EXPERIMENTER’S CORNER
Acid-Base Indicators (15 points)
by Kathleen J. Dombrink and David O. Tanis

BACKGROUND:
Acid-base indicators are highly colored dyes that change colors when an acid or a base is added.  Various plants, such as red cabbage, contain certain dyes that can act as acid-base indicators. You will determine if some plant dyes are indicators by their color changes in acids and bases.


MATERIALS
White vinegar
household ammonia
small saucepan
measuring cup,
2 saucers or bowls or small glasses
1 tablespoon
1 teaspoon
red cabbage
varous household substances


PROCEDURE

PART 1: Preparation of the Indicator:

  1. Tear 2 or 3 large red cabbage leaves into small pieces; place them in  a small saucepan
  1. Add about 1/2 cup (about 125 mL) of water to the saucepan.
  1. Slowly boil the cabbage leaves until the water turns a dark color and the leaves almost lose their color completely. You may need to add  more water to keep the cabbage from burning during the boiling.
  2. Let the boiled cabbage cool. Then carefully pour off the liquid into a cup. What color is the cabbage juice? (Note: If the color is pale, you
    may concentrate the juice by boiling the juice to evaporate more water.) 
  3. Into a saucer, place a tablespoon (about 15 mL) of white vinegar (a solution of acetic acid, C2H3O2.)
  4. Into another saucer, place a tablespoon of household ammonia, a solution of ammonia gas in water:
    NH3 (g) + H2O (l) --> NH4 + (aq) + OH- (aq)
  5. To each solution in the saucers add about 1/2 teaspoon (about 2-3 mL) of the cabbage juice indicator. What is the color of the indicator
    in the vinegar? In the household ammonia?  (If your experiment is successful, you should have different color in each saucer)
  6. Try to find to another substance that acts as an acid-base indicator.  Extract possible indicators from flower petals, radish skins, or
    berries. Follow the same procedure as above. Heating may be unnecessary in some cases.
 PART 2: Using Indicators to Test Household Substances

  1. Choose at least 8 other household liquids to test for acids or bases.
  2. Set up a data table to record your results of testing your 10 substances (ammonia, vinegar, + 8 other items).
  3. Use your cabbage juice indicator to test your 8 other substances.  Record the results on your data table.
  4. Use your other extracted indicator to test 8 other substances.  Record the results on your data tale.

PART 3:  Using your homeade indicator to measure pH

  1. Bring a sample of both of your indicators into school during flex, before, or after school. 
  2. Have your instructor assist you in using the Vernier pH probes and determine the endpoint (pH range) of each color in both of your indicators.  Make a chart for each indicator showing the corrleation between pH and color.

ANALYSIS
Answer the following questions in your Lab Report and then turn in to your Instructor:

  1. Indicate on your data table which substances are acids, which are bases, and which you think are neutral.
  2. Did both of your indicators give you the same results for which substances were acids and bases? Explain.
  3. Use your data from part 4 to assign a pH to all of your household substances.