Objective
To determine the concentrations of the active ingredients in 2 commercial bleaches.
Introduction
Sodium hypochlorite is usually found in bleaching solutions. It is the active ingredient of bleaching solutions. It bleaches by oxidation. When it is added to dye, the following reaction occurs:
ClO- + dye —————-> Cl- + (dye + O)
If the oxidized form of the dye is colorless, then the color of the dye would fade away.
In the presence of acid, the hypochlorite ions from the bleaching solution reacts with the iodