Name
Date
Period
XBIO Lab: Testing pH of Water and Biological Materials
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 concentrations 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.
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 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 substances derived from organisms.
Could some substance present in biological materials account for the difference in
pH change from that of nonbiological materials? Frequently in biological
investigation 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 (a nonliving chemical solution) will be used as a model to obtain
data with which to answer these two questions.
Materials
 Lab apron
 Safety goggles
 50-mL beaker
 50-mL graduated cylinder
 0.1M HCl dropper bottle
 0.1M NaOH dropper bottle
Procedure
1. Put on goggles and a lab apron.
2. Pour 25 mL of tap water into a 50-mL beaker.
3. Record the initial pH of the water in Table 1.






Stirring rod
Forceps
pH paper
tap water
biological material _______________
sodium phosphate pH 7 buffer solution
4. ACID GROUP:
 Add 0.1M hydrochloric acid (HCl) a drop at a time, swirling to mix after each drop.
 Determine the pH after each series of 5 drops has been added until 30 drops have
been used.
 Record the pH measurements in the table after each 5 drops in Table 1.
BASE GROUP:
 Add 0.1M sodium hydroxide (NaOH) a drop at a time, swirling to mix after each
drop.
 Determine the pH after each series of 5 drops has been added until 30 drops have
been used.
 Record the pH measurements in the table after each 5 drops in Table 1.
CAUTION: 0.1M HCl and 0.1M NaOH are irritants and may destroy clothing. Avoid skin/eye
contact; do not ingest. Immediately flush spills and splashes with water & call teacher.
5. Rinse the beaker thoroughly and pour in another 25 mL of the biological material into the
beaker. Record the initial pH of the biological material in Table 1. Repeat step 4 (make
sure you use HCl if you are in the ACID GROUP and NaOH if you are in the BASE GROUP).
6. Rinse the beaker thoroughly and pour in 25 mL of the buffer solution. Record the initial
pH in Table 1. Repeat step 4.
7. If you are in the ACID GROUP share your answers with the BASE GROUP (and vice
versa).
8. Return your materials to the appropriate location and wash your hands thoroughly before
leaving the lab.
Name
Date
Period
XBIO Lab: Testing pH of Water and Biological Materials
Data
Table 1
Tests with 0.1M HCl
pH after addition of
Solution
Tested
0
5
10
15
20
25
Tests with 0.1M NaOH
pH after addition of
30
pH
∆
0
5
10
15
20
25
Tap Water
Buffer
Bio Mat
Analysis
1. Create ONE graph that includes… (make acid line straight and base line dashed)
 Water + acid
Blue
Legend:
 Water + base
 Buffer + acid
Red
 Buffer + base
 Biological material + acid
Green
 Biological material + base
30
pH
∆
2. Summarize your graph in words.
Conclusions
1. Write a hypothesis that the original experimenters were trying to test.
2. What is/are the control groups?
3. What is/are the experimental groups?
4. Summarize the effects of HCl on tap water.
Summarize the effects of NaOH on tap water.
5. How do biological materials respond to changes in pH?
6. With respect to changes in pH, is the response of the biological material more like that
of water or of the buffer? Justify your answer with data from the lab.
7. Would buffers help or hinder your body trying to maintain homeostasis if it was exposed
to a strong acid or base? Explain you answer.