Natural Indicators
How can natural substances indicate an acid or a base?
Student Learning Objectives:
 Students will be able to observe the color changes produced by natural and commercial
indicators in solutions with various pH values.
 Students will compare the results of these natural indicators with the results of using a
commercially available universal indicator.
 Students will observe the acid/base behavior of pH indicators.
 Students will use their data and observations to determine the color associated with each
indicator in its acid and base form and will approximate the pH at which the color
changes occur.
Affiliated Learning Standards and Science Practices-Next Generation Science Standards:
 ETS1.A: Defining and Delimiting an Engineering Problem
 PS3.D: Energy in Chemical Processes
 LS1.A: Structure and Function
Materials:
- Safety goggles
- Assorted fruits and flowers that can be used as pH indicator sources (list of these attached)
- Universal pH Indicator solution or pH test paper
- Litmus paper (optional)
- pH meter (optional)
- Dropper pipettes
- Plastic sheet protector or plastic wrap to use as reaction surface
- Mortars and pestles (optional)
- Hot plates (optional)
- Isopropyl (rubbing) alcohol
- Colored pencils, crayons, or markers
- Beakers, 100 or 150 ml
- Various household solutions to test such as:
*Skim milk
*Diluted lemonade
*Apple juice
*Ammonia solution
*Light-colored soda
*Vinegar solution
*Detergent solution
*Window washer fluid
Teacher Tips:
Students may choose their own plant source and extract their own indicator or you may do the
extraction of an indicator for them to investigate. There is a wide variety of plant sources, so it is
possible to allow students a great deal of freedom in choosing flowers and fruits to test. Extraction
of the indicator solutions can be easily carried out in roughly 15 minutes. Students can chop up
the plant source into small pieces (or grind using mortars and pestles) then soak the plant parts in
alcohol to extract the indicator. This could easily be done the day before the rest of the
investigation and the solution left to soak overnight.
Extraction methods:
 Extraction may be done using hot water, 95% denatured ethyl alcohol or 70% isopropyl
alcohol. There seems to be no difference obtained from using different solvents, but the
alcohols do have an odor so they should only be used in a well-ventilated room. Using
isopropyl alcohol allows the extraction to be done overnight, with students setting up the
extraction one day and performing the rest of the experiment the next day.
 Another option is herbal tea; e.g., Red Zinger (2 teabags in 100 ml hot water steeped for
15 minutes) also works great, giving red for pH 1–4, golden tan at pH 5–6, and green
above 7.
Plant options:
Natural Fruit and Vegetable Indicators
Natural Flower Indicators
Apple skin (Red)
Dahlias
Beets
Daylilies
Blueberries
Geraniums
Red cabbage
Hibiscus
Cherries
Hollyhocks
Cranberries
Hydrangeas
Red or purple grapes
Blue iris
Red onions
Morning glories
Peaches
Purple mums
Plums
Pansies
Radish skin
Peonies
Rhubarb skin
Petunias
Strawberries
Poppies
Tomato leaves
Red or pink roses
Turnip skin
Violets
Alternate student leading question options:
 What kind of investigation can we do to extract natural pH indicators and see their color
changes?
 How can we get the natural indicator substances out of the plant matter?
Name: ____________________________________________
Natural Indicators
How can natural substances indicate an acid or a base?
Background:
Red cabbage (Brassica oleracea) has dark reddish-purple leaves due to a
pigment called anthocyanin. This water-soluble pigment is also found in apple skin,
plums, poppies, cornflowers, and grapes. Because of the way this pigment reacts to
chemicals in the environment, the plant itself changes its color according to the pH
value of the soil. This explains the fact that the very same plant is known by different
colors in various regions throughout the world, such as Northern Europe, Northern
America and China.
In this lab, we will determine the pH of several common household chemicals
using both universal indicator and a natural indicator extracted from red cabbage
leaves. We will then use the pHs of these chemicals to compare the colors produced
by universal indicator with the colors produced by red cabbage indicator in order to
determine its effective range.
Prediction
Which of the household chemicals do you think will be the most acidic? Basic?
Procedure:
Design an experiment that allows us to to observe the color changes of our natural
pH indicators, and to compare these color changes to those of commercially
available pH indicators. Begin with a leading question, and follow up:
 What is the difference in colors for cabbage juice and universal indicator?
 How do the colors of cabbage juice and universal indicator var yed on pH
 What pH values do bases have?
All student procedures should be approved by the teacher before students begin
any lab work.
General procedure:
a. Obtain a reaction surface (plastic sheet protector or sheet of plastic wrap) and
place a sheet of white paper in or under this surface.
b. Use droppers to place several small drops of each solution to be tested on this
reaction surface, making sure these drops are far enough apart to avoid joining
together.
c. Use two samples of each solution being tested. Test one with the pH test
paper or universal indicator solution and the second with the natural indicator
students made.
d. Record the data and observations from this experiment in an appropriate data
table.
e. Determine approximate pH values from the color chart provided with the
commercial pH indicator (or by using the pH meter). Use this information to make
your own color chart for their extracted indicator.
Data table:
Questions:
1. a) Describe the acid/base behavior of their natural dye.
b) Identify the color of the acid and base form of the dyes.
c) What is the approximate pH at which the form of this dye changes from
acid to base?
2. a) Describe the pH’s of your household substances. In general, what types
of items were acids and what types were bases?
b) Within your results, what provided the most unexpected value?
3. Students may present their findings to their classmates and compare their
results with those gathered using a different plant indicator source.