Lab 9: Analysis of Commercial Bleach
Many common products are effective because they contain oxidizing agents. Some products, which contain oxidizing agents, are bleaches, hair coloring agents, scouring powders, and toilet bowl cleaners. The most common oxidizing agent in bleaches is sodium hypochlorite, NaClO (or NaOCl). Commercial bleaches are made by bubbling chlorine gas into a sodium hydroxide solution. Some of the chlorine is oxidized from the molecular form (Cl2) to the hypochlorite ion, ClO-. Some of the molecular form is also reduced to the chloride ion, Cl-. This type of reaction, where the same type of element is both oxidized and reduced, is called a disproportionation reaction. The solution remains strongly basic. The net ionic chemical equation for the process is:
Cl2(g) + 2OH-(aq) → ClO-(aq) + Cl-(aq) + H2O(l)
The amount of hypochlorite ion present in a solution of bleach is determined by an oxidation-reduction titration. One of the best methods is the iodine-thiosulfate titration procedure. Iodide ion, I-, is easily oxidized by almost any oxidizing agent. In acid solution, hypochlorite ions oxidize the 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.
2H+(aq) + ClO-(aq) + 2I-(aq) → Cl-(aq) + I2(aq) + H2O(l)
from: HCl bleach KI solution
(2) Iodine is only slightly soluble in water. 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-brown when concentrated. I2(aq) + I-(aq) → I3
iodine iodide triiodide
(3) The triiodide is titrated with a standard solution of thiosulfate ions, which reduces the iodine back to iodide...
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