Rev 2016-10-21
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3. Standardizing your EDTA solution
a. Pipet 25.00 mL of your CaCO
3
standard solution into each of three 250 mL
Erlenmeyer flasks. Add ~1 ml of buffer solution using a 10 mL graduated
cylinder. The pH of your solution should be ~10 after the addition of buffer. Add
~2-3 drops of calmagite indicator solution. Titrate the solution until the last trace
of red color disappears upon addition of just a fraction of a drop of EDTA. The
final color change should be from a violet color to a pale blue. The change should
be fairly sharp. If it is not, it could mean that the indicator is old. Record each
endpoint volume to the nearest 0.01 mL.
b. Prepare a sample for a blank titration as follows:
Pipet a 25.00 mL sample of DI water into a clean 250 mL Erlenmeyer flask. Add
~ 1-2 mL of the buffer. Add a few drops of calmagite indicator solution. If the
solution turns blue, there is no measureable magnesium or calcium in the solution
and you will not have a blank correction. If the solution stays red or violet, titrate
with the EDTA solution until there is no trace of red or violet in your solution.
The color change on reaching the endpoint is very subtle. Ask the TA to help you
determine if you are near your endpoint. One thing that might help is to carry your
solution over to the window and look at it in natural light. The fluorescent lights may
make the solution have a pinkish tinge to your eye, even at the endpoint. Also, try to
reach the same color for each titration of your standards and unknowns. The
consistency of your technique will improve the precision of your measurements.
B Performance Evaluation
1. You will have a performance evaluation sample given to you by your TA or by Lab
Services. This sample will have a total volume of 300 mL and should be well shaken.
If you take more than 300 mL your lab grade will be decreased by 50%.
2. Use a volumetric pipet (either 2 x 50.00 mL or 4 x 25.00 mL) to place 100.00 mL of
your water sample in each of three 250-mL Erlenmeyer flasks. Add 5 - 10 drops of 3
M hydrochloric acid to each flask, or until the pH is ~2 (test using indicator paper),
and gently boil the solution for about 5 min. Dissolved carbon dioxide is removed
from the solutions during the boiling. The acid is added to convert dissolved
carbonate to carbon dioxide. Also prepare a solution for a blank titration.
3. Cool each solution to near room temperature. Use ice to speed things up. Add 3 M
sodium hydroxide solution dropwise to each flask until the pH is ~7 (use indicator
paper to test the pH). This may take anywhere from 5-15 drops of 3 M NaOH
solution. The hydrochloric acid which was added in step 2 is neutralized in this step.
4. Add 1 or 2 mL of pH 10 buffer and about 2-3 drops of calmagite indicator. The pH
should be ~10. Use indicator paper to check the pH. The solutions should themselves
be red in color.
5. Fill a 50-mL buret with the standardized EDTA solution and use the solution to titrate
each water sample to the endpoint. At the endpoint the titration solution changes from
red to blue. Record each endpoint volume to the nearest 0.01 mL.
6. Carry out a blank titration. Proceed as in steps 2 - 6, but using 100 mL of DI water as
a sample.