pH and Carbon Dioxide Lab
OBJECTIVES
Participants will be able to:
1.
2.
3.
4.
5.
6.
Perform an instrument calibration
Name some acids and bases in everyday use
Draw the pH scale and explain what it represents
Explain how carbon dioxide can cause the pH of water to drop
Explain the role of carbon dioxide in the biosphere
Devise an experiment to test a simple hypothesis
PROCEDURE
General Guidelines:
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You will work in groups of 2 or 3.
Glassware may be used, washed well, rinsed with distilled water, and used again.
Take care not to cross-contaminate any of the materials. How might that be prevented?
The instructor will present minimal background information at this time. S/he will have materials
available, and will permit you to explore on your own in small working groups.
You will find a glossary of terms for this activity in the manual.
General Instructions for Using the Benchtop pH Meter
1. DO NOT PUSH ANY BUTTONS EXCEPT:
ON/OFF
CAL/MEAS
CON (or ENTER)
2. The bulb of the pH electrode is glass; handle with care.
3. Do not touch the bulb with your fingers nor attempt to dry it.
4. Keep the bulb of the electrode wet at all times. When not using it to measure pH, immerse it in
tap water in the soaking beaker.
5.
Calibrate the pH meter before taking any measurements. Instructions for calibrating a pH meter
are on the next two pages
introduction to tribal air quality
revised 6/27/14
Calibrating Cole-Parmer pH meters
1. Pour a small amount of pH 7 buffer into a CLEAN beaker. Label the beaker.
2. Pour a small amount of pH 4 buffer into a CLEAN beaker. Label the beaker.
3. Pour a small amount of pH 10 buffer into a CLEAN beaker. Label the beaker.
4. Rinse the pH electrode with distilled water as follows:
 Hold the electrode over a sink or dishpan and spray the bulb with distilled water from the
plastic wash bottle.
 DO NOT WIPE THE ELECTRODE DRY. Wiping causes static electricity and will
cause problems with the calibration.
5. Immerse the pH electrode 1 to 2 inches into the pH 7 buffer.
6. Press the ON/OFF button. All the LCD segments will display for a few seconds. The LCD
then will switch to the pH mode. In pH mode, a black box appears around pH on the display.
7. Press the CAL/MEAS button to enter the calibration mode. CAL will appear on the top of
your LCD screen to show that the meter is in calibration mode.
8. When the READY indicator displays in the left hand corner, press the CON button. The
calibration point will then be stored in the meter.
9. Do not turn off the meter. The display changes so the larger number corresponds to the
calibration buffer value (at or around 7.00) and the smaller display automatically changes to
the next buffer value (pH 4).
10. Do not turn off the meter. Rinse the bulb again with distilled water from the plastic wash
bottle. Immerse the electrode 1 to 2 inches into the pH 4 buffer and proceed as you did for pH
7.
11. Do not turn off the meter. When the smaller display automatically changes to the next buffer
value (pH 10), rinse the bulb again with distilled water from the plastic wash bottle. Immerse
the electrode 1 to 2 inches into the pH 10 buffer and proceed as you did for pH 7 and pH 4.
12. To end the calibration, press the CAL/MEAS button to return to measurement mode.
13. Rinse the bulb again and return the electrode to the soaking beaker.
14. Now you are ready to begin taking pH measurements.
Note: This calibration procedure is sequential. Stay in CAL mode throughout the calibration
sequence. Do not turn off the pH meter between steps. If you accidentally turn off the meter, start the
calibration again.
introduction to tribal air quality
revised 6/27/14
Calibrating Oakton pH meters
1. Pour a small amount of pH 7 buffer into a CLEAN beaker. Label the beaker.
2. Pour a small amount of pH 4 buffer into a CLEAN beaker. Label the beaker.
3. Pour a small amount of pH 10 buffer into a CLEAN beaker. Label the beaker.
4. Rinse the pH electrode with distilled water as follows:
 Hold the electrode over a sink or dishpan and spray the bulb with distilled water from the
plastic wash bottle.
