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F-10
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 0.02 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 0.02 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
CH2Cl2 layer?) Discard as much of the top layer in the graduated cylinder as can be poured off easily and
transfer the dichloromethane layer to the buret. Some of the aqueous layer will also be transferred, however
it will be on top of the CH2Cl2 layer and will not interfere. Deliver a few millilitres of the dichloromethane
layer from the buret into a small beaker in order to fill the tip with dichloromethane, and discard the solution
in the special waste container.
Deliver about 10 mL, accurately measured (to 0.02 mL), of the CH2Cl2 layer from the buret into a clean
250 mL Erlenmeyer flask. Add 10.0 mL of 0.10 M KI and 2.0 mL of 1.0 M HCl to the flask and begin the
titration. In this case, you will need to swirl the flask rapidly during the thiosulfate addition, because the
reaction occurs only as the I2 is extracted from the CH2Cl2 layer back into the aqueous layer. When the
