pH and Buffers Lab
Name:
6.0 Biology
Date:
Period:
Prelab:
1) Read the instructions carefully and develop the OBJECTIVE of this experiment.
2) Describe or define the following:
a. Chemical and physical properties of an acid.
b. Chemical and physical properties of a base.
c. Buffer
3) Hypothesize: What will happen to the pH of a solution if the following happens…
a. Mix HCl with water?
c. Mix HCl with a buffer?
b. Mix NaOH with water?
d. Mix NaOH with a buffer?
4) Write a summary procedure for Part 1 and Part 2 of the lab.
5) Create data tables for Part 1 and Part 2 of the lab.
Part 1 - Introduction to pH
Indicators are special chemicals that can show whether a given substance is an acid, a base, or neither.
Indicators usually react with an acid or base to form a slightly different chemical with a different color. One
type of pH paper turns a different color at each of several pH values ranging from 1 to 11. In this investigation,
you will test a number of substances using litmus and pH paper.
MATERIALS:
Well plates
Red Litmus Paper
6 Different Substances
Blue Litmus Paper
Forceps
pH paper
Procedure:
1. Record the name of the substances in the data table. There will be six substances.
2. Put 3 drops of the substance in a well of the well plate.
Use the forceps when dipping the pH paper and the litmus paper in the substances.
3. Dip the Red and the Blue litmus paper in the substance and record the color change.
4. Use pH paper to determine the pH of each substance. Based on the color change you observe in the pH
paper, record the approximate pH of the substance.
5. Repeat steps 1-4 with the other substances.
Part 2 - Introduction to BUFFERS
Individual organisms must maintain a relatively stable internal environment. Both organisms and cells respond
to many environmental factors that otherwise would change their internal environment. One factor is the
relative concentration of hydrogen (H+) and hydroxide (OH-) ions. Biochemical activities of living tissues
frequently affect the pH, yet life depends on maintaining the pH range that is normal for each tissue or system.
Testing pH of Water, Buffers and Biological Materials
How do organisms maintain the pH of their tissues within a normal range despite activities that tend to change
the pH? You can begin to answer that question by comparing the response of a nonbiological material and a
biological material to the addition of an acid and a base. The nonbiological material is tap water, and the
biological material is one of several substance derived from organisms – liver, potato, egg white and gelatin.
Frequently in biological investigations, it is difficult to study living tissue. Investigators have found that they
can learn as much, and sometimes more, by substituting a model for the real thing. A buffer solution (nonliving
chemical solution) will be used as a model to obtain data.
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MATERIALS:
50 mL beaker
0.1 M HCl
0.1 M NaOH
50 mL graduated cylinder
pH paper
pH 7 buffer
Tap water
Liver homogenate
Warm gelatin
Egg white solution
Potato homogenate
CAUTION: 0.1 M HCl (hydrochloric acid) and 0.1 M NaOH (sodium hydroxide) are irritants. Avoid
skin/eye contact; do NOT ingest. Notify teacher immediately. Flush spills and splash with water for 15
minutes; rinse mouth with water.
PROCEDURE:
1. Pour 20 mL of tap water into a clean 50 mL beaker. Record the initial pH of the water. Place all used
pH paper on a paper towel that can be thrown away at the end of lab.
2. Add 0.1 M hydrochloric acid (HCl) one drop at a time, swirling to mix after each drop. Record the new
pH with a new strip of pH paper.
3. Continue to add in 3 drop increments and record the pH each time until 18 drops of HCl have been
added. Swirl to mix and be sure to use new pH paper each time.
4. Empty contents into sink. Clean and rinse the beaker thoroughly.
5. Pour 20 mL of tap water into the clean beaker. Record the initial pH of the water. Add 0.1M sodium
hydroxide (NaOH) drop by drop, swirling to mix after each drop. Record the new pH with a new strip of
pH paper.
6. Continue to add in 3 drop increments and record the pH each time until 18 drops of NaOH have been
added. Swirl to mix and be sure to use new pH paper each time.
7. Clean and rinse the beaker thoroughly.
8. Pour 20 mL buffer solution into the beaker. Record the initial pH of the buffer. Add HCl in 3 drop
increments. Record the pH after each addition of 3 drops. Once you reach 18 drops, you may discard
the solution.
9. Clean and rinse the beaker thoroughly.
10. Pour 20 mL of fresh buffer solution into the beaker. Record the initial pH of the buffer. Add NaOH in
3 drop increments. Record the pH after each addition of 3 drops. Once you reach 18 drops, you may
discard the solution.
11. Obtain 20 mL the biological material assigned to your group by your teacher. Take the initial pH of the
solution. Record the change in pH as you add 3 drop increments of HCl. Once you reach 18 drops,
discard the biological material (any liquid can go into the sink, but solid material should be thrown
away).
12. Clean and rinse beaker.
13. Obtain 20 mL of the same biological material. Take the initial pH of the solution. Record the change
in pH as you add 3 drop increments of NaOH. Once you reach 18 drops, discard the biological material
(any liquid can go into the sink, but solid material should be thrown away).
14. Clean all lab equipment and return materials to original location. Clean lab tables and wash your hands
thoroughly before leaving the lab.
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ANALYSIS
Part 1 - Introduction to pH
1.
2.
3.
4.
Which substance had the highest concentration of H+ ions? Give evidence to support your answer.
Which substance is probably the strongest base? Give evidence to support your answer.
What does it mean if a substance is considered neutral? List any substances that had a neutral pH.
Use the data from the table to explain how red and blue litmus paper can be helpful in determining if a
substance is an acid or base?
5. Predict what would happen if you were to mix together an acid and a base? Explain your answer.
Part 2 - Introduction to BUFFERS
Make a graph (on graph paper) of the changes in pH in tap water versus the number of drops of acid and base
solutions added. Plot two lines - a solid line for changes with acid and a dashed line for changes with base.
 On the same graph, graph the results for your biological material. Use different colored solid (with acid)
and dashed lines (with base) to represent changes in pH for the samples.
 On the same graph used for tap water and the biological material, graph the reaction of the buffer
solution. Use a different colored solid (with acid) and dashed line (with base) for clarity.
1.
2.
3.
4.
5.
6.
Summarize the effects of HCl and NaOH on tap water.
What was the total pH change after 18 drops of HCl added to the biological material?
What was the total pH change after 18 drops of NaOH was added to the biological material?
How do these data compare with the changes in tap water?
How do biological materials respond to changes in pH?
How does the buffer system respond to HCl and NaOH? With respect to changes in pH, is the response of
the buffer system more like that of water or of the biological material?
7. Would buffers aid or hinder the maintenance of a stable internal environment in a changing external
environment?
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