Experiment 8: Determination of Concentration
by Oxidation-Reduction Titration and an Actual
Student Lab Write-Up*
Background: Commercially available bleaching solutions contain NaOCl, sodium hypochlo-
rite. Sodium hydroxide is reacted with chlorine gas to produce the hypochlorite ion, OCl–.
Cl (^22) ]gg++ 2 OH---]]aqgg"OCl aq +Cl H O]lg
In solution, NaOCl dissociates into sodium ions (Na+) and hypochlorite ions (OCl–). Bleaching
involves an oxidation-reduction reaction in which the Cl in the OCl–ion (oxidizing agent) is re-
duced to the chloride ion (Cl–). The reducing agent is either a dye, which fades, or the stain be-
ing removed.
The amount of hypochlorite ion present in bleach can be determined by an oxidation-reduction
titration. In this experiment, an iodine-thiosulfate titration will be utilized. The iodide ion is ox-
idized to form iodine, I 2. This iodine is then titrated with a solution of sodium thiosulfate of
known concentration. Three steps are involved:
- An acidified solution of iodide ion and hypochlorite ion is oxidized to iodine:
22 H+---]]]]aqgggg++OCI aq I aq "Cl aq ++I (^22) ]aqg H O]lg
- However, iodine is not very soluble in water. Therefore, an aqueous solution of iodide ion
is added to the iodine to form the complex ion, triiodide ion, I 3 – (aq). In dilute concentra-
tions, the triiodide ion is yellow and in concentrated solutions it is a dark reddish-brown.
II I 2 aq + aq " 3 aq
--
] g ]]gg- Finally, the triiodide ion is titrated with a known solution of thiosulfate ions, which forms
iodide ions:
I- (^3) ]]]]aqgggg++ 23 SO 232 - aq " I- aq SO 462 - aq
In this step the reddish brown color of the triiodide begins to fade to yellow and finally to clear,
indicating only iodide ions present. However, this is not the best procedure for determining
when all of the I 3 – has disappeared since it is not a sensitive reaction and the change from pale
yellow to colorless is not distinct. A better procedure is to add a soluble starch solution shortly
prior to reaching the end point, since if it is added to soon, too much iodine or triiodide ion
may be present forming a complex that may not be reversible in the titration. The amount of
thiosulfate is proportional to the amount of hypochlorite ion present.
The experimental error involved in measuring small volumes of liquids is usually greater than
the error when measuring larger volumes. Diluted samples will be prepared, called aliquots, to
improve the accuracy.
Laboratory Experiments