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pH Lab
Name: _____________________
Biology I
period and date: __________________
Purpose: To determine acidity or basicity in solutions and to determine the approximate pH of each.
Introduction: pH can be defined as how acidic or basic a substance is. Boiled red cabbage extract can be used as an indicator
to determine if solutions are acidic or basic. In the presence of an acid, the extract will change to a bright pink
color. In a base, the extract becomes yellow and/or green. This type of indicator gives what is called a
qualitative measurement—the quality of or what’s in the substance tested. To get numerical data, pH paper or a
pH meter must be used. These methods yield what is termed quantitative measurement—the actual value such
as pH, length, concentration, etc. In this lab, we will do both qualitative (cabbage extract) and quantitative
measurement (pH paper).
Materials:
Red cabbage extract, pH paper, test tube rack, numbered test tubes containing small amount of liquid
substances (0.1 M HCl, 0.1 M NaOH, ammonia, alcohol, vinegar, lemon juice, water), plastic pipet, waste
container for used pH paper, goggles
(WARNING: Liquids in this lab are hazardous—Wear protective eyewear at ALL TIMES!!!)
Liquid # key:
1. HCl
2. NaOH 3. Ammonia
4. alcohol 5. vinegar 6. lemon juice
7. water (control)
Procedure: Part A
1. Obtain tray with materials.
2. Dip the end of a single strip of pH paper into the liquid in test tube #1.
3. Compare the color of the paper to the color key on the side of the pH paper vial.
4. Record the pH in Part A Data Table (below).
5. Place used pH paper into waste container.
6. Repeat steps 1-5 for test tubes 2-7.
Part A Data Table:
Tube # & substance
Color of pH paper
pH
1. HCl
2. NaOH
3. Ammonia
4. Alcohol
5. Vinegar
6. Lemon juice
7. water (control)
Procedure: Part B
1. Using your pipet, add 10-20 drops of red cabbage extract to tube #1.
2. Record the color change (if any) in Part B Data Table (on next page). Based on the color change, determine
if the liquid is an acid or a base. (See introduction above)
3. Repeat steps 1-2 for tubes 2-7.
Part B Data Table
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Tube # & substance
Color of solution after
adding cabbage extract
Acidic or
basic???
1. HCl
2. NaOH
3. Ammonia
4. Alcohol
5. Vinegar
6. Lemon juice
7. water (control)
Questions:
1. What is the range of the pH scale. What values indicate acid? base? neutral?
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2.
What is the difference between qualitative and quantitative data? Which method did we use for each in this lab?
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3.
How can pH testing be used in the real-world?
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4.
What is meant by the term “indicator?”
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5.
Which liquids did you find to be acidic? Basic? Neutral? Were any of them strong acids or bases? Which ones?
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6.
Did you notice any variation in the color of the Universal indicator after adding it to the different liquids? Examples:
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7.
Why does water make a good control for this experiment?
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8. HCl (hydrochloric acid) is used in your stomach to break down food. What do you think the pH of your stomach is?
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9. Urine contains ammonia compounds. Would you expect it to be acidic or basic? Why?
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