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F-8
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 0.01 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
aqueous layer turns a pale yellow (exact point is not too important here), add 3 drops of starch solution.
Then continue the titration until the aqueous layer and the CH2Cl2 layer become colorless at the same time.
If the aqueous layer becomes colorless and the CH2Cl2 layer is still colored, stop the titration and
swirl the flask vigorously, until I2 is transferred from the CH2Cl2 phase to the aqueous phase. If you do
not do this, you may overshoot the end point. Test the end point to be sure it is accurate. Read and record
the buret level (to 0.01 mL).
Dispose of the CH2Cl2 titrated solution in the special ORGANIC TOXIC WASTE container in
the fumehood.
Repeat the titration on a second sample. A third titration must be done if the results of the first two titrations
are not consistent. Again, to consider the results consistent for the titrations in this Part, the difference
between the ratios of
volume
titrant
volume
analyte
for the runs must be less than
0.015.
Dispose of the remaining CH2Cl2 in the ORGANIC TOXIC WASTE container in the fumehood
Have you remembered to record the concentration of the standardized sodium thiosulfate solution and the
initial concentration of KI in the aqueous triiodide solution?
When you have finished all of your titrations, and have disposed of all of the wastes
properly, wash your burets with hot tap water (four times) and rinse once with deionized
water. Leave them at your work station filled with deionized water.
Before leaving the laboratory, check to make sure that all of your equipment is clean, and back in your
drawer. As well, all communal equipment must be cleaned and returned to the designated location in the
