CABBAGE PATCH PRIVATE EYE

PROBLEM:

You work for a hazardous waste company. You just received a shipment of household products. Before disposing of the products, you must know if the products are acids or bases. Unfortunately for you, you are out of the universal indicator you normally use to check acidity and basicity. However, you noticed a head of red cabbage in your refrigerator. Could this be used to conduct your tests?

BACKGROUND INFORMATION:

Acids and bases were recognized as important and unusual substances even before our modern theories of chemistry were proposed in the 18 th and 19 th centuries. They were often identified by their ability to cause color changes in certain dyes, which are today called indicators. When testing the acidity level of substances with an indicator, color changes are observed due to changes in the molecular structure of the indicator.

Acid-base indicators are actually weak acids and bases themselves. They change colors because they accept or lose a proton when placed in the test solution. The form of the indicator that exists depends on the concentration of the H + ion present in the solution. The higher the H + ion concentration (or the lower the OH - ion concentration), the greater the tendency for color #2 to predominate. The lower the H + ion concentration (or the higher the OH - ion concentration), the greater the tendency for color #1 to predominate.

The pH scale is a measure of acidity and basicity. A pH of 7 indicates a neutral solution, a solution that exhibits neither the characteristic properties of an acid or a base (for example, pure water). Acidic solutions have pH values less than 7 and basic solutions have pH values greater than 7. By using different indicators that change color at different pH values, the pH of a solution can be determined. Testing with a single indicator generally only tells us that the pH is less than or greater than the pH at which the color change takes place.

In this activity you will extract an acid-base indicator from red cabbage to test various household substances. The indicators are pigments called anthocyanins. Red cabbage juice is an unusual indicator in that it has several different color changes. Actually, red cabbage juice is a mixture of several indicators. Each individual indicator in cabbage juice changes color at a specific pH value. Because the juice is a mixture of indicators, each showing a specific color at a specific pH, a range of colors is possible.

OBJECTIVE:

The student will extract the juice from red cabbage to use as a universal acid-base indicator. The student will then use the indicator to test household chemicals for acidity or basicity. pH will be determined by comparing the sample color to the colors of several solutions of known pH.

MATERIALS:

  • Red cabbage
  • Knife
  • One of the following:
  • Water
  • Strainer / Funnel
  • Large jar or beaker for cabbage juice
  • Materials (clear solutions work best) to test such as:
  • Eyedroppers
  • Large test tubes (150mm or 6 in. long)
  • Test tube rack
  • Meter pH (extension)
  • Universal indicator (extension)
  • Standard solutions of pH 1,3,5,7,9,11,13
  • One or more of the following materials:
    (Optional Activity)

    SAFETY AND DISPOSAL:

    Be careful when dealing with the low and high pH solutions as they may be corrosive or caustic.

    Dust pellets and concentrated solutions of sodium hydroxide are very caustic. They can cause severe chemical burns and destroy cell membranes. Contact with the skin and eyes must be prevented. Should contact occur, rinse the affected area with water for 15 minutes. If the contact involves the eyes, medical attention should be sought while the rinsing occurs.

    Dissolving sodium hydroxide in water is an exothermic (heat producing) process. The solutions can get very hot. Handle carefully. It is advisable to dissolve sodium hydroxide near a sink so water is readily available for rinsing in case sodium hydroxide comes in contact with your skin.

    Household ammonia or its vapors can damage the eyes. Eye protection is required. It is recommended that contact lenses not be worn when working in the lab as gaseous vapors may condense on the contact lens and cause damage to the eye. Use ammonia only in a well ventilated area. Should contact with eyes occur, rinse the affected area with water for 15 minutes. Medical attention should be sought while the rinsing occurs.

    The above precautions must be followed when preparing the dilute solutions used in this activity. Eye protection is required even when using dilute solutions. When diluting acids, ALWAYS ADD ACID TO WATER , and not the reverse. The heat given off in the dissolving process can cause splattering if the procedure is not carried out in the correct sequence.

    The sodium hydroxide, citric acid, and ammonia solutions used in the activity can be diluted with water and flushed down the drain. Unused solutions can be saved for further use.

    Dispose of the cabbage juice within a day or two to prevent spoilage. It will keep longer if frozen or stored in a refrigerator.

    TIME:

    Getting Ready: 25-30 minutes (with an additional 45 minutes to prepare stock solutions of known pH.)

    Doing the Activity: 5-20 minutes.

