Sometimes the simplest ideas actually work. Shocking, I know. Having just taught about the idea of buffers, I felt like the students needed a brief hands-on interaction so they could build their understanding better. I made up this lab … are you ready… make a buffer and then test it. Wow. To make it competitive I gave each group a slightly different challenge. Here is how I did it.
Materials to make a buffer
I gave the groups the following materials.
0.1 M acetic acid, 0.1 M boric acid, 0.1 M sodium carbonate
Then I gave them the conjugates calcium acetate, sodium carbonate, and sodium borate (just plain borax). How did I come up with these chemicals? I had them. That is all I had them so I used them. Sometimes those are the best labs. What you have. I also did not separate the acids from the conjugates on the list I gave them. They had to figure out what was available and what would make a buffer. Next time I might throw in a couple red herrings as well.
Make a buffer
I didn’t tell the students which buffer to make or which chemicals to use. Instead, each group drew a pH from a hat. Their challenge was to use the Henderson-Hasselbalch Equation to create a recipe for their buffer. I made sure that each of the pH values given were slightly different than the pKA of the various acids. That way they could not put equimolar amounts of acid and conjugate base. They had to figure out which chemicals to use and then how to manipulate the H-H equation to make the buffer the correct pH.
I have students compete for a few points every lab. The winner here was the team that could get their buffer closest to the prescribed pH. They were not allowed to use their pH probes until they had finished their buffer. It made for a fun and intense “reveal”.
Test the buffer
I had only lectured on buffers for one day, so their understanding of how they worked was limited. I decided to let them test their buffer as compared to water. They put 25 mL of water in a beaker. Recorded the pH. Then they added 10 drops of 0.1 M HCl and recorded the pH after 1, 2, 5 and 10 drops. Obviously, the pH dropped like crazy. They repeated that process but with 0.1M NaOH. The pH went up.
This was the part I was not expecting. They repeated each of the above processes but this time with their buffer. It felt like Christmas morning. “Mr. Koch, it’s working!” They couldn’t believe that the thing that they had just made actually did what chemistry told them it would do. It’s funny, but it really opened their eyes. They were so proud of their little 50 mL buffer.
We finished with a few net ionic equations and analysis of the various buffers in the class. We discussed how to change the buffering capacity. I will absolutely repeat this one. And before you ask, no, I don’t have a write up. I just pulled this one out of my rear before school! You can too.