Experiment 13
Acids and Bases
For this experiment, you will examine pH using a dye indicator that you will make yourself from red cabbage, which changes color at different pH values. You will then determine the approximate pH of several household chemicals using your dye indicator and standardized buffers at different pH's.
Part A: Preparation of pH dye indicator
In a 250-mL beaker, add about 100 mL of DI water. Add enough cut-up red cabbage to fill the beaker to about the 200-mL marking. Heat the containing until the solution just starts to boil. Remove the beaker from the heat and let it sit for about 10 min to cool and to absorb color from the leaves. This dye indicator will be used in the remainder of Part A and for Part B of this experiment.
To prepare a pH reference set, arrange 13 test tubes in two test tube racks. You may need to combine your test tube set with your neighbor's set. Pour 3-4 mL of each buffer in a separate test tube to create a set of pH values of 1-13.
Caution: Low pH values ar strongly acidic; high pH values ar strongly basic (alkaline). Work with care!
To each test tube, add 2-3 mL of the cooled red cabbage solution. If you wish a deeper color, add more cabbage solution, but try to be consisent and add the same amount of indicator to each tube. Describe the colors of the pH solutions. (You may wish to create a single reference set for two groups, who might wish to work together during thid part of the experiment.)
Keep this reference set for the next part of the experiment.
Part B: Measuring pH
Place 3-4 mL of each chemical you plan to determine the pH on in a clean test tube. After you have added the solutions to the test tubes, add about 2-3 mL of red cabbage solution into each test tube (be sure to use the same amount of the red cabbage solution as used for the standard solutions prepared above). Describe the color and compare to the colors of the pH reference set you have made. The pH of the buffer in the reference set that gives the best color match is the pH of the sample.
Record your observations carefully in your lab notebook. Test several samples.
Samples may include the following, or other solutions prepared by the stockroom: shampoo, conditioner, mouthwash, antacids, detergents, fruit juice, vinegar, cleaners, aspirin, soda, etc.
Part C: Effect of Buffers on pH
Buffers are solutions that resist a change in pH. Buffers are most often composed of acid-base conjugate pairs in various concentrations. When either acid or base is added to a buffer, the pH does not change very much (there will always be a slight change in pH, however). You may find it convenient for one group of students to do the acid addition experiment, and another group of students do the base addition experiment. Then, compare results.
Effect of Adding Acid
Place about 10 mL of one of the following solutions into a test tube:
Add 2-3 mL of cabbage indicator. Describe the color. Determine the pH of each sample, using the pH reference set. Record your observations in your lab notebook.
For acid addition: Add 5 drops of 0.1 M HCl (acid) to each of the tube listed above. Mix the contents. Determine the pH. Add 5 more drops of 0.1 M HCl. Record any color change in the indicator solutions. Determine the pH.
Repeat this procedure with each of the solutions above.
For base addition: Add 5 drops of 0.1 M NaOH (base) to each of the tube listed above. Mix the contents. Determine the pH. Add 5 more drops of 0.1 M NaOH. Record any color change in the indicator solutions. Determine the pH.
Repeat this procedure with each of the solutions above.
Determine the change in pH, if any for each of the solutions above. Identify the solutions that are buffers.
Record all data in your laboratory notebook.
Disposal of chemicals: Place all liquid waste in the liquid waste container. Be sure to clean up any spills on the lab bench and work area.
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