Introduction:
The figure pH expresses the acidity or alkalinity of a solution on a logarithmic scale where
neutral is 7 below that is acidic and above that value is alkaline. pH levels are important for
maintaining a constant environment of any kind. Changes in pH levels can effect Plant
Uptake of Benzotriazoles (ie. by Sunflowers in Hydroponic Culture) (Baggs, Smales et al.
2010), change homeostasis of human and animal bodies (Bronshtein, Smith et al. 2005). The
affinities of enzymes for substrates and inhibiters can be affected by pH. Enzymes may
become denatured if the pH is out of the range suitable affecting the normal functions of
the body including metabolism (DIXON 1952). For this reason it was interested in finding out
what solutions could indicate the pH of various substances. Knowing that HCL is acidic,
water is neutral and baking powder mixed in water is basic I tested various solutions to find
out which ones would indicate changes in pH levels by changing colour.
Aim: To test whether or not cabbage, geraniums, carrots and onions can be used as a pH
indicators.
Hypothesis: My hypothesis is that the cabbage and geraniums will make a colour change in
the various pH levelled solutions and therefore be good pH indicators. However, I do not
expect carrots and onions to be good pH indicators and therefore not change the colour of
the various pH levelled solutions.
Equipment:
Red Cabbage, Geraniums, Carrots, Onion, Stove, Sauce pan, Cutting board, Gloves, Lab
coat, Tap water, Spoon, 7 beakers, Syringe, HCL acid, Baking powder, Strainer, Permanent
marker, Safety glasses
Procedure:
1. Put on lab coat, safety glasses and gloves.
2. Label the beakers ‘Acid’; ‘Base’ and ‘Neutral’.
3. Label the other beakers ‘geranium solution’; ‘onion solution’; ‘carrot solution’; and
‘cabbage solution’.
4. Label the remaining 3 beakers ‘test acid, ‘test basic’ and ‘test neutral’.
5. Add 200mL of water into a saucepan and boil the cabbage until the water becomes
coloured.
6. Drain the water, using the strainer, into the beaker labelled ‘cabbage solution’.
7. Clean the sauce pan and repeat the procedure for the onion, carrot and geraniums
and pour into appropriately labelled beakers.
8. Pour 500mL of acid into the beaker labelled acid and 500Ml of water into both the
beaker labelled ‘neutral’ and ‘basic’.
9. Add 4 table spoons of backing powder to the beaker labelled ‘basic’ and stir.
10. Using the syringe measure 10mL of acid into the beaker labelled ‘test acid’, 10mL of
the basic solution into the beaker labelled ‘test basic’ and 10mL of water into the
beaker labelled ‘test neutral’. Be sure to rinse the syringe to avoid contamination.
11. Add 5mL of geranium solution to each of the test beakers and record any colour
change.
12. Rinse the beakers and dispose of waste appropriately then repeat step 10.
13. Measure 5mL of cabbage solution into the test beakers and record any colour
change.
14. Rinse the beakers and dispose of waste appropriately then repeat step 10.
15. Measure 5mL of carrot solution into the test beakers and record any colour change.
16. Rinse the beakers and dispose of waste appropriately then repeat step 10.
17. Measure 5mL of onion solution into the test beakers and record any colour change.
18. Repeat experiment 5 more times.
19. Dispose of waste appropriately and clean materials used.
Safety/Risk:
This experiment requires a lab coat, gloves, safety glasses and lab shoes to be worn as the
experiment deals with HCL acid and hot water. It is important to keep in mind the corrosive
nature of HCL and to avoid contact with it. It would also be a good idea to know what to do
if you come into contact with HCL or how to treat burns. It is essential when disposing of the
HCL to flush it down the sink with lots of water.
Results:
Various Solutions Indicating pH Levels by Colour Change
Basic
Colour change
to purple
Neutral
No colour
change
Acidic
Colour change
to red
Carrot Solution
no colour
change
No colour
change
No colour
change
Geranium
Solution
No colour
change
No colour
change
Colour change
to red
Onion Solution
No colour
change
No colour
change
Colour change
to yellow
Cabbage
Solution
Picture
Discussion:
My hypothesis that the cabbage would change in the basic and acidic solutions was correct.
However, the geraniums did not change the colour of the solution in the basic solution but
did in the acidic solution. Contrary to what I expected the onion solution did change the
colour of the acid solution.
The basic solution I used was baking powder mixed with water which is not a strong base
therefore may not have shown the colour changes like a stronger basic solution would.
The neutral solution was the control and was used to compare the colour changes in the
basic and acidic solutions. It was important in this experiment to avoid contamination by
rinsing equipment as not to tamper with the results.
To understand the reaction that takes place indicating the pH level changes I researched
further. A pigment molecule in red cabbage called flavin (an anthrocyanin) when in contact
with acidic solutions will turn red. The pigment turns purple in neutral solutions and
greenish-yellow in basic solutions. This colour change is a result of hydrogen ion
concentrations changing due to acids donating hydrogen ions in an aqueous solution and
having a low pH(Gluckman 2013). Onions are olfactory indicators (Education 2012) which
means its smell varies depending on whether it is mixed with an acidic or basic solution
however also changes yellow in acidic solutions (2001) I was only observing the colour
changes and not observing the smell of the onion during this test. However if I were to do
this experiment again, knowing that onion is an olfactory indicator, I would pay more
attention to the smell change as well as the colour.
Conclusion:
This experiment was successful in testing whether or not cabbage, geraniums, carrots and
onions can be used as pH indicators. It is clear from this test that cabbage makes the best
colour indicator of the pH changes and that carrots don’t work as pH indicators. Both
geraniums and onions indicate to a certain degree the pH changes.
(2001). New Oxford Dictionary of English. United Kingdom, Oxford University Press.
Baggs, E. M., et al. (2010). "Changing pH shifts the microbial sourceas well as the magnitude of
N2O emission from soil." Biology and Fertility of Soils 46(8): 793-805.
Bronshtein, I., et al. (2005). "The Effect of pH on the Topography of Porphyrins in Lipid
Membranes¶." Photochemistry and Photobiology 81(2): 446-451.
DIXON, M. ( 1952). "The Effect of pH on the Affinities of Enzymes
for Substrates and Inhibitors." from http://www-02.all-portland.net/bj/055/0161/0550161.pdf.
Education, S. (2012). "Acids and Bases:Cabbage Juice pH Indicator Teacher Version." from
www.stanford.edu/group/lpchscience/cgi-bin/wordpress/images/2012/10/Acids-and-BasesT.pdf.
Gluckman, J. (2013). pH Indicators pH Indicators