6
The reaction of EDTA with either calcium or barium ions will be followed by a process
called titration in which an indicator changes color when the number of moles of EDTA
molecules (in an aqueous solution) added equals that of the cations. Students will
practice their titration technique with a solution of known calcium ion concentration.
Procedure (Students will work in pairs).
1. Preparation of saturated solutions of CaCO3, CaSO4, and BaSO4. Use a spatula to
transfer an amount equivalent to about 2 teaspoons of each solid to each of three separate
50 ml capped centrifuge tubes. Add approximately 40 ml of distilled water. Put the 3
tubes on the wrist-action shaker and shake vigorously for at least a half-hour. Remove
the tubes from the shaker and allow the undissolved solid to settle to the bottom of the
tube. The aqueous layers are saturated with the appropriate salts.
2. Titration procedure. Note that each student pair will do four sets of 3 titrations: From
the Ca2+ standard and from the three saturated solutions (CaCO3, CaSO4, and BaSO4).
Working with standard. Pour about 20 ml of the Ca2+ standard ( 2.50 x 10-3 M) into a
labeled 50 ml plastic beaker. In a separate labeled 50 ml plastic beaker, pour 20 ml of
1.5 x 10-2 M EDTA Obtain and label a 5 ml syringe (with needle): "Calcium standard."
The titration itself. RECORDING THE EXACT WEIGHT OF THE TITRATION
BOTTLE AND ITS CONTENTS AT EACH STEP OF THIS PROCEDURE ARE
CRITICAL. MAKE SURE YOU UNDERSTAND THE PROCEDURE
COMPLETELY. The most efficient and convenient form for organizing your titration
is a table. Here is a suggested format (report the exact concentration of EDTA):
Table 1. Titration of Ca2+ standard (2.50 x 10-3 M) with 1.5 x 10-2 M EDTA
Trial 1
Trial 2
Trial 3
Sample
Weight, grams
Weight, grams
Weight, grams
Empty Bottle
Bottle + standard
Bottle + standard + reagents
Bottle + standard + reagents +
EDTA
a. Weigh a small titration bottle and record its weight (or if no others students are using
the same balance, tare the bottle.)
b. Remove the bottle from the balance, and use the 5 ml syringe to add between 4 and 5
ml of the calcium standard to the bottle. Weigh the bottle again to determine the exact
amount of calcium standard added.
c. Add 2-3 drops of the pH 10 ammonia buffer and 1 drop of the Calmagnite indicator to
the titration bottle and weigh the bottle again. The solution should be red at this point.
d. Remove the bottle from the balance. Take a disposable Pasteur pipette and add the
EDTA solution dropwise to the bottle and swirl it carefully after each addition. Continue
to add EDTA until the solution turns blue. This point is called the endpoint in the
