Acid-Base Indicators (15 points)
by Kathleen J. Dombrink and David O. Tanis
|Acid-base indicators are highly colored dyes
change colors when an acid or a base is added. Various plants,
as red cabbage, contain certain dyes that can act as acid-base
You will determine if some plant dyes are indicators by their color
in acids and bases.
2 saucers or bowls or small glasses
varous household substances
PART 1: Preparation of the Indicator:
- Tear 2 or 3 large red cabbage leaves into small pieces;
place them in
a small saucepan
- Add about 1/2 cup (about 125 mL) of water to the saucepan.
PART 2: Using Indicators to Test Household Substances
- Slowly boil the cabbage leaves until the water turns a dark
color and the leaves almost lose their color completely. You may need
more water to keep the cabbage from burning during the boiling.
- Let the boiled cabbage cool. Then carefully pour off the
into a cup. What color is the cabbage juice? (Note: If the color is
may concentrate the juice by boiling the juice to evaporate more
- Into a saucer, place a tablespoon (about 15 mL) of white
vinegar (a solution of acetic acid, C2H3O2.)
- Into another saucer, place a tablespoon of household
ammonia, a solution of ammonia gas in water:
NH3 (g) + H2O (l) --> NH4 + (aq) + OH- (aq)
- To each solution in the saucers add about 1/2 teaspoon
2-3 mL) of the cabbage juice indicator. What is the color of the
in the vinegar? In the household ammonia? (If your experiment is
successful, you should have different color in each saucer)
- Try to find to another substance that acts as an acid-base
indicator. Extract possible indicators from flower petals, radish
berries. Follow the same procedure as above. Heating may be unnecessary
in some cases.
- Choose at least 8 other household liquids to test for acids
- Set up a data table to record your results of testing your
10 substances (ammonia, vinegar, + 8 other items).
- Use your cabbage juice indicator to test your 8 other
substances. Record the results on your data table.
- 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
- Bring a sample of both of your indicators into school
flex, before, or after school.
- Have your instructor assist you in
the Vernier pH probes and determine the endpoint (pH range) of each
color in both of
indicators. Make a chart for each indicator showing the
corrleation between pH and color.
|Answer the following questions in your Lab
Report and then turn in to your Instructor:
- Indicate on your data table which substances are acids,
which are bases, and which you think are neutral.
- Did both of your indicators give you the same results for
which substances were acids and bases? Explain.
- Use your data from part 4 to assign a pH to all of your