    PROCEDURE:

    Activity 1

    1. Preparation of Cabbage Indicator (Choose one method):
      1. Cut up a quarter head of red cabbage and put it into a blender or food processor. Just cover with water and blend the cabbage into a slurry (1-3 minutes). Pour the slurry through a strainer collecting the juice in a beaker. This method does not produce as strong an odor.
      2. Chop a half head of red cabbage and place it in a container for boiling. Add water to just cover the cabbage. Boil 15 minutes, then remove the cooked cabbage and discard it. Add the other half of the uncooked cabbage to the juice in the pan and boil it for an additional 15 minutes. Pour the cooked slurry through a strainer collecting the juice in a beaker.
    2. Preparation of standard solutions of known pH. Prepare solutions of the following pH values: 1,3,5,7,9,11,13. For the lower grades the teacher should prepare these standard solutions. The students could prepare the solutions in an upper level chemistry class. A 500-ml amount should be enough for several demonstrations throughout the day. This procedure will give the approximate required pH. For the exact pH check the solutions with a pH meter.
      1. For safety and precision use the following when preparing the standard solutions. Measure 400 ml of distilled water into a 500-ml beaker. Add the necessary amount of either solid sodium bicarbonate or solid citric acid monohydrate and stir to dissolve. Then add the necessary amount of 1M NaOH with gentle stirring. Finally add the necessary amount of distilled water to make a total of 500 ml of solution. This 500 ml stock solution can be divided into smaller amounts for several demonstrations

        Needed pH 1M NaOH Citric Acid Monohydrate
        1 0 ml 5.25 g
        3 12 ml 5.25 g
        5 48 ml 5.25 g
        7 Use distilled water  
          1M NaOH Sodium Bicarbonate NaHCO 3
        9 2.5 ml 2.1 g
        11 23 ml 2.1 g
        13 70 ml 2.1 g

      1. To be done by the teacher as a demonstration for younger students:
        1. Label clean test tubes 1,3,5,7,9,11,13.
        2. Fill each tube half full with the appropriate solution of known pH. Add one dropper full or approximately 1 ml of cabbage juice.
        3. Shake to mix. These solutions could be used as the control comparison for the remaining activities.
      2. To be done by the student in upper level classes:
        1. Label clean test tubes 1,3,5,7,9,11,13.
        2. Fill each tube half full with the appropriate solution of known pH. Add one dropper full or approximately 1 ml of cabbage juice and mix.
        3. Shake to mix. Compare your results with the chart in Appendix A or with the teacher's demonstration.
        4. To obtain the approximate pH of an unknown solution, take a test tube ½ full of the solutions you wish to test. Add one dropper full or approximately 1 ml of cabbage juice and mix. Compare the resultant color to those obtained for the solutions of known pH. Estimate the approximate pH of the substance. The color comparison should be made as soon as possible after mixing the test solution and the cabbage juice; the colors will change upon sitting and will deteriorate badly after about an hour.

    EXTENSIONS:

  • Have students obtain the actual pH using a pH meter and compare the results to their estimated pH from the cabbage juice.
  • Have students use universal indicator in addition to cabbage juice and compare results.
  • Have the students extract juice from other common plants and vegetables to use as a pH indicator. Examples of such plants and vegetables are listed in the materials section. To extract, add a few tablespoons of rubbing alcohol (70% isopropyl alcohol) over about 5 g of berry pulp or flower petals that have been pulverized using a mortar and pestle. Filter the juice through a paper towel or cheese cloth.

    QUESTIONS:

    1. You have a mixture of two indicators. Indicator A is red above pH 7. Indicator B is blue below pH of 10. What color will show at a pH of 9?



    2. Describe a real-life situation where a person would find use for red cabbage juice indicator?





    FOR ADDITIONAL INFORMATION CONTACT:

    Dr. Phil Crandall

    Anna McCain - Research Specialist II, Food Science U of A Fayetteville,
    amccain@comp.uark.edu

    REFERENCES:

    Hanshumaker, W. "A Head for Chemistry" Science and Children 1987, 25 (3), 24-46.

    Newell, S. B. Chemistry: An Introduction, 2 nd . ed.; Little Brown: Boston, MA, 1980.

    Mebane, R.C.; Rybolt, T. R. "Edible Acid-Base Indicators" J. Chem. Educ. 1985, 62, 285.

    Shakhashiri, B.Z. Chemical Demonstrations; University of Wisconsin Press: Madison, WI 1989; Vol. 3, pp. 50 - 57.
    Substance being tested Color of cabbage indicator Color of universal indicator Estimated

    pH

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