Anthocyanins as Acid/Base Indicators: Applications and
Activities for High School Students
Brett Hale
Heath Dawley
Southeast Missouri State University
BioQUEST 2008
Fruits_and_vegetables2. (Picture). URL:
http://www.firodiyaheartfoundation.org/images/fruits_and_vegetables2.jpg
USING PLANTS IN HIGH
SCHOOL CHEMISTRY
*Wanted to use anthocyanins as acid/base indicators
*Were familiar with the famous red cabbage acid/base
indicator.
*Decided to try out other anthocyanins as acid/base
indicators, having students explore the uses of chemicals
derived from plants.
*Designed 4 activities to help students explore this area
PRE-ACTIVITY: CHEMISTRY
AND
MATH
*For later activities reference solutions at the unit pHs from 1-14 are needed.
*Though the teacher can pre-make these, it is also a good activity to have
students do.
*They will take 1.00 M HCl and 1.00M NaOH, and add it to distilled water,
while exploring the math relationships of dilution.
*Goal here is to see that pH is a logarithmic pattern. Taking 5 mL of each acid
or base and diluting it to 50 mL and repeating the pattern gives you what you
need.
*Students are required to show all their work, or figure out the pattern and
explain it.
pH=-log [H3O+]
ACTIVITY ONE- PLANT
PIGMENT
CHROMATOGRAPHY
*Students will be given leaves from plants with a high anthocyanin concentration.
*Students will perform paper chromatography to separate out the plant pigments in
the leaves.
*Students will roll a coin over the leaf to place the pigments on the filter paper.
*Suggested solvent is a petroleum ether/acetone mixture.
*Students will calculate Rf values for the pigments and compare to some standard Rf
values.
*Using this information the students will identify the plant pigments.
Chorophyll a and b, Xanthophylls, Carotenes,
Anthocyanins
ACTIVITY ONE CONT.- WHICH
ACT AS ACID/BASE
INDICATORS?
•Using the chromatographs produced in part one, students will test
which pigments can be used as acid/base indicators
•The pH range samples generated in the pre-activity are used
•The students dip the paper from each pigment type into the
standard solutions
•They will discover that anthocyanins change color in very acidic and
basic environments
Which of the pigments will change color?
ACTIVITY TWO:
CONSTRUCTION OF A PH
SCALE
*Many high school students have used pH paper, but now they will construct their own
scale based on anthocyanins from plants.
*Each student group will be given an anthocyanin rich plant. Examples include red
cabbage, grapes, blueberries, tomatoes, strawberries, plums, or raspberries. The plants
should be different for each group.
*Each will then place some of their plant material in a beaker and boil it to extract the
anthocyanins. Decanting the liquid will give them their own acid/base indicators
(Withers, 2001).
*The students will then take a little of each of the pH reference solutions and put them
into 14 test tubes, adding a small amount of their natural indicator.
A single group of plant pigments provides a lot of
different colors.
ACTIVITY TWO CONT.
FINDING THE PH OF AN
UNKNOWN
*Students need a reason to use their new color scale, so we will use it to
find a pH range of an unknown acid or base.
*Placing a small amount of their natural indicator into a sample of the
unknown, students will compare their color to their color scale to get an
estimated pH range.
*Students will then collaborate all of their estimates to get the most precise
estimate possible.
*Students must record all their observations and reasoning.
Plants + Acid or Base -> Color Change
ACTIVITY THREE:
QUANTITATIVE ANALYSIS
*Now students will test the accuracy of their pH measurement using a handheld pH meter.
*Students will also graph real data as they titrate their unknown. The main data they will be
collecting will be pH and volume of titrant added.
*Using Excel, students will construct a graph of pH vs. mL of titrant, and analyze the graph to
determine the unknown’s concentration. If their anthocyanin indcator changes color they will
indicate that, too.
Time to use some math!
ACTIVITY THREE
CONTINUED
*Students will then be required to come up with a hypothesis for the
existence of yellow tomatoes based on the data they have learned.
At halfway to the equivalence point pKa=pH
EXTENSIONS
*Students could look at spectroscopy of the
anthocyanins to measure absorbance at different
wavelengths of light, and use a database to help
identify specific anthocyanins (See Kalb and Howard).
*While fresh fruit was used in our trials, it would be a
great variation to try frozen and canned, to see if these
processes had any effect on the anthocyanins.
There’s lots more to learn!
QUESTIONS??
???
REFERENCES
Blueberry_Cluster. (Picture). URL:http://www.oregonberry.com/Blueberries.htm
Accessed:
June 19, 2008.
Fruits_and_vegetables2. (Picture). URL:
http://www.firodiyaheartfoundation.org/images/fruits_and_vegetables2.jpg
Kalb, Dee A., and G. W. Howard. "Analysis of Plant Pigments Using Paper
Chromatography and Visible and/or UV Spectroscopy." Purdue University
Instrument Van Project. Purdue University.
<http://www.chem.purdue.edu/teacher/table_of_contents/UVVUS/UVVIS.Plant%20Pig
ments_CH.pdf>. Accessed: June 19, 2008.
LYC11. (Yellow tomato picture). URL:
http://www.magicgardenseeds.com/images/product/LYC11/LYC11.jpg. Accessed
June 19, 2008.
Learning requires you to get help from others.
REFERENCES
Marsden, Steve. "Separation of Plant Pigments by Paper Chromatography."
Chemtopics.Com. 19 Aug. 2007. <http://www.chemtopics.com/unit06/pchrom.pdf>.
Accessed: June 19, 2008.
Massengale. Chromatography of plant pigments. URL:
http://www.biologyjunction.com/chromatography_plant_pigments.htm
NASA. Chromatography of leaf pigments: Lesson 2 of 2. URL:
http://media.nasaexplores.com/lessons/01-037/9-12_2. Accessed: June 18, 2008.
Plant Pigment Chromotography. Milanareaschools.Org.
<http://milanareaschools.org/~wbenya/Biology/documents/chromatography%20lab.pdf
>. Accessed June 19, 2008.
Red Acre Cabbage. (Picture). URL:
www.seedfest.co.uk/seeds/cabbage/cabbage.html
Accessed: June 19, 2008.
So does teaching!
REFERENCES
Red Cabbage Acid Base Indicator. 27 Feb. 2002. University of Minnesota.
<http://www.chem.umn.edu/services/lecturedemo/info/Cabbage_Indicator
.html>. Accessed: June 19, 2008.
Red Cabbage pH paper. URL:
http://www.ipse.psu.edu/activities/paper/red_cabbage.pdf
Accessed: June 19, 2008.
Reiss, Carol. Experiments in Plant Physiology. Englewood Cliffs, NJ: Prentice Hall,
1994.
Senese, Fred. Water to Wine. General Chemistry Online. 31 July 2007. Frostburg.
<http://antoine.frostburg.edu/chem/senese /101/features/water2wine .shtml>.
Accessed: June 19, 2008.
In fact, teaching is learning!
REFERENCES
Strawberry. (Picture). URL:
http://www.arabidopsisthaliana.com/strawberry/strawberry.jpg
Accessed: June 19, 2008.
Tomato_pd. (Picture). URL:
http://collectingtokens.files.wordpress.com/2007/08/tomato_pd.jpg. Accessed: June
19, 2008
Withers, G. (2001). Natural indicators: How do they work? GIPSE. URL:
http://www.cmu.edu/gipse/materials/pdf-2001/natural_ph_indicators.pdf. Accessed:
June 19, 2008.
Thank you!