 DO NOT WIPE THE ELECTRODE DRY. Wiping causes static electricity and will
cause problems with the calibration.
5. Immerse the pH electrode 1 to 2 inches into the pH 7 buffer.
6. Press the ON/OFF button. All the LCD segments will display for a few seconds. The LCD
then will switch to the pH mode. In pH mode, a black box appears around pH on the display.
7. Press the CAL/MEAS button to enter the calibration mode. CAL will appear on the top of
your LCD screen to show that the meter is in calibration mode.
8. When the READY indicator displays in the left hand corner, press the ENTER button. The
upper display flashes the calibration value momentarily. The calibration point will then be
stored in the meter.
9. Do not turn off the meter. The display changes so the larger number corresponds to the
calibration buffer value (at or around 7.00) and the smaller display automatically changes to
the next buffer value (pH 4).
10. Do not turn off the meter. Rinse the bulb again with distilled water from the plastic wash
bottle. Immerse the electrode 1 to 2 inches into the pH 4 buffer and proceed as you did for pH
7.
11. Do not turn off the meter. When the smaller display automatically changes to the next buffer
value (pH 10), rinse the bulb again with distilled water from the plastic wash bottle. Immerse
the electrode 1 to 2 inches into the pH 10 buffer and proceed as you did for pH 7 and pH 4.
12. To end the calibration, press the CAL/MEAS button to return to measurement mode.
13. Rinse the bulb again and return the electrode to the soaking beaker.
14. Now you are ready to begin taking pH measurements.
Note: This calibration procedure is sequential. Stay in CAL mode throughout the calibration
sequence. Do not turn off the pH meter between steps. If you accidentally turn off the meter, start the
calibration again.
introduction to tribal air quality
revised 6/27/14
Part I. Using a pH meter to measure the pH of household substances
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Use the Worksheet for Part I to keep a record of substances tested and the pH for each.
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Choose 4 or 5 substances from those available for testing.
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Pour about 30 milliliters of a liquid substance into a clean beaker and label the beaker with the
name of that substance.
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If you choose a powdered substance (baking soda or powdered milk), use a glass stirring rod
to mix ½ teaspoon into 30 milliliters of distilled water.
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To measure pH, follow this sequence of steps:
1. Rinse the pH electrode with distilled water as you did when calibrating the pH meter.
2. Immerse the electrode ½ to 2 inches into the solution to be tested.
3. Press the ON/OFF button.
4. Press the CAL/MEAS button.
5. The display should say MEAS and pH.
6. When READY appears on the display, record the pH value.
7. Turn the meter OFF.
8. Rinse the bulb again and return the electrode to the soaking beaker.
introduction to tribal air quality
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Part II. Investigating the effect of carbon dioxide on pH
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Use the Worksheet for Part II. Keep a careful record of what you do and the results you see.
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Your group will design and run an experiment using distilled water and your own
breath. Your task will be to test this hypothesis: exhaled breath can change the pH
of distilled water.
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Write the hypothesis on the Worksheet for Part II.
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Before doing anything else, plan your experiment. When your group agrees on the plan, write
down the steps of your plan in the Methods section of the Worksheet for Part II. Now you are
ready to carry out the experiment and record the Results.
Part III. Investigating the effect of a buffer (using an acid)
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Use the Worksheet for Part III. Keep a careful record of what you do and the results you see.
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Perform an experiment to test this hypothesis: a buffer will resist pH change.
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There are several ways that a person could test this hypothesis. Three possible experiments
are described below. Before doing anything else, study Experiments A-C outlined below and
CHOOSE ONE. Also choose the acid you want to use in your experiment.
NOTE:
It is critical to include a distilled water control (distilled water with no buffer added) as part of the
experimental design. Why is this important?
Experiment A
1. Use two beakers; label one “DW” and label the other “pH 7 buffer.”
2. Into the DW beaker pour 50 ml of distilled water and into the other beaker pour 50 ml of pH 7
buffer.
