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F-7
Clean a buret, then rinse it with 2 x 5 mL portions of the aqueous layer from Part I. Fill the buret with the
aqueous layer, being careful not to transfer any of the organic layer. If some CH2Cl2 does go into the buret,
allow it to settle to the bottom of the buret and then drain it from the bottom of the buret back into the
graduated cylinder. In this experiment the meniscus is difficult to read because the liquid is colored. You
may read the top of the meniscus or any fixed point that is more convenient. Deliver about 15 mL,
accurately measured, of the aqueous layer from this buret into a clean (but not necessarily dry) 125 mL
Erlenmeyer flask. Record the initial and final readings on the buret (reminder: all buret readings
must be made to 0.01 mL or marks will be deducted). Add 2.0 mL of 1.0 M HCl solution to the flask.
NOTE: The HCl does not participate in the reaction but prevents conversion of I
2(aq)
to IO3
-
(aq)
, which
occurs in basic solutions.
Clean a second buret, then rinse it with 2 x 5 mL portions of the standardized sodium thiosulfate (Na2S2O3)
solution.
NOTE:
The standardized sodium thiosulfate solution is located in squeeze bottles on the
instructor’s bench. Use these bottles to dispense this solution.
Fill the buret with the standardized sodium thiosulfate solution and record the concentration of this solution
on your Observations Sheet. Read the initial level of the buret containing sodium thiosulfate (to 0.01 mL),
and record this on your Observations Sheet. Begin the titration of the aqueous layer aliquot by slowly adding
the thiosulfate titrant to the Erlenmeyer flask. Continue adding thiosulfate, while swirling the flask, until the
solution turns a pale yellow (exact point is not too important here). Add 3 drops of starch solution and then
continue the titration until the solution turns from blue to colorless on one drop of titrant. Near the end, add
thiosulfate one drop at a time. Test the end point to be sure it is accurate. Read and record the final buret
level of the titrant (to 0.01 mL).
Repeat the titration on a second sample. A third titration must be done if the results from the first two
titrations are not consistent.
NOTE:
To determine if your results are consistent, find the ratio of
volume
titrant
volume
analyte
for each run. To
consider your results consistent for the titrations in this experiment, the difference between
these ratios must be less than
0.015.
III. Titration of the Dichloromethane (CH2Cl2) Layer.
When the titrations of Part II are completed, drain the buret containing the aqueous iodine-triiodide solution.
Do not clean out the buret at this time. (Why is it not necessary to clean the buret before adding the
