Buffers are solutions that resist a change in pH. In this activity, students will add HCl dropwise to a measured amount of water, monitoring the pH. Students then repeat the process, using a simple buffer prepared by combining baking soda and tonic water. A comparison of the results will allow students to visualize the nature of a buffer in terms of its ability to maintain a stable pH level.
Before the Activity
1. Prepare 1.0 M HCl. Place the acid into dropper bottles or labeled pipets. Each lab group will need just 10 drops of acid.
2. Prepare 1.0 M NaHCO3 solution (84 g baking soda per liter of solution). Alternatively, you can have each lab group dissolve 2.1 g of baking soda into 25 mL of water.
During the Activity
Hand out the activity directions, which are explict.
In overview, students will be asked to set up the CBL 2 unit with a pH probe. Students will collect data in the selected entry format using the EasyData App. The pH of 50 mL of water will be monitored as 5 drops of 1 M HCl are successively added. This results of this run are stored. The procedure is repeated, replacing the 50 mL of water with a buffer made by combining 25 mL of tonic water(H2CO3) with 25 mL of NaHCO3. Both graphs are viewed together in order make a comparison between how water responds to the addition of acid versus the response of a buffered system. The initial pH of the buffer is close to that of water.
After the Activity
Students will display both graphs simultaneously. Students will be asked to formulate a working description of a buffer based on the results of the experiment.