3. Measure the pH of the distilled water. Then add 5 drops of an acid and measure the pH again.
Add 5 more drops of the same acid and measure the pH for the third time.
4. Measure the pH of the buffer. Then add 5 drops of the same acid and measure the pH again. Add
5 more drops of the same acid and measure the pH for the third time.
5. A diagram of this procedure follows.
distilled water (DW)
DW + 5 drops acid
DW + 10 drops acid
-----> measure pH
-----> measure pH
-----> measure pH
pH 7 buffer
pH 7 buffer + 5 drops acid
pH 7 buffer + 10 drops acid
-----> measure pH
-----> measure pH
-----> measure pH
introduction to tribal air quality
revised 6/27/14
Experiment B
1. Use two beakers; label one “DW” and label the other “DW + buffer.”
2. Into the DW beaker pour 50 ml of distilled water and into the other beaker pour 40 ml of distilled
water and 10 ml of pH 7 buffer.
3. Measure the pH of the distilled water. Then add 5 drops of an acid and measure the pH again.
Add 5 more drops of the same acid and measure the pH for the third time.
4. Measure the pH of the DW + buffer. Then add 5 drops of the same acid and measure the pH
again. Add 5 more drops of the same acid and measure the pH for the third time.
5. A diagram of this procedure follows.
distilled water (DW)
DW + 5 drops acid
DW + 10 drops acid
-----> measure pH
-----> measure pH
-----> measure pH
DW + pH 7 buffer
DW + pH 7 buffer + 5 drops acid
DW + pH 7 buffer + 10 drops acid
-----> measure pH
-----> measure pH
-----> measure pH
Experiment C
1. Use two beakers; label one “DW” and label the other “DW + buffer.”
2. Into the DW beaker pour 50 ml of distilled water and into the other beaker pour 40 ml of distilled
water and 10 ml of pH 7 buffer.
3. Measure the pH of the distilled water. Keeping the pH electrode in the water, add an acid one
drop at a time, recording the pH after each drop. How many drops are required in order to see a
change in pH?
4. Measure the pH of the DW + buffer. Keeping the pH electrode in the solution, add the same acid
one drop at a time, recording pH after each drop. How many drops are required in order to see a
change in pH?
distilled water (DW)
add acid one drop at a time
-----> measure pH
-----> measure pH after each drop
DW + pH 7 buffer
add acid one drop at a time
-----> measure pH
-----> measure pH after each drop
Part III. Investigating the effect of a buffer (using a base)
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Use the second Worksheet for Part III. Again, keep a careful record of what you do and the
results you see.
Repeat your experiment but this time use a BASE instead of an acid.
introduction to tribal air quality
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Worksheet for Part I
Name
________________
Using a pH meter to measure the pH of household substances
Item Tested
introduction to tribal air quality
pH (from pH meter)
Acid or Base?
revised 6/27/14
Worksheet for Part II
Name
________________
Investigating the effect of carbon dioxide on pH
Hypothesis to be tested:
Methods:
(List the materials used; list the steps in your experiment.)
Results:
(As you perform your experiment, record the pH numbers here.)
Discussion:
(What do the results mean? What can you conclude from the experiment?)
introduction to tribal air quality
revised 6/27/14
Worksheet for Part III
Name ________________________
Investigating the effect of a buffer (using an acid)
Hypothesis being tested:
Experiment chosen (A, B, or C):
Results:
(As you perform your experiment, record the pH numbers here.)
Discussion:
(What do the results mean? What can you conclude from the experiment?)
introduction to tribal air quality
revised 6/27/14
Worksheet for Part III
Name ________________________
Investigating the effect of a buffer (using a base)
Hypothesis being tested:
Experiment chosen (A, B, or C):
Results:
(As you perform your experiment, record the pH numbers here.)
