NAME______________________________
DATE_______________
SR LAB AQUA 8
DRAFT DOCUMENT
ACID RAIN AND BUFFERING CAPACITY
BACKGROUND INFORMATION
The acidity of a solution can be expressed using the pH scale, which ranges from 0 to 14. As
shown below, solutions with a pH above 7 are basic, solutions with a pH below 7 are acidic, and
a neutral solution has a pH of 7.
Due to the interaction of water droplets with carbon dioxide gas (CO2) in the atmosphere, normal
precipitation is slightly acidic. Natural rainwater has an average pH value of 5.6. Therefore, rain
is not considered acidic until its pH drops below 5.6.
Acid rain is a significant problem in Maryland and poses a serious threat to aquatic organisms.
pH values in the eastern United States average below 4.4. Freshwater lakes are usually slightly
basic are usually slightly basic. pH values between 6.5 and 8.2 are optimal for most organisms.
In Maryland, streams with a pH below 5.0 typically have no fish, while streams with a pH higher
than 6.0 average more than 9,000 fish per mile. Streams with pH values between 5.0 and 6.0
average about 500 fish per mile.
Distribution of Acid Rain
(precipitation-weighted pH)
This means that “pure” rainwater in the eastern United States is acutely lethal to all native fish
species and is equivalent to murder. So, how is it that fish and other aquatic organisms survive in
Maryland waterways? This is the central issue in this lab – acid waters and the buffering
capacity of substrates.
The susceptibility of lakes to changes in pH will vary depending on buffering capacity. A buffer
is a substance, which can absorb acids or bases without changing its own pH. The buffering
capacity is controlled by the concentration of naturally occurring ions such as carbonates and
828
NAME______________________________
DATE_______________
SR LAB AQUA 8
DRAFT DOCUMENT
bicarbonates. As carbon dioxide gas dissolves in water droplets of unpolluted air, the following
reaction occurs:
carbon dioxide + water  carbonic acid
 H2CO3
CO2 + H2O 
Carbonic acid (H2CO3) is a weak acid that causes the rain from unpolluted air to be slightly
acidic. This source of “acid rain” is not usually considered to be a pollutant, since it is natural
and usually does not have a great effect on the pH of rainwater. Other compounds also
contribute to acid rain as indicated in the diagram below.
CO2 (carbon
(nitrogen dioxide) NO2
dioxide)
H 2 CO3
H NO2
H NO3
H2SO3
SO2
(sulfur dioxide)
Oxides of sulfur (SO2) and nitrogen (NO2) dissolve in water droplets to cause more serious
problems. For example, sulfur dioxide dissolves in water to produce sulfurous acid (H2SO3).
sulfur dioxide + water  sulfurous acid
 H2SO3
SO2 + H2O 
In areas with underlying limestone (mostly calcium carbonate, CaCO3), surface waters have high
concentrations of carbonate and bicarbonate ions. These ions are able to resist changes in pH and
serve as buffers. The pH of a well-buffered lake does not change dramatically following a storm
or snowmelt period because the acidity becomes neutralized by these ions.
Any buffer does have a limited capacity, however. In other words, as increasing amounts of acid
are continually added to aquatic ecosystems, the buffering capacity is consumed. As a result, the
pH will drop if more acid is added to the system.
In this lab, you will study the effect of sulfuric acid on the pH of different water types. One
approach to restore an acidic lake is to add lime (pulverized limestone) to the lake itself, to the
influent streams, or to the watershed land. This laboratory will simulate such buffering effects.
OBJECTIVES
When students have completed this lab, they will be able to:
1. Study the effect on pH of dissolving sulfuric acid (H2SO4) in various waters
2. Describe why some bodies of water are more vulnerable to acid rain than others
in order to identify various threats to water resources and their impact on those aquatic
environments.
829
NAME______________________________
DATE_______________
SR LAB AQUA 8
DRAFT DOCUMENT
MATERIALS (when using calculators)
LabPro interface
dilute H2SO4
TI graphing calculator
ring stand and utility clamp
DataMate program
Vernier pH sensor
wash bottle with distilled water
lake water
buffer solution
graphical analysis or graph paper
100 mL beaker
250 mL beaker
safety goggles
PROCEDURE (when using calculators)
Note: All glassware must be clean in this experiment
1. Obtain and wear goggles.
2. Before each use of the pH sensor, you need to rinse the tip of the sensor thoroughly with
distilled water. To do this, hold the pH sensor above a rinse beaker, and use the rinse
bottle to thoroughly rinse the sensor tip.
Note: Do not let the pH sensor dry out. Keep it in a 250 mL beaker with about 100 mL of
tap water when not in use. The tip of the sensor is made of glass – it is fragile. Handle
with care!
3. Plug the pH Sensor into Channel 1 of the LabPro interface. Use the link cable to connect
the TI Graphing Calculator to the interface. Firmly press in the cable ends.
4. Turn on the calculator and start the DATAMATE program. Press CLEAR to reset the
program.
5. Set up the calculator and interface for a pH System.
a. If the calculator displays PH in CH 1, proceed directly to Step 6. If it does not,
continue with this step to set up your sensor manually.
b. Select SETUP from the main screen.
c. Press ENTER to select CH 1.
d. Select PH from the SELECT SENSOR menu.
e. Select OK to return to the main screen.
6. Wash a 100 mL beaker with tap water and dry it with a paper towel. Put 50 mL of
distilled water into this clean beaker. Using a ring-stand and utility clamp, secure the pH
sensor in the beaker as shown below.
Experimental Set Up
830
NAME______________________________
DATE_______________
SR LAB AQUA 8
DRAFT DOCUMENT
7. Set up the data-collection mode.
a. Select SETUP from the main screen.
b. To select MODE, press
(the up arrow key) once and press ENTER .
c. Select EVENTS WITH ENTRY from the SELECT MODE menu.
d. Select OK to return to the main screen.
8. Select START to begin taking pH measurements. Monitor the pH readings displayed on
the calculator screen. When the readings are stable, press ENTER on the calculator.
9. The calculator will prompt you for the number of drops of acid you added to the beaker.
Type “0” and press ENTER .
10. Add 1 drop of acid to the beaker. Stir the solution thoroughly after addition. CAUTION:
Handle the sulfuric acid with care. It can cause painful burns if it comes in contact with
the skin. Avoid spilling it on your skin or clothing.
11. When the readings are stable, press ENTER on the calculator. Enter the total number of
drops of acid added to the water in the beaker and press ENTER .
12. Repeat Steps 10 – 11, adding 1 drop each time until a total of 10 drops has been added.
13. On the calculator, press STO to stop data collection. A graph will be displayed with pH
along the y-axis and the number of drops added along the x-axis. Move the flashing
cursor from point to point and record the pH values in the Data Table. When finished,
press ENTER to return to the main screen.
H2SO4 and Lake Water
14. Complete these steps:
a. Clean the pH electrode.
b. Wash and dry the 100 mL beaker.
c. Put 50 mL of “Lake Water” in the 100 mL beaker, lower the pH electrode into
this water, and then briefly swirl the water about the electrode.
d. Repeat Steps 8 – 13 using “Lake Water” in place of distilled water.
H2SO4 and Buffer Solution
15. Complete these steps:
a. Clean the pH electrode.
b. Wash and dry the 100 mL beaker.
c. Put 50 mL of “Buffer Solution” in the 100 mL beaker, lower the pH electrode into
this water, and then briefly swirl the water about the electrode.
d. Repeat Steps 8 – 13 using “Buffer Solution” in place of distilled water.
831
NAME______________________________
DATE_______________
SR LAB AQUA 8
DRAFT DOCUMENT
MATERIALS (when using computers)
Computer
LabPro interface
Logger Pro software
dilute H2SO4
Vernier pH sensor
ring stand and utility clamp
wash bottle with distilled water
lake water
buffer solution
graphical analysis or graph paper
100 mL beaker
250 mL beaker
safety goggles
PROCEDURE (when using computers)
Note: All glassware must be clean in this experiment
1. Obtain and wear goggles.
2. Before each use of the pH sensor, you need to rinse the tip of the sensor thoroughly with
distilled water. To do this, hold the pH sensor above a rinse beaker, and use the rinse
bottle to thoroughly rinse the sensor tip.
Note: Do not let the pH sensor dry out. Keep it in a 250 mL beaker with about 100 mL of
tap water when not in use. The tip of the sensor is made of glass – it is fragile. Handle
with care!
3. Plug the pH Sensor into Channel 1 of the LabPro interface.
4. Prepare the computer for data collection by opening the file “18b Acid Rain” from the
Biology with Computers folder of Logger Pro.
5. Wash a 100 mL beaker with tap water and dry it with a paper towel. Put 50 mL of
distilled water into this clean beaker. Using a ring-stand and utility clamp, secure the pH
sensor in the beaker as shown below.
