Red Cabbage Lab: Acids and Bases
Introduction:
Liquids all around us have either acidic or basic (alkaline) properties. For
example, acids taste sour; while, bases taste bitter and feel slippery. However,
both strong acids and strong bases can be very dangerous and burn your skin,
so it is important to be very careful when using such chemicals. To measure how
acidic or basic a liquid is, one can use a pH scale as illustrated below:
The strength of the pH scale is determined by the concentration of hydrogen ions
(H+) where a high concentration of H+ ions indicate a low pH and a high
concentration of OH- ions indicate a high pH. The pH scale ranges from 1 to 14
where 1 to 7 is classified as acidic, 7 neutral (neither a base or an acid) and 7 to
14 is classified as basic.
In this lab, you will use the juice from red cabbage as a pH indicator. You will mix
cabbage juice with different household liquids and see a color change produced
by a pigment called flavin (an anthocyanin) in red cabbage.
Procedure:
Isolate cabbage juice:
Tear a handful of red cabbage leaves into small pieces
Place leaves in a 400ml or 600ml beaker
Add enough water to cover the cabbage
Bring to slight simmer on hot plate
Once the liquid is uniformly dark, ask your teacher to remove the beaker
Allow to cool
Turn off and unplug hotplate
Prepare solutions to test:
You going to create 7 test tubes of liquid to test each about 2/3 full
Arrange them in the test-tube rack 1-7
Rinse the droppers several times with plain water between
making tube solutions
Tube 1: fill about 2/3 with vinegar
Tube 2: a dropper full of liquid from tube 1 mixed with enough
water to fill the test tube about 2/3 full
Tube 3: a dropper full of liquid from tube 2 mixed with enough
water to fill the test tube about 2/3 full
Tube 4: plain water
Tube 5: Make tubes 7 and 6 first. Add a dropper full of liquid from tube 6
mixed with enough water to fill the test tube about 2/3 full
Tube 6: Make tube 7 first. Add a dropper full of liquid from tube 7
mixed with enough water to fill the test tube about 2/3 full
Tube 7: In a separate beaker, mix a spoonful of baking soda with about
50ml of water, fill tube 7 with this liquid
Testing the solutions:
Add 5-10 droppers of cabbage extract to each tube – record color changes
Combine tube 6 and 2 in a clean beaker -record color change
Combine tube 1 and 7 in a clean beaker -record color change
Write a paragraph or 2 describing the procedure and results. Which substance
was an acid? Which was a base? Explain what happened when you combined
the contents of the test tubes in the last 2 steps.
The Chemistry (and Physics) Behind The Process:
Red cabbage and other plants (beets, blueberries, radishes,
and many more) contain a class of compounds
called anthocyanins. Anthocyanins have different colors that
depend upon the numbers of removable protons that remain
attached to the molecule.
Anthocyanins are weak organic acids. Remember, B-L acids
are proton donors, who give up protons to become conjugate bases. Different
forms of some molecules (not all molecules) will reflect light differently causing a color change. The compounds that form molecules that do reflect
light differently can be used as acid-base indicators.
Molecule 1 represents the anthocyanin with two protons (the 2
red H) to donate - a B-L acid.
The molecule reflects red light.
Molecule 2 represents the B-L conjugate base of molecule 1,
but it still has one proton to donate.
The molecule reflects blue light.
This molecule can either act as a B-L conjugate base and
accept a proton - changing its color back to red - or act as an
B-L acid and donate its other proton.
Molecule 3 represents the B-L conjugate base of molecule 2,
with no more protons to donate.
The molecule reflects greenish-yellow light.
This molecule can no longer donate protons, but as a B-L
conjugate base it can accept protons. Accepting 1 proton
changes its color back to blue - while accepting 2 protons
changes its color back to red.
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The higher the hydrogen ion concentration, [H+], in the solution
the original Anthocyanin (molecule 1) is placed in, the fewer
molecules donate protons.
The lower the hydrogen ion concentration, [H+], in the solution the
original Anthocyanin (molecule 1) is placed in, the more
molecules donate protons.
Unless the [H+] is very high - (few molecules donate protons), or
very low - (most molecules donate protons), the solution will reach
an equilibrium with a combination of these three molecules in it.
The visible color of the solution containing the indicator would be
something like this:
[H+]
Color
very high
red
high
mixture of red and blue
moderate
blue
low
mixture of blue and greenish-yellow
very low
greenish-yellow
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The pH indicated by colors:
pH
Color
2
red
4
purple
6
violet
8
blue
10
blue-green
12 greenish-yellow
Application:
Red cabbage is not used in the production of commercial acid-base
indicators. What are some possible reasons for this?
Extension:
Your cabbage solution can be used to make pH paper.
1.
2.
3.
4.
Soak filter paper in the cabbage juice for several hours.
Remove and allow to dry.
Cut into strips.
Use the strips on the same solutions you tested with the liquid.
Compare the results.