Name ____________________________
pH and Homeostasis Lab
I. BACKGROUND INFORMATION
One feature that defines all living organisms is their ability to maintain a relative stability; this is known as
HOMEOSTASIS. There are many examples of this in your own body -- the maintenance of constant body
temperature (98 ̊F), keeping your blood pH within a certain range, and balancing food/water intake with waste
removal, to name a few.
Many factors affect the internal stability of plants and animals, and one of them is the relative concentration
of hydrogen ions (H+) and hydroxide ions (OH-). The biochemical activities of living tissues produce these ions
that affect pH, yet life depends on maintaining a pH range that is within certain limits for each tissue or system.
Using pH paper, you will be looking at the responses of several substances to the addition of acid and base.
Some of these substances will be from actual living tissue and others will not. The question you should be
considering throughout this lab is: "How do organisms survive and function despite metabolic activities that
tend to shift pH toward either acidic or basic ends of the pH scale?" This hypothesis should address how you
believe the water will respond to the addition of the HCl and NaOH vs. how you believe biological materials will
respond to the addition of HCl and NaOH.
II. MATERIALS
50mL tap water
0.1M NaOH
0.1M HCl
Potato homogenate
2 small beakers
Milk
pH paper
Egg whites
III. PROCEDURE
PART I: Mixing water with HCl and with NaOH
1. Pour 25 mL of tap water into a beaker.
2. Record the initial pH of the water using a small strip of pH paper.
3. Add 0.1M HCl one drop at a time.
4. GENTLY SWIRL THE MIXTURE AFTER EACH DROP.
5. After adding FIVE DROPS, use a strip of pH paper to record the pH in your chart.
6. Repeat this procedure 5 more times so that 30 drops have been added in total. Record all data in your data table.
7. Rinse the beaker really well.
8. Pour another 25 mL of water into the beaker.
9. Record the initial pH.
10. Add 0.1M NaOH drop-wise (one drop at a time) just like above with the HCl.
11. Record your data for each 5 drops added until 30 drops have been added in total.
12. Rinse the beaker really well.
PART II: Mixing biological materials with HCl and with NaOH
1. Repeat the above procedure, replacing water with one of the following materials: potato homogenate, egg white,
and/or milk.
2. Record data in data table.
PART III: Manipulating and organizing your data
1. Obtain class data & average pH values after each set of 5 drops was added to the water.
2. Record these average values in your data table.
3. With the other group(s) who used the same biological material as you, obtain average pH values after each set of
5 drops were added to that biological material.
4. Record these average values in your data table.
5. Record all class data (for all materials) in your own data table.
IV. DATA/OBSERVATIONS
Title: __________________________________________________
Drops
Added
Group
pH Acid
+ Water
Class
pH Acid
+ Water
Group
pH Acid
+Potato
Class
pH Acid
+ Potato
Group
pH Acid
+ Egg
Class
pH Acid
+ Egg
Group
pH Acid
+Milk
Class
pH Acid
+Milk
Group
pH Base
+Milk
Class
pH Base
+Milk
0
5
10
15
20
25
30
Title: __________________________________________________
Drops
Added
Group
pH Base
+ Water
Class
pH Base
+ Water
Group
pH Base
+Potato
Class
pH Base
+ Potato
Group
pH Base
+ Egg
Class
pH Base
+ Egg
0
5
10
15
20
25
30
Create a graph of your class average data showing the addition of acid to each of the liquids (water, potato, egg
white, milk) and the addition of base to each of the liquids. You will decide whether a line graph or a bar graph is
appropriate for your data.
Title ____________________________________________
Use the graph that you made to briefly summarize the effects of HCl and NaOH on the tap water and biological
materials.
** Note: Because you are still in the results section, all comments here must be OBSERVATIONAL,
not analytical. Use these questions to help GUIDE your summary: What happens to the pH as base or acid is
added? Does the pH level off after a while? Compare the pH changes you observed in the tap water with the pH
changes you saw overall in the biological materials (the potato, egg white, and milk) - do you notice any difference?
V.ANALYSIS/DISCUSSION Answer the following questions.
1. Based on what you can see from your data, does your data support your hypothesis? Why or why not?
2. Relate your findings to the theme of homeostasis, and comment upon the question posed at the beginning
of this lab - how do you think that organisms survive and function despite an organism’s activities that tend
to shift pH toward the acidic or basic ends of the pH scale?
3. Describe homeostasis in your own words. Also discuss how buffers work and that might relate to your data.
VI. SOURCES OF ERROR Discuss 3 sources of error and how they could have affected your results. Then give
recommendations for fixing these sources of error.
1.
2.
3.