1 M NaOH(aq) is added incrementally to beakers containing deionized water, and 0.01 M, 0.1 M, and 1 M acetic acid-acetate buffer solutions to which Universal Indicator has been added. Also, 1 M HCl(aq) is added incrementally to the water and to the acetic acid-acetate buffer solutions. The color of the indicator in the deionized water changes dramatically with the addition of a single drop of acid or base. The three buffer solutions of 0.01 M, 0.1 M, and 1 M all will resist the change in pH when small amounts of acid and or base are added. The 1 M buffer solution, on the other hand, has to be transferred to a larger beaker to accommodate the acid/base addition before the solution begins to change color!
Buffer capacity is defined after the presentation of the demonstration.
Curriculum Notes
This demonstration is good for showing the concept of buffer capacity in a general chemistry class. About 10 minutes of class time is necessary.
One day of lead time is required for this project.
Web page authors: Randy Sullivan and T. Greenbowe, University of Oregon. This page is under construction.
Buffer capacity of a buffer solution is the ability of the buffer to neutralize incoming acid and or base and maintain a pH. The concentration of the weak acid and the conjugate base, in the case of an acidic buffer, determines the buffer capacity.
Preparation
As described above.
Materials
- 4 ea. 150 mL beakers containing 100 mL of the following solutions: 0.5 M Acetic Acid/0.5 M Sodium Acetate, 0.05 M Acetic Acid/0.05 M Sodium Acetate, 0.005 M Acetic Acid/0.005 M Sodium Acetate, each with a couple of dropperfuls of universal indicator solution added and a stir bar in the beaker.
- stir plate
- 1 ea. large dropper bottle of 1 M NaOH.
- 1 ea. 250 mL bottle containing 1 M NaOH.
- 1 ea. 400 mL beaker containing a stir bar.
Procedure
- Place the beaker of DI water on the stir plate and turn on and adjust the stir plate.
- Add a couple of drops of 1 M NaOH to the solution until it changes to violet.
- Repeat with the remaining solutions, proceeding to the next higher buffer concentration each time. it will require more and more base to effect the change each time.
- By the time you get to the IM buffer solution, you will need to transfer the solution to the 400 mL beaker to accomplish the change.
Safety Precautions
- Always wear goggles when performing chemistry demonstrations.
- If you get base on your hands, rinse thoroughly with water.
- I you get base in your eyes flush with copious amounts of water.
Prep. Notes
As described above.
Randy's Notes
This demonstation has a great "wow" factor. The demonstration maintains the attention of students. I like to ham it up by acting like I'm getting bored with adding the acid or base dropwise to the 1 M acetic acid-acetate buffer solution, and then add it by the dropperful. Then I get bored with adding it by the dropperful and pour directly from the dropper bottle. When that runs out, I pull out the 250 mL bottle and start pouring from that.
An alternate presentation of buffer capacity using a didactic approach is found on the followintg URL:
