What is the effect of acids and bases on biological
solutions?*
INTRODUCTION
It is important for organisms to be able to protect themselves from changes in
internal and environmental pH. Any changes could disrupt metabolic activities.
Organisms have developed ways in which to cope with these changes. Today
you will look at how some biological materials can help moderate drastic swings
in pH. Working with your bench mates, you will use a pH sensor attached to a
computer to see changes in pH as you add acid or base to water, to buffers, and
to several biological solutions. You will then share your information with the other
students in the class.
OBJECTIVES
In this experiment, you will
 Add an acid to a solution and note the extent that it resists changes in pH.
 Add a base to a solution and note the extent that it resists changes in pH.
 Pool you data with other members of the class.
PRE-LAB
Read about pH in Life on Earth. Review class notes on pH. Define: acid, base,
neutralization, buffer
MATERIALS
You will work with your bench mates. The jobs for the bench mates are
1. Add acid/base
2. Hold and swirl the beaker
Please rotate the jobs as you work
3. Record data
with different liquids
4. Go-fer
Each group should have:
Computer with attached pH sensor
Plastic box containing a test tube rack with two tubes-label one ACID, label the
other BASE
Micropipette – 100 ul or one that is set at 100 ul plus tips as needed
50 ml beakers for solutions
250 ml beaker for rinsing sensor tip
0.1N HCl solution – on teacher’s desk
0.1 N NaOH solution – on teacher’s desk
Squeeze bottle of water (“rinse bottle”)
pHydrion paper
Various biological materials such as apple juice, cranberry juice, potato extract,
and liver solution
Buffer solutions of pH 4, pH 7, and pH 10
PROCEDURE
I. Testing the effect of acid on water
1. Before each use of the pH sensor, you need to rinse the tip of the electrode
thoroughly with distilled water. To do this, hold the pH electrode above a rinse
beaker and use the rinse bottle to thoroughly rinse the electrode. Gently blot
with paper towel.
Important: The electrode tip is made of glass and is fragile. Handle with
care!
2. Fill one of the small beakers with about 20 ml of water.
3. Take the initial pH by carefully placing the electrode tip into the water.
4. Wait a few seconds for the pH to stabilize. Record the pH in your table.
5. Using the micropipette, add 100 ul of ACID to the water. Swirl the beaker
carefully to mix.
6. Wait a few seconds until the pH stabilizes. Take the pH measurement and
record it.
7. Rinse the electrode tip.
8. Repeat steps 5 through 7 nine more times for a total of 1000 ul of acid added
to the water.
9. Throw out water and rinse beaker well. Rinse the electrode tip..
II. Testing athe effect of base on water
Repeat steps 3 through 9 above using BASE instead of acid.
III. Testing the effect of acid and base on a buffer solution
1. Each bench in a triad will choose one of the three buffers. My group buffer is
pH ___
2. Pour about 20 ml of buffer into a small beaker. Repeat steps 3 through 9
using ACID as in section I.
3. Pour about 20 ml of a buffer into a clean beaker and repeat steps 3 through 9
using BASE as in section II.
IV. Testing the effect of acid and base on a biological liquid.
Important: If you are using liver solution, please use pHydrion paper and
not the pH sensor. The protein of the liver will stick to the electrode. See
teacher for instructions.
1. Each bench in the triad will choose one biological liquid or liver solution. If you
have time, you can try a second liquid. Our biological solution is
________________________.
2. Pour about 20 ml of liquid into a small beaker. Repeat the steps 3 through 9
as for ACID in section I.
3. Pour 20 ml of liquid into a clean beaker and repeat steps 3 through 9 as for
BASE in section II.
DATA TABLE - Don’t forget to get the data from other members of your
triad and the class. Make sure you have data for all the buffers and all the
biological solutions.
GRAPHING THE DATA
1. Make a series of separate graphs using the information you have gathered on
the biological materials. Which is the independent variable? The dependent
variable?
2. Each graph should include the data for the solution with acid added and with
base added. The acid and base data you obtained for water should be
included in each graph for comparison. (total of 4 lines in each graph –
include a legend)
QUESTIONS –make sure you answer each part of the question
1. Describe the effect of adding ACID to each solution? Was this true for every
solution? Why did you think this happened the way it did?
2. Describe the effect of adding BASE to each solution? Was this true for every
solution? Why do you think this happened the way it did?
3. Compare the various graphs of each substance. Why was it of value to
include the plot of water in acid and water in base with every experiment?
4. Which of the materials tested today is the best buffer? Explain.
5. Which of the materials is the poorest buffer? Explain.
6. Read about the carbonate buffer system in humans. Explain how this system
helps maintain our stable pH.
*This lab is based on the Acids and Bases lab from Biology with Vernier.