Acid-Base & Buffer Investigation
Part A – Identifying Acids and Bases
Pre-lab Discussion:
Many items commonly found at home are acids or bases. For example, many of the foods you eat are acidic or
alkaline (basic). Many people are familiar with pH paper. It contains indicators -special chemicals that show
whether a given substance is an acid, a base or neutral. Indicators react with a solution to form a slightly
different chemical with a different color specific to a given pH level. Scientific probes are instruments that
measure pH with much greater accuracy. In this investigation you will use pH probes to determine the degree of
acidity or alkalinity of common substances.
Question: Which foods and household substances are acids and which are bases?
Hypothesis: In Table A, predict whether each substance is acidic, neutral, or basic.
Materials (per group): Substances to be tested
Wash bottle
pH probe, interface, computer
Beaker with H2O
Procedure:
1. You will find test tubes at your lab station that contain each of the substances you will test. The test
tubes are labeled numerically and a key, identifying each substance, is written on the board.
2. Dip the pH probe into the test tube containing the tap water.
3. Record the pH in your data table.
4. Taking care to rinse the pH probe in distilled water between every test, repeat steps 2 and 3 with the
other substances.
Data Table A:
Substance
pH
Prediction
pH of
Substance
Tap water
Lemon Juice
Vinegar
Cola
Mouth Wash
Antacid
Coffee
Milk
Baking Soda
Answer the following questions in your composition notebook.
Part A:
1. List substances that are acids, bases and neutral.
2. How did the actual pH of the substances compare with your predictions?
3. Which substance is the strongest acid and strongest base? Explain your conclusion.
4. Research and describe two situations in which acid-base indicators might be useful in everyday life.
5. You may have found that the results you obtained were different from those of other groups. What
variables might have affected your results?
Part B - Buffers
Prelab discussion:
The body must maintain an internal environment that is very constant. This includes temperature, structural
characteristics and pH levels, among others. Maintenance of a stable internal environment is called homeostasis.
The internal environment is mostly fluid. Body fluid plays a central role in homeostasis. Homeostatic
mechanisms function throughout the body to keep the chemistry of the body fluids constant. Changes in
optimum pH levels of body fluids can interfere with important chemical reactions in the body. Buffers are
compounds dissolved in our body fluids that minimize pH changes and thereby help maintain homeostasis.
Question: How does the addition of an acid affect the pH of water and liver homogenate?
Hypothesis:
Materials: (2) 50 mL beakers
water
pH probe, interface, computer
liver homogenate
HCl dropper bottle
Beaker with H2O
Wash bottle
Procedure:
1. Pour 20 mL of water into a beaker.
2. Using the same technique as in Part A, measure the pH and record it in Data Table B.
3. Using the acid dropper bottle, add 1 drop of acid into the beaker containing water and stir gently with
the pH probe.
4. Measure and record the pH.
5. Repeat steps 3 and 4 two more times for a total of 3 drops of acid in solution.
6. Pour 20 mL of the homogenate into a clean beaker.
7. Repeat steps 2 – 5 for the beaker containing the homogenate.
Data Table B:
Substance
Initial
pH
pH w/ 1 pH w/ 2 pH w/ 3
drop
drops
drops
acid
acid
acid
Answer the following questions in your composition notebook.
Part B:
1. Which fluid is more likely to contain buffers? Explain why you think this is true.
2. Why would substances from living things contain buffers?
3. Do some research to explain how a specific buffer works in a living organism. Connect this learning to
the concept of homeostasis.