Discussion:
(What do the results mean? What can you conclude from the experiment?)
introduction to tribal air quality
revised 6/27/14
Glossary of Terms for pH and Carbon Dioxide Lab
Acid: any substance that releases hydrogen ions in a solution, making the solution more acidic.
Acid precipitation: rain or snow that has a pH below 5.6.
Base (alkali): any substance that absorbs hydrogen ions or releases hydroxide ions in a solution,
making the solution more basic (alkaline).
Buffer: a substance that tends to stabilize the pH of a solution.
Carbon dioxide (CO2): odorless, colorless, tasteless gas that is a by-product of combustion and of
cellular respiration; also a “greenhouse” gas.
Carbonic acid: a weak acid formed when carbon dioxide dissolves in water.
Hypothesis: a conjecture about some phenomenon; provides the framework for a scientific
investigation; can be tested by means of an experimental design.
Neutral: having a pH of 7.0, neither acidic nor basic.
pH (household definition): a measure of how acidic or basic (alkaline) a substance is.
pH (technical definition): a measure of the hydrogen ion (H+) concentration of a solution.
introduction to tribal air quality
revised 6/27/14
pH and Carbon Dioxide
QUESTIONS FOR DISCUSSION
1. Why is it important to calibrate the pH meter before taking any measurements?
2. What have you learned about the pH of everyday substances such as vinegar and baking soda?
3. How did you test the hypothesis that exhaled breath will change the pH of water?
4. What change occurred when you exhaled into water?
5. What is in exhaled breath that could change the pH of water?
6. What else do you know about pH?
7. What else do you know about carbon dioxide?
8. How does carbon dioxide fit into the big picture of life on our planet?
9. Why are high elevation lakes in granite rock formations particularly sensitive to the effects of
acid precipitation?
introduction to tribal air quality
revised 6/27/14
introduction to tribal air quality
revised 6/27/14
Background Information about pH
Water exists predominantly as an H2O molecule, but a small percentage of any body of water exists
in a dissociated form. This means that the water molecule breaks into a hydrogen ion (H+) and a
hydroxide ion (OH-). An ion is an atom or molecule with an electric charge.
H2O —————————> H+
+ OH-
In pure water, the concentration of hydrogen ions is exactly equal to the concentration of hydroxide
ions. However, when other substances dissolve in water, they may release hydrogen or hydroxide
ions, and the proportions of these ions in the resulting solution might not be equal.
Scientists use pH as a measure of the hydrogen ion concentration in a solution; pH is measured on a
scale of 0 to 14. The scale is exponential. This means that a pH change of one unit reflects a 10-fold
change in hydrogen ion concentration.
A solution in which the concentration of hydrogen ions and hydroxide ions is exactly equal has a pH
of 7 and is said to be neutral.
A solution that has more hydrogen ions than hydroxide ions will have a pH less than 7 and is said to
be acidic.
A solution that has more hydroxide ions than hydrogen ions will have a pH greater than 7 and is
said to be alkaline or basic.
An acid is any substance that releases hydrogen ions in a solution, making the solution more acidic.
For example, hydrochloric acid (HCl) dissociates in water as follows :
HCl ————————> H+
+ Cl-
When hydrochloric acid is dissolved in water, there will be an excess of hydrogen ions compared to
hydroxide ions in the solution, and the pH will go down.
An alkali or base is any substance that releases hydroxide ions in a solution, making the solution
more alkaline or basic. For example, sodium hydroxide (NaOH) dissociates in water as follows :
NaOH ————————> Na+ + OH-
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If sodium hydroxide is dissolved in water, there will be an excess of hydroxide ions compared to
hydrogen ions in the solution, and the pH will go up.
Buffers are molecules that can take up and release excess hydrogen or hydroxide ions in a solution,
stabilizing the pH. If a solution contains a buffer and acid is added, the buffer will absorb excess
hydrogen ions and they will not affect the pH of that solution. Likewise, if a base is added, the buffer
will absorb the excess hydroxide ions and, again, the pH will not change.
