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F-4
The end point of a titration, however, is the point at which the indicator changes color. In order for an
indicator to be useful in a titration, it must change color at or very near the equivalence point of the
titration.
In Part I you will establish the distribution equilibrium of equation (3) by adding CH2Cl2 to an aqueous
solution that contains I
-
(aq)
, I
2(aq)
, I3
-
(aq)
solution. After ~15 minutes of vigorous swirling, let the mixture
stand for several minutes to allow the two immiscible layers to separate.
The concentrations of iodine in the dichloromethane layer and (iodine + triiodide ions) in the aqueous layer
are determined by titrating samples of each layer with sodium thiosulfate. Two thiosulfate ions are needed
to react with one molecule of I2 or with one triiodide ion. Note: The iodide ion does not react with
thiosulfate. The pertinent equations are:
2 S2O3 ²
-
(aq)
+ I
2(aq)
S
4
O
6
2-
(aq)
+ 2 I
-
(aq)
(8)
2 S2O3 ²
-
(aq)
+ I3
-
(aq)
S
4
O
6
2-
(aq)
+ 3 I
-
(aq)
(9)
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
