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F-5
In these titrations, the thiosulfate ion will be added from a buret until exactly all of the I3
-
(aq)
and I
2(aq)
has
reacted. This is the equivalence point of the titration. The equivalence point of this titration is determined
using a starch indicator, which changes color at or very near the equivalence point. Iodine and triiodide
ions have a reddish-brown color in water. Iodine in CH2Cl2 is purple. The iodide ion is colorless. At the
equivalence point of the aqueous layer titration in Part II, the number of moles of thiosulfate added will
equal twice the number of moles of iodine plus triiodide ion present in the aqueous sample that was
titrated. At the equivalence point for the CH2Cl2 layer titration in Part III, the number of moles of
thiosulfate added will equal twice the number of moles of iodine present in the CH2Cl2 sample that was
titrated.
As the titration proceeds in water, the color in the aqueous layer changes from dark reddish-brown to
pale yellow to colorless. The pale yellow to colorless transition is not very distinct, therefore, a few drops
of starch solution are added to act as an indicator. Starch reacts with triiodide anion to give a complex,
which has a deep blue color. The end point in the aqueous layer titration is determined by a change of
color from blue to colorless.
In the CH2Cl2 layer, the color changes from purple to faint pink to colorless as the titration proceeds.
Starch will again be used to help mark the equivalence point; therefore, at the end point of the titration, the
color changes from pale blue to colorless in the aqueous layer, simultaneously with the color of the
CH2Cl2 layer changing from pale pink to colorless. Be sure to add the thiosulfate solution slowly as you
get close to the equivalence point.
NOTE:
Like triiodide and iodide ions, thiosulfate ions are not significantly soluble in the CH2Cl2 layer,
therefore, the reaction between S2O3²
-
(aq)
and I
2(aq)
occurs in the aqueous layer. For the
iodine in the CH2Cl2 layer to react with thiosulfate, it must be extracted back into the aqueous
layer. Both layers must become colorless at the same time. To ensure that this happens, and
that all of the I2 in the CH2Cl2 layer does react, the thiosulfate must be added slowly near the
equivalence point of the titration of the CH2Cl2 layer. Also, the flask must be swirled
vigorously and continuously to mix the layers, so that all the I2 is extracted back into the
aqueous layer.
