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F-4
EXPERIMENTAL METHOD
Chemists are often required to determine ( or to verify ) the amount of a substance (S) in a sample. If S
reacts with a reagent R according to the following hypothetical the reaction:
aS + bR
products
then the amount of S in the sample can be determined by adding R to a stoichiometrically equivalent
amount.
This procedure of adding R until it is in a stoichiometrically equivalent amount to S is known as a
titration. The amount of R used is controlled by adding the solution of R from a buret.
In a titration, the equivalence or stoichiometric point is the point at which the moles of substance
added, the titrant, is stoichiometrically equivalent to the number of moles of substance being titrated, the
analyte. This point is usually made apparent by the use of some type of colored indicator which gives an
endpoint
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, you will then 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)