Experimental Set Up
6. You are ready to begin the measurements. Start measuring the pH by clicking
Note that a new button
is available.
.
7. Click
to take a pH measurement. A text box will appear. You should enter the
number of drops of acid you add to the water in the beaker. Since you didn’t add any acid
yet, type “0” in the text box and press ENTER.
8. Add 1 drop of H2SO4 (sulfuric acid) to the water. Stir thoroughly, until the pH is stable.
CAUTION: Handle the sulfuric acid with care. It can cause painful burns if it comes
832
NAME______________________________
DATE_______________
SR LAB AQUA 8
DRAFT DOCUMENT
into contact with skin.
9. Click
. Enter the number of drops of acid added to the beaker. Type “1” in the
text box and press ENTER.
10. Repeat Steps 8 – 9, adding 1 drop at a time, until you have added 10 drops of acid.
11. Click
when all measurements have been made.
12. From the table window, record the pH values in the Data Table.
13. Store your data by choosing Store Latest Run from the Experiment menu.
H2SO4 and Lake Water
14. Complete these steps:
a. Clean the pH electrode.
b. Wash and dry the 100 mL beaker.
c. Put 50 mL of “Lake Water” in the 100 mL beaker, lower the pH electrode into
this water, and then briefly swirl the water about the electrode.
d. Repeat Steps 6 – 13 using “Lake Water” in place of distilled water.
H2SO4 and Buffer Solution
15. Complete these steps:
a. Clean the pH electrode.
b. Wash and dry the 100 mL beaker.
c. Put 50 mL of “Buffer Solution” in the 100 mL beaker, lower the pH electrode into
this water, and then briefly swirl the water about the electrode.
d. Repeat Steps 6 – 13 using “Buffer Solution” in place of distilled water.
833
NAME______________________________
DATE_______________
SR LAB AQUA 8
DRAFT DOCUMENT
DATA
Drops
pH of this water type
distilled
lake
buffer
0
1
2
3
4
5
6
7
8
9
10
pH
ANALYSIS
1. Calculate pH for the distilled, lake, and buffered water samples. Subtract the final pH
from the initial pH and take the absolute value. Record this information in the Data
Table.
2. Using Graphical Analysis, or by hand, make a plot of pH vs. drops of acid added. Place
all three graphs (distilled, lake, and buffered water) on the same plot.
3. List the solutions tested in order from most pH change to least pH change.
4. What happened to the pH of the buffer solution as more and more sulfuric acid was
834
NAME______________________________
DATE_______________
SR LAB AQUA 8
DRAFT DOCUMENT
added?
5. Based on your answer to question #4, develop a definition for a buffer.
6. Many aquatic life forms can only survive in water with a narrow range of pH values. In
which body of water – with or without a buffer – would living things be more threatened
by acid rain? Explain.
7. If you steadily add acid, what would be the affect on the pH of a well-buffered solution?
(Circle the correct answer)
A. It will increase immediately.
B. It will initially remain constant, and then begin decreasing.
C. It will decrease exponentially.
D. It will remain constant, and then begin increasing.
8. Acid rain may cause a greater change in lake acidity in one area because of differences in
(Circle the correct answer)
A. the type of bedrock and soil.
B. the size and shape of the lake.
C. the biodiversity of the lake.
D. the temperature of the lake.
835
NAME______________________________
DATE_______________
SR LAB AQUA 8
DRAFT DOCUMENT
9. Based on the information you discovered in this lab, suggest a way to protect vulnerable
lakes from the effects of acid rain.
APPLICATION
1. Analyze the data in the chart below.
Mirror Lake
Loon Lake
pH
7.5
6.0
Temperature
6 °C
4 °C
50 ppm
260 ppm
Geologic Substrate
Sandstone –
siltstone
Limestone shale
Dissolved Oxygen
13 ppm
7 ppm
Alkalinity
a. Which lake is more likely to experience large drops in pH due to acid precipitation?
Explain.
b. Alkalinity is a measure of the capacity of bases to neutralize acids. How does alkalinity
relate to buffering capacity?
836
NAME______________________________
DATE_______________
SR LAB AQUA 8
DRAFT DOCUMENT
2. Study the following chart. It gives the range of pH in which the named aquatic organisms
can survive. If there is an “x” present, then the organism survives in that pH range.
a. In the space below, construct a food web for a lake having a pH of 4.0.
b. In a short paragraph, explain how environmental pH limits the distribution of
some aquatic populations and why buffers are so important.
837