Experiment 14: Measuring Acidity with Red Cabbage Juice
Many common household chemicals can be described as acids or bases. For example, many cleaning
supplies are basic (sometimes called alkaline). Vinegar is a 5% solution of acetic acid (the remainder is
water). Shampoos and cosmetic products are usually pH balanced to a level which minimizes irritation of
the skin. In this experiment, you will test various substances and designate them as an acid, a base, or
neither (neutral).
Acids are substances that can donate a hydrogen ion in water; bases are substances that can accept a
hydrogen ion or produce hydroxide ion in water. The amount of hydrogen ion donated or accepted
determines the strength of the acid or base. The concentration of hydrogen ion in a solution is described by
its pH value. The pH of a solution is mathematically defined as
pH   log[ H  ]
where, [H+] is the concentration of the hydrogen ion (i.e. the amount of hydrogen ion per liter). The negative
logarithmic relationship means that every time the pH increases by 1, the hydrogen ion concentration
decreases by a factor of 10. A lower pH value
corresponds to a higher acidity of the solution. A Logarithms & Earthquakes
pH above 7 is basic, and pH below 7 is acidic. For
a neutral solution, the pH is 7. Many naturally The Richter Scale measures the relative strengths
occurring molecules are influenced by the level of of earthquakes. Like pH, the Richter Scale is a
the acidity around them. The hydrangea flower has logarithmic scale. For example, an earthquake
measuring 6.0 is ten times stronger than one that
a red color in acidic soil, but blue in basic soil. measures 5.0; similarly, an earthquake that
Molecules that change color in response to changes measures 5.0 is ten times stronger than one
in acidity level are called indicators.
Some which measures 4.0. The negative sign in the pH
indicators change color only at one particular pH, formula reverses the effect, so a solution with pH
whereas others exhibit numerous numbers of colors 4.0 is ten times less acidic than one with pH 3.0.
over a broad pH range. Red cabbage leaves contain Logarithmic relationships are more complex than
a compound that exhibits different colors at you might expect. For example, an earthquake
different pH values. In this experiment, you will measuring 4.5 is not five times stronger than one
measuring 4.0, as many people might assume (in
examine the color changes which accompany the fact it is only 3.16 times larger).
pH changes.
Procedure:
Note that your instructor may recommend that you work in pairs for some or all of this experiment.
A. Determine the indicator’s color at various pH values
1. Obtain about 5 large red cabbage leaves and tear them into small pieces. Place them in a 400mL beaker
and cover them with DI water.
2. Heat the cabbage water until the water boils. Boil it for 5 to 7 minutes, then turn off the burner.
3. Pour the cabbage juice into a 250-mL beaker and discard the leaves. Allow the juice to cool to room
temperature.
4. Obtain 12 clean test tubes and a test tube rack. Label the test tubes from 1 to 12 to correspond with the
pH values of the solutions you will be testing.
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5. Add about 2 mL of the cabbage juice to each of the test tubes. Then, add about 2mL of the standard
solution labeled pH=1 to test tube 1, and pH=2 to test tube 2, and so on. Think of a way to do this step
without measuring the volume of solutions repeatedly.
6. Record the colors of the solutions in the test tubes in the data table on the report sheet Save these test
tubes to use as standards for comparison to the household substances you will be testing in Part B.
B. Determining the pH values of household substances
1. Obtain 10 additional test tubes and label them to indicate which household substance is in each.
2. Add 2mL of cabbage juice to each of the 10 test tubes.
3. If the substance is a liquid, add 2ml of it to a test tube. If the substance is viscous (i.e. syrupy, like baby
oil), add only few drops to a test tube and use a stirring rod to swirl the substance in the test tube. If the
substance is a solid, add a small spatula tip of it (about the size of a pea) to the test tube containing the
cabbage juice.
4. Record the color of each household substance with cabbage juice on the report sheet. Compare these
solutions to the standard solutions from part A and estimate the pH based on the color.
5. Categorize your household substances as acids, bases, or neutral.
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Report Sheet
Name:___________________________
Measuring Acidity with Red Cabbage Juice
A. Determine Indicator’s Color at Various pH
pH
1
2
3
4
5
6
Color
pH
7
8
9
10
11
12
Color
B. pH of Household substances
substance
Color in Cabbage Juice
pH
Acid. Base or
Neutral?
Vinegar
Ammonia
Lemon Juice
Apple Juice
7-UP
Ivory Liquid
Detergent
Shampoo
Hair Conditioner
Antacid
Aspirin
Mouthwash
Windex
Wisk Cleaners
Ajax
Corn Syrup
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Questions
Suppose that you have two samples of vinegar, although each contains a different volume of it. Sample A
contains 100 mL, while Sample B contains 500 mL. Other than the difference in volume, the vinegar sample
are identical.
Is one sample “more acidic” than the other? If so, then identify the more acidic one. Give a brief
explanation of your answer.
Five drops of an indicator is added to each of the samples in the previous question, and both solutions turn
orange. The color in Sample B is fainter than that of Sample A, although the actual shade of the color is the
same. Explain these two observations.
Does the information given in the second question seem to be consistent with what you would expect from
your answer to the first question? Explain briefly.
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Materials for this experiment:
Please refer to chem 110 lab -We will use the same standard buffers and the same household good for this experiment.
Cabbage
We will not use pH paper nor pH meters.
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