Acids, Bases, and Indicators
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Edwin A. Metzl Lincoln Park High School
2001 North Orchard Street Mall
Chicago IL 60614
This is a multileveled approach to learning some of the common
characteristics of acidic and basic solutions and use of some of the
Apron Distilled water
Safety goggles 200 mL 0.1 M HCl
13 small or medium sized test tubes 200 mL 0.01 M NaOH
Test tube racks to hold 12 test tubes a few leaves from
1 glass stirring rod Red Cabbage
12 250mm beakers Heat source such as
1 glass marking pencil a hot plate
1 10 ml graduate cylinder Pot or 1 L beaker
1 100 ml graduate cylinder Various solutions of
At least 6 acid-base indicators which unknown pH
should include: Blue & Red Litmus Paper 13 Eye Droppers
Universal indicator paper
Phenol Red + others
The teaching strategy is both multifacetted and multileveled. At the
lowest level, litmus paper testing of different household products might be an
effective means of introducing the student to acid and bases. However, for
purposes of this discussion, it will be presumed that the class is very advanced
and the teacher may decide how far and how fast to proceed.
Acid-base indicators are substances that change color as a function of pH,
usually over a range of 1 to 2 pH units. These indicators are weak acids or
weak bases which disassociate and change colors when equilibrium shifts per
Le Chatelier's principle. Indicators are available that shift color for every
value of the pH scale. The earliest indicators were extracted from plants. In
this lab, we will compare indicator colors to known pH's; use the juice from red
cabbage as an indicator; use red cabbage indicator to find the pH of unknowns;
and lastly, do an acid-base titration.
Preparation of standard solutions
-Obtain 12 250 mL beakers and label them 1 through 12. To beaker 1,
add 100 mL of 0.1 M HCl. This solution has a pH of 1. Measure 10 mL
of this solution and add it to beaker 2 with 90 mL distilled water.
The resulting solution has a pH of 2. Follow this procedure for
beakers 3, 4, 5, and 6. Of course, beaker 7 will have only distilled
water with a pH of 7.
-To beaker 12, add 100 mL of 0.01 M NaOH solution. This has a pH of 12.
Successive dilutions by factors of ten will produce pH values of 11,
10, 9, and 8.
Comparing indicators with known pH's
-Label 12 test tubes and take samples of each known pH solution. Use
whichever indicators are available to compare indicators with these
knowns. Note which indicators appear to be best for given ranges
Extraction of an indicator from red cabbage
-Cut leaves from the red cabbage into small pieces and place them into
a clean large beaker with enough distilled water to cover the pieces.
Boil the cabbage and water until the solution is a deep purple. Use
the stirring rod to keep all leaf pieces in the boiling water. Permit
the solution to cool.
Using red cabbage as an indicator
-Place 5 mL of known pH solutions in each of your labelled test tubes.
To these, add 5 drops of red cabbage indicator. You are now ready to
compare your unknowns with your knowns by placing 5 mL of an unknown
into a clean test tube, adding 5 drops of red cabbage juice and
comparing the unknown with your knowns.
Using red cabbage for titration
-Only a few words on this subject ... As all titrations that this
writer is familiar with use phenolphthalein as an indicator, there
is no reason for not using red cabbage instead. For those not
familiar with titrations, it is a method of determining quantities
necessary to change a solution of one pH into a solution of a
A comparison should be made by the students of the cabbage indicator with
the "store bought" varieties plus a comparison between the various indicators.
Questions, such as, "Which is the best indicator?" should be more specific with
questions such as, "Which has the most discernible color change?" and "Which
might be used for a specific range?"
Of course, with teacher approval only, students could try extracting other
colored substances from leaves and flower petals to test as indicators...try tea
and beet juice as examples.
Cabbage juice appears to be a useful indicator, as do other some
naturally colored materials. Materials serve best within specific pH ranges.