrogen chloride enters the water, it is protolyzed. Then one could imagine that the hydrogen ion concentration increases, and that the pH then decreases. But the buffer is made so that there are quite a few free acetate ions in the solution. Instead of dissolving in the water, the hydrogen ions that are released will bind to the acetate ions, like this. This keeps the hydrogen ion concentration at the same level as before, at least if only a moderate amount of acid is added. (And now you can draw this picture!) We can do the same thing and look at what happens when you add a moderate amount of base to a buffer. Here we have a beaker with the same acetate buffer, but now we add some base in the form of hydroxide ions instead. The hydroxide ion ends up in the water. Then it is the case that the hydroxide ion here first takes up a hydrogen ion from the solution so that water is formed. Then one can imagine that the concentration of hydrogen ions in the solution would drop, and that the pH should therefore rise. But now the buffer is made so that there are quite a few acetic acid molecules in it too. They will then be protolyzed and compensate for the hydrogen ions that are taken up by the hydroxide ions, so that the hydrogen ion concentration is also maintained at the same level as before. But we can also use Le Chatelier's principle to explain how a buffer works. To do that, we're going to draw a little graph here, with the concentration on the y-axis, and then we're going to put HAc, which is in equilibrium with H⁺ and Ac⁻, here on the x-axis. But wait a moment before drawing this, because here too there will be some animations that I will show first! In any case, some bars may show the approximate concentration of the substances. In a buffer, it's typically like this, that we have fairly high concentrations of the acid and base, and lower concentrations of hydrogen ions. What happens when you add a little acid? Yes, the acid, in this case, is hydrogen ions, so I'm increasing the stack of hydrogen ions like this. But what happens to this equilibrium? Well, according to Le Chatelier's principle, we know that when we disturb an equilibrium, a reaction occurs that counteracts the disturbance. So, what we fill in at one end, flows out at the other, so to speak. This means that this equilibrium is now shifting to the left. And when the equilibrium shifts to the left, the concentrations of H⁺ and Ac⁻ decrease while the concentration of HAc increases. And when the system reaches equilibrium again, the H⁺ concentration has dropped so much that it is basically as high again as it was at the beginning. We'll draw in some arrows showing how the concentration has changed as well, and then this picture is ready for you to draw! Now I'm making another picture where I'm going to show what happens when a base is added. Wait to draw this until I tell you again. Just as before, we put the concentration on the y-axis, and then we have the equilibrium HAc ⇌ H⁺ + Ac⁻ on the x-axis, with some bars showing the concentrations of the different substances. If I add a base (in this case hydroxide ions, OH⁻), they will react with the hydrogen ions and form water. This means that the H⁺ concentration will decrease – like this. What happens to the equilibrium then? Yes, again we reason according to Le Chatelier's principle, and conclude that now the reaction will go to the right. This means that the concentrations H⁺ and Ac⁻ will increase, while the concentration HAc decreases. When the system has reached equilibrium again, the hydrogen ion concentration is again about as high as it was at the beginning. We draw some arrows to show how the concentrations change, and then this picture is ready for you to draw too! Now you know a little bit about how buffers work, let's now write a little more about what buffers consist of, i.e. how to make buffers. This is done by mixing a weak acid or base plus its corresponding salt. Everything must be in relatively high concentrations. Then you get good buffers. As an example, we can take acetic acid and sodium acetate, which I have already talked about a lot. Here, the acetic acid is an acid (of course) and in the salt here, sodium acetate, the acetate ion acts as a base. The sodium ion does not react with the water, but is only a spectator ion. Another buffer can be obtained by mixing ammonia with ammonium chloride. Ammonia is a base, and ammonium chloride is its corresponding salt. The ammonium ion then acts as an acid, while the chloride ion here does not react with the water either, but is only a spectator ion. A third example of a buffer is obtained by mixing carbonic acid with sodium bicarbonate. Carbonic acid is of course acidic, and sodium bicarbonate contains bicarbonate ions that can act as a base. The sodium ions here are again just spectator ions. Finally, I thought I'd finish by telling you why buffers are so important. It's like this, that in your body and in your cells you have lots of different proteins. It is the same for all living things. These proteins only work at a certain pH, that is, at a certain hydrogen ion concentration. If the pH in the cell differs too much from the proteins' optimal pH, then the proteins will stop functioning, leading to the death of the cell and eventually the entire organism. Yes, you can actually die from having too low a pH in your body! However, in our body we have a buffer system consisting primarily of carbonic acid and bicarbonate ions. This means you won't die from drinking a Coca-Cola or another acidic soft drink. The body's buffer system protects you from dying from just one soda. However, if you ingest too much ethanol, things can get really bad. When ethanol is broken down in the liver, it is first converted into acetic acid. If a person has drunk too much, so much acetic acid can form that the body's proteins collapse. Nothing good! So buffers are vital, but not just for us, but also for plants and everything else living on Earth. And in the soil there are also quite a few different buffer systems. Different soils have different buffering capacities, something you may have already heard about in Biology 1. In the soil there are something called humus particles. They consist of decomposed organic matter, and they can buffer the soil's pH. They can do this, among other things, because they are very small, and therefore have a large total surface area. On the right here we now draw the soil surface and then a humus particle in very high magnification. Thanks to the large surface area, positive ions can now bind to the humus particle, such as hydrogen ions, magnesium ions, iron ions and potassium ions. The bound ions are then in equilibrium with the ions dissolved in the water in the soil. But what happens if acid rain comes? Well, of course it rains down a lot of hydrogen ions. The concentration of hydrogen ions in the soil increases, which causes the equilibrium here to be pushed to the left. Then a lot of important metal ions are leached out of the soil and flow further down. For example, it then becomes more difficult for plants to access magnesium ions, which they need for their chlorophyll. Thanks to the buffering capacity of the soil, plants can withstand small amounts of acid rain. But the buffering capacity is not infinite! Therefore, acidification and acid rain are serious environmental problems that must be controlled and curbed.