Ph and Carbon Dioxide Lab

pH and Carbon Dioxide Lab

OBJECTIVES

Participants will be able to:

1. Perform an instrument calibration

2. Name some acids and bases in everyday use

3. Draw the pH scale and explain what it represents

4. Explain how carbon dioxide can cause the pH of water to drop

5. Explain the role of carbon dioxide in the biosphere

6. Devise an experiment to test a simple hypothesis

PROCEDURE

General Guidelines:

· 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

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.

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.

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.

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.

Part I. Using a pH meter to measure the pH of household substances

· Use the Worksheet for Part I to keep a record of substances tested and the pH for each.

· Choose 4 or 5 substances from those available for testing.

· Pour about 30 milliliters of a liquid substance into a clean beaker and label the beaker with the name of that substance.

· 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.

· 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.

Part II. Investigating the effect of carbon dioxide on pH

· Use the Worksheet for Part II. Keep a careful record of what you do and the results you see.

· 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.

· Write the hypothesis on the Worksheet for Part II.

· 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)

· Use the Worksheet for Part III. Keep a careful record of what you do and the results you see.

· Perform an experiment to test this hypothesis: a buffer will resist pH change.

· 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) -----> measure pH

DW + 5 drops acid -----> measure pH

DW + 10 drops acid -----> measure pH

pH 7 buffer -----> measure pH

pH 7 buffer + 5 drops acid -----> measure pH

pH 7 buffer + 10 drops acid -----> measure pH

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) -----> measure pH

DW + 5 drops acid -----> measure pH

DW + 10 drops acid -----> measure pH

DW + pH 7 buffer -----> measure pH

DW + pH 7 buffer + 5 drops acid -----> measure pH

DW + pH 7 buffer + 10 drops acid -----> 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) -----> measure pH

add acid one drop at a time -----> measure pH after each drop

DW + pH 7 buffer -----> measure pH

add acid one drop at a time -----> measure pH after each drop

Part III. Investigating the effect of a buffer (using a base)

· 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.

Worksheet for Part I Name ________________

Using a pH meter to measure the pH of household substances

Item Tested pH (from pH meter) Acid or Base?

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?)

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?)

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?)

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.