Analysis of Bleach

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Redox Titration
Analysis of a Commercial Bleach

Purpose
To perform a redox titration.
To determine the amount of hypochlorite ion present in commercial bleach.

Introduction
Many commercial products, such as bleaches and hair coloring agents, contain oxidizing agents. The most common oxidizing agent in bleaches is sodium hypochlorite, NaClO (sometimes written NaOCl). Commercial bleaches are made by bubbling chlorine gas into a sodium hydroxide solution. Some of the chlorine is oxidized to the hypochlorite ion, ClO-, and some is reduced to the chloride ion, Cl-. The solution remains strongly basic. The chemical equation for the process is:

Cl2(g) + 2 OH-(aq) ( ClO-(aq) + Cl-(aq) + H2O(l)

The amount of hypochlorite ion present in a solution of bleach can be determined by an oxidation-reduction titration. One of the best methods is the iodine-thiosulfate titration procedure. The iodide ion, I-, is easily oxidized by almost any oxidizing agent. In acid solution, hypochlorite ions oxidize iodide ions to form iodine, I2. The iodine that forms is then titrated with a standard solution of sodium thiosulfate. The analysis takes place in a series of steps: 1. Acidified iodide ion is added to hypochlorite ion solution and the iodide is oxidized to iodine.

2 H+(aq) + ClO-(aq) + 2 I-(aq) ( Cl-(aq) + I2(aq) + H2O(l)

2.Iodine is only slightly soluble in water, but it dissolves very well in an aqueous solution of iodide ion, in which it forms a complex ion called the triiodide ion. Triiodide is a combination of a neutral I2 molecule with an I- ion. The triiodide ion is yellow in dilute solution and dark red-brown when concentrated.

I2(aq) + I-(aq) ( I3-(aq)

3.The triiodide is titrated with a standard solution of thiosulfate ions, which reduces the iodine back to iodide ions.

I3-(aq) + 2 S2O32-(aq) ( 3 I-(aq) + S4O62-(aq)

During this last...
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