Natural pH Indicators
Subject Area: Chemistry
Grade Level: High School Chemistry
Lesson Title: Natural pH Indicators
National Science Education Standards:
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Science as Inquiry: 9–12
Structure and Properties of Matter: 9–12
Chemical Reactions: 9–12
Physical Science: 9–12
Suggested Prior Knowledge: concepts of acids and bases, pH
Purpose: To understand pH indicators, pH, and properties of acids and bases
Key Vocabulary:
acid—substance that has a pH less than 7, and produces hydrogen ions in solution (it
might be helpful to connect to a lesson on water that would help students visualize
where hydrogen and hydroxide ions come from).
base—substance that has a pH greater than 7; produces hydroxide ions in solution
extract—to draw out or to separate a substance from a mixture
indicator—compound that changes color in different conditions; in this case, we are
looking at pH indicators which are compounds that give different colors in solutions
of different pH. We will use In− to represent the Indicator ion
pH—measure of the acidity of a substance; a pH of less than 7 is acidic (lower
number = more acidic) and a pH of greater than 7 is basic, in general the scale goes
from 1 (very acidic) to 14 (very basic). pH = −log[H3O+]
strong acid/base—an acid or base that completely dissociates into ions (H+ or OH−
and other ions) in solution
weak acid/base—an acid or base that only partially ionizes in solution
Objectives:
1. Students will be able to extract natural indicators from a plant source.
2. Students will be able to observe the color changes produced by natural and
commercial indicators in solutions with various pH values.
3. Students will compare the results of these natural indicators with the results of
using a commercially available universal indicator.
4. Students will observe the acid/base behavior of pH indicators.
5. 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.
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
 Apple juice
 Light-colored soda
 Detergent solution
 Ammonia solution
 Vinegar solution
 Diluted orange juice
 Diluted lemonade
Procedure:
1. Discuss with students the concept of pH indicators that give different colors in
different pH conditions. Review briefly pH, acids and bases, and pH indicators. A
demonstration of how litmus paper changes color in acidic and basic solutions is a good
place to start this discussion. Introduce the idea that many naturally occurring substances
behave the same way. These natural pH indicators are themselves weak acids and bases.
They are dyes that change color depending upon whether they donate or accept protons.
In a sufficiently acidic solution, these indicators accept a proton to take the acid form
(HIn in general) and in a sufficiently basic solution, they donate a proton to take the basic
form (In− in general). Different pH indicators have different acid strengths and will
donate or lose a proton at a specific pH. Litmus paper is red in its acid form and blue in
its basic form.
HIn  In− + H+
(red)
(blue)
2. Discuss with students how we can design an experiment that allows us to create our
own natural pH indicators, 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:
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What is the difference between an acid and a base?
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What pH values do acids have?
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What pH values do bases have?
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What kind of investigation can we do to extract natural pH indicators and see
their color changes?
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How can we get the natural indicator substances out of the plant matter?
3. Work with students to design a valid investigation similar to the one outlined in this
lesson that will allow them to extract a natural dye and observe its color in solutions of
different pH. Comparison to the results given with the universal pH indicator solutions or
pH paper will allow them to determine approximate pH values for the solutions. Use of a
pH meter is optional, but will give more precise pH values for the solutions. Students
should be sure their procedures are detailed and they should create appropriate data tables
for their data and observations. All student procedures should be approved by the teacher
before students begin any lab work.
4. Lab safety equipment and protocols should be followed. Goggles should be worn at all
times. Be especially careful if you are extracting the indicators with alcohol. The room
should be well ventilated to avoid buildup of fumes, and ensure that there are no open
flames in the laboratory. Students should use care if they are using hot plates.
5. 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.
6. 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 wellventilated 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.
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Plant options:
Natural Fruit and Vegetable
Indicators
Apple skin (Red)
Beets
Blueberries
Red cabbage
Cherries
Cranberries
Red or purple grapes
Red onions
Peaches
Plums
Radish skin
Rhubarb skin
Strawberries
Tomato leaves
Turnip skin
Natural Flower Indicators
Dahlias
Daylilies
Geraniums
Hibiscus
Hollyhocks
Hydrangeas
Blue iris
Morning glories
Purple mums
Pansies
Peonies
Petunias
Poppies
Red or pink roses
Violets
7. 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. Students can 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. Have the students record the data and observations from this experiment in an
appropriate data table.
e. Students can determine approximate pH values from the color chart provided with the
commercial pH indicator (or by using the pH meter) and use this information to make
their own color chart for their extracted indicator.
8. After the students have finished their investigations, they should be able to describe the
acid/base behavior of their natural dye. They should be able to identify the color of the
acid and base form of their dyes and they should know the approximate pH at which the
form of this dye changes from acid to base.
9. Students may present their findings to their classmates and compare their results with
those gathered using a different plant source.
Additional Resources:
http://www.flinnsci.com/Sections/Safety/safety.asp
http://chemistry.about.com/od/acidsbases/Acids_Bases_and_pH.htm
http://www.erowid.org/archive/rhodium/chemistry/equipment/ph-indicator.html
http://www.ehow.com/list_5931608_flowers-natural-indicators.html
http://wow.osu.edu/experiments/chemistry/pH.html
Student Worksheet for Natural pH Indicators
Experiment Title: _____________________________Date: _______Name: __________
Plant source: _____________________________________________________________
Student Hypothesis: _______________________________________________________
Materials: _______________________________________________________________
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Procedure:
Wear safety goggles for all lab work.
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Data and Observations:
Analysis of Data: _________________________________________________________
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Conclusion: _____________________________________________________________
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