In this video, Legos® are used to create possible molecular level models of a buffer. This is done to better understand how a buffer works and the components a buffer must contain in order to be effective. Students evaluate these models against a set of criteria determined at the beginning of the video and then consider other parameters that must be taken into account when designing a buffer.
After watching this video students will be able to:
Funding provided by the Singapore University of Technology and Design (SUTD)
Developed by the Teaching and Learning Laboratory (TLL) at MIT for SUTD
MIT © 2012
Here are two solutions, A and B, to which I've added universal indicator. Here's the color key. As you can see, both of these solutions are around pH 6, very slightly acidic.
Now I'm going to add concentrated sodium hydroxide, a strong base, to each solution.
It only took one drop of strong base to dramatically raise the pH of solution A. The pH of the other solution hasn't changed.
Let's reset. Here are the same two starting solutions, A and B, with the same indicator. But this time, I'm going to add concentrated hydrochloric acid, a strong acid, to each solution.
It only took one drop of strong acid to dramatically lower the pH of solution A. The pH of the other solution hasn't changed.
OK, let's look at our pairs of solutions. When we added a strong acid or base to solution A, its pH changed dramatically after only one drop! When we added one drop of acid or base to solution B, its pH stayed the same. Let's add more acid and more base to solution B and see what happens.
It takes much more acid or base to change the pH of this solution by the same amount! How is this solution able to resist changes to its pH when strong acids and bases are added? How could we make and use such a solution? In this video, you'll find out.
This video is part of the Structure-Function-Properties video series. The structure, function, and properties of a system are related and depend on the processes that define or create the system.
Hi, my name is George Zaidan and I am [attribution]
Before watching this video, you should know what an acid is, what a base is, be familiar with the concept of chemical equilibria, understand what distinguishes strong acids or bases from weak ones, and be able to define pH, Ka and pKa.
Chapter 1: Making a model
In chemistry, solutions that resist changes to their pH when acids or bases are added are called "buffers." Solution B in our demo was a buffer solution. Let's develop a molecular-level model of solution B to try and figure out how buffers work. First, let's review our experimental data and list the observations our model must satisfy:
Chapter 2: Buffer Design
Why would you want to make a buffer solution? Well, let's say you're modeling a reaction that occurs in human blood. Blood is a buffered solution with a pH of about 7.4, so you'd want to make sure that your experimental system is also buffered at this same pH. Or suppose you study Helicobacter pylori, a bacterium which colonizes the human stomach. Your experimental system would need to be buffered at around pH 2. And no matter what your target pH, you'd want your system to have a high buffer capacity: in other words, you want it to be as resistant to pH changes as possible. In designing a buffer solution, you have a lot of choices to make. Pause the video and suggest a few factors you should consider when designing a buffer solution.
First, you have to choose your specific acid/conjugate base or base/conjugate acid pair. Then, you have to decide how much of the weak acid or base you want to use. Finally, you have to decide how much of the conjugate species you want to have at equilibrium. Each of these decisions affects the pH and buffer capacity of your final buffer solution. Let's look at each in turn.
We know that a buffer solution has to have either a weak acid or weak base; but of course "weak" encompasses a range of strengths. For example, acetic acid is much stronger than boric acid, even though both of them are considered "weak" compared to a strong acid like HCl. The strength of the weak acid used will influence the final pH of the buffer: as you might guess, the stronger the weak acid, the lower the pH of the final buffer.
But we also need sufficient conjugate base to make the solution function as a buffer. And so you might also correctly guess that the more of the conjugate base we add, the higher the pH of the final buffer.
But again, that's not all. Remember the physical significance of our weak acid and its conjugate base: the acid is a reserve of extra H+ ions that could react with added base, and the conjugate base is a sink, a place for extra H+ ions from added acid to go. Would a system with an acid to conjugate base ratio of say, 20:1 be an effective buffer? Pause the video.
Since the acid reserve is 20 times larger than the conjugate base sink, this buffer would be very good at resisting pH if base were added, but not very good if acid were added. So, it would be a good buffer in only one direction. Intuitively, you might expect that a buffer with an acid:conjugate base ratio of 1:1 provides the widest range over which the pH is considered buffered, and you'd be right. Many real-life buffers don't necessarily have a 1:1 ratio, because of other design considerations (for example, target pH).
And of course, it's not just the ratio between the acid and its conjugate base that influences buffer capacity. Can you imagine a situation in which the acid and conjugate base are present in a 1:1 ratio, but the buffer is still not an effective one? Pause the video.
Suppose we have a buffer system in which the concentrations of weak acid and conjugate base are very low, in the micromolar range. Even though the acid to conjugate base ratio is 1:1, their absolute amounts are so small that the system would get overwhelmed by the addition of even dilute acids or bases.
So designing a buffer system requires a delicate balance to make sure that the pH is where you want it to be, the ratio between the acid and conjugate base is close to 1:1, and that there is enough of each species to provide adequate buffering capacity.
In this video, we created a conceptual model of a buffer. We saw that to effectively resist changes in pH, a buffer must contain a weak acid and its conjugate base or a weak base and its conjugate acid. We also discussed some of the choices that need to be made when designing a buffer and how those choices may impact the properties of the buffer.
We hope that by better understanding the function of various buffer components, this video will give you some context for many of the calculations you'll need to carry out when dealing with buffer solutions.
It is highly recommended that the video is paused when prompted so that students are able to attempt the activities on their own and then check their solutions against the video.
During the video, students will: