Chemistry 120: Experiment 3
Preparation of Standard Sodium Thiosulfate Solution and
Determination of Hypochlorite in a Commercial Bleach Product
Iod
ine can be used as an oxidizing agent in many oxidation-reduction titrations and iodide
can be used as a reducing agent in other oxidation-reduction titrations:
I
2
+ 2 e
–
= 2 I
–
(1)
If a standard iodine solution is used as a titrant for an oxidizable analyte, the technique is
iodimetry. If an excess of iodide is used to quantitatively reduce a chemical species while
simultaneously forming iodine, and if the iodine is subsequently titrated with thiosulfate, the
technique is iodometry. Iodometry is an example of an indirect determination since a product of
a preliminary reaction is titrated.
The use of iodine as a titrant suffers from two major disadvantages. First, iodine is not
particularly soluble in water, and second, iodine is somewhat volatile. Consequently, there is an
escape of significant amounts of dissolved iodine from the solution. Both of these disadvantages
are overcome by adding iodide (I
–
) to iodine (I
2
) solutions. In the presence of iodide, iodine
reacts to form triiodide (I
3
–
) which is highly soluble and not volatile.
I
2
+ I
–
= I
3
–
(2)
The major chemical species present in these solutions is triiodide. The reduction of
triiodide to iodide is analogous to the reduction of iodine.
I
3
–
+ 2 e
–
= 3 I
–
(3)
Triiodide reacts with thiosulfate to yield iodide and tetrathionate.
2 S
2
O
3
2–
+ I
3
–
= S
4
O
6
2–
+ 3 I
–
(4)
thiosulfate
↑ ↑ tetrathionate
Dilute triiodide solutions are yellow, more concentrated solutions are brown, and even
more concentrated solutions are violet. Iodide solutions are colorless. If all of the other solution
components are colorless, it is possible to detect the endpoint of titrations involving triiodide
without the use of an indicator. Endpoint detection is considerably easier, however, with an
indicator. The indicator that is usually chosen for titrations involving iodine (triiodide) is starch.
Starch forms a dark blue complex with iodine. The end point in iodimetry corresponds to a
sudden color change to blue. Likewise the end point in iodometry corresponds to a sudden loss
of blue color due to the complex. Potato starch, rather than corn starch, is preferred for making
the indicator solution since the color change due to the starch complex at the end point is sharper.
In iodometry the starch is added only after the color due to triiodide has begun to fade, i.e., near
the endpoint, because starch can be destroyed in the presence of excess triiodide.
In the first portion of this experiment a sodium thiosulfate solution is prepared and
standardized with the primary standard potassium iodate. Iodate (IO
3
–
) reacts with an excess of
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