Demonstrating the Effect of a Strong Acid or Base on a Buffer System

Set-up of the Buffer Demo

How do you demonstrate the workings and effectiveness of buffers in your acid-base unit?

In our acid-base unit, we define buffers as a two-component system that prevents a major change in pH. More specifically, a buffer is comprised of a weak acid and its conjugate base or a weak base and its conjugate acid. Any small amount of strong acid or base added to a buffer is converted to the weaker acid or base already comprising the buffer system, thus, minimizing pH change. We have several examples of where buffers are at play: living organisms, seawater, certain foods or other items meant for human consumption, etc. But, how do YOU demonstrate how a buffer system works?

Last month as we wrapped up our acid-base unit and approached buffers, I did some brainstorming. Here’s a description of what I devised (and it worked well!).

1. Set up 4 beakers. Two with distilled water and two with a buffer of your choice. I “borrowed” a pre-made buffer from the biology teacher. Note: a common misconception among students is that buffers have a neutral pH. Remember that buffers can be slightly acidic, slightly basic, or neutral depending on the composition.

Figure 1 - Buffer demo set-up.

2. Obtain a dropper bottle of 1M HCl and 1M NaOH.

3. Attach a pH probe to a sensor interface. Our department has provided a LabQuest Mini to each science teacher for demos and we have a set available for labs. What’s great about these interfaces is that you can plug into a PC laptop or MacBook that has Vernier software downloaded or plug into a Chromebook that has the “Graphical Analysis” app set up. I used my staff Chromebook and Google Cast for Education to display the data collection on the projector in real-time.

       

Figure 2 - Vernier LabQuest Mini  

 

  

Figure 3 - Projection  of graph using Google Cast for Education

4. Insert the pH probe (with data collection on) into a beaker with distilled water. Show them how the pH is constant (for some reason it was not 7 for me, but I cared more about the qualitative data students were about to observe). Then, add a drop or two of a strong acid. Observe the sharp decline in pH.

5. Now, insert the pH probe into a beaker with the buffer. Observe a constant pH data collection before adding a drop or two of a strong acid. You’ll notice the change in pH is minimized compared to the control experiment (strong acid in distilled water).

6. Repeat steps 4 and 5, this time using strong base.

Figure 4 - Projection of data using Google Cast for Education

7. Encourage a class discussion or ask probing questions (no pun intended) about this phenomenon of buffers. Please note that in upper-level classes you’ll be able to bring in the Henderson-Hasselbalch equation. I did not do this for my non-Honors course.

I hope sharing this demo (and use of a Chromebook) is helpful to you. Do you have anything you would add or do differently for your students?