Analysis of bleach by redox titration
Introduction: Bleaches are one of many products that contain chemicals that work as oxidizing agents (others include hair coloring, scouring powders, and toilet bowl cleaners). The active ingredient in laundry bleach is sodium hypochlorite, (specifically the hypochlorite ion, ClO¯). This ion is readily reduced to the chloride ion under many conditions. The amount of sodium hypochlorite in bleach can be determined by a series of redox reactions involving forms of iodine and culminating with titration by a solution containing the thiosulfate ion, S2O32-.. These unbalanced reactions are described here:
(1) A large excess amount of the iodide ion is added to the bleach and the solution is acidified. The following reaction occurs.
H+ (aq) + ClO¯(aq) + I¯ (aq) ---> Cl¯ (aq) + I2 (aq) + H2O (l)
(2) The excess iodide can form a polyatomic ion (called “triiodide”) with the iodine formed in reaction (1). This helps the iodine dissolve and gives the solution a color that varies from yellow to dark red-brown, depending on how concentrated it is. The formation of triiodide is shown here:
I2 (aq) + I¯ (aq) --> I3¯ (aq)
(3) Finally, the triiodide ion undergoes titration by a solution of thiosulfate ions of known molarity. This redox reaction reforms iodide and creates dithionate ions....
I3¯ (aq) + S2O32¯(aq) ---> I¯ (aq) + S4O62¯
The color of the triiodide ion can serve as its own indicator. When it is all reduced by the thiosulfate, it will appear colorless again. However the color change can be made much more striking by use of a starch indicator. Starch forms a dark blue-black complex with either iodine or triiodide. The disappearance of the triiodide by titration will be signaled by the loss of color from this complex. Care must be taken in this procedure however... starch added to a high concentration of iodine may lead to an irreversible complex... one in which a color change would...
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