Buffers have their limits. If we continue to add acid to a buffered solution, we will exceed the ability
of the buffer to absorb hydrogen ions and the pH will drop quickly. Similarly, if we continue to add
alkali to a buffered solution, we will exceed the ability of the buffer to absorb hydroxide ions, and the
pH will rise quickly.
The pH of blood is closely regulated by buffers in the bloodstream. The pH of blood must remain
constant for all the biological processes in the body to be conducted properly.
Similarly, the pH of soil and water helps to determine what organisms can live and reproduce there.
In areas where there are natural limestone deposits, soil and water are well buffered. Such areas will
not be as susceptible to damage from acid precipitation as areas that lack natural buffering.
Background Information about Carbon Dioxide
Carbon dioxide (CO2) is an odorless, colorless, tasteless gas. It is a by-product of combustion
(burning) and of cellular respiration in plants, animals, fungi, and microorganisms.
During combustion, oxygen from the air rapidly combines with the burning material and energy is
released.
During cellular respiration, living cells use oxygen to help them break down sugar and release energy
for life’s activities. The overall equation for cellular respiration is written as:
C6H12O6 + 6 O2 ——————> 6 CO2 + 6 H2O + energy
sugar + oxygen ——————> carbon dioxide + water + energy
The carbon dioxide released from cells during their respiration can be used by plants in
photosynthesis. This is a process whereby green plants use raw materials from their environment
and energy from the sun to make sugar. The raw materials are carbon dioxide (from the air), and
water (from damp soil or groundwater).
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The overall equation for photosynthesis is written as:
sunlight + 6 CO2 + 6 H2O ——————> C6H12O6 + 6 O2
sunlight + carbon dioxide + water ——————> sugar + oxygen
Notice that oxygen (also an odorless, colorless, tasteless gas) is a by-product of photosynthesis, just
as carbon dioxide was a by-product of cellular respiration. Thus, one process is the reverse of the
other and the gaseous by-products are recycled. Nature seldom wastes anything!
When carbon dioxide (from the air or from aquatic organisms) dissolves in water, it forms a weak
acid called carbonic acid. The equation for this chemical reaction is written as:
CO2 + H2O ——————> H2CO3
carbon dioxide + water ——————> carbonic acid
The carbonic acid will release some hydrogen ions into the water. So, if the water is not buffered, the
addition of carbon dioxide will cause the pH to decrease. This happens when carbon dioxide in the
air dissolves in rainwater, giving normal rain a pH of 5.6 (slightly acidic).
Certain pollutants in our air rise high into the atmosphere where they react with water vapor to form
additional acids such as nitric acid and sulfuric acid. This decreases the pH of rain below 5.6,
causing acid rain. If acid rain (or acid snow) falls on an area where there is little natural buffering,
lakes, streams, forests, and cropland can be severely damaged. Such acid precipitation has even
damaged buildings and statues in cities.
The concentration of carbon dioxide in Earth’s atmosphere is about 0.03 percent by volume.
Although it is not found in great quantities, carbon dioxide plays a vital role in making our planet
livable. It is an important greenhouse gas. This means that it acts like the glass in a greenhouse,
allowing sunlight to reach the surface of our planet, but holding in the heat that might otherwise
escape from the planet to outer space. Without carbon dioxide in the atmosphere, the average
temperature on Earth would be below the freezing point of water.
Since carbon dioxide is released during combustion, and humans have been burning great quantities
of fossil fuels (coal, oil, natural gas) since the Industrial Revolution began, the concentration of
carbon dioxide in our atmosphere is increasing. Scientific studies show that this is enhancing the
natural greenhouse effect and causing the average temperature on Earth to increase. An increase of
even a few degrees will cause major changes in climate. This, in turn, will impact sea level, river
flows, agriculture, forests, endangered species, the spread of disease organisms, human population
distribution, national economies, and political boundaries.
introduction to tribal air quality
revised 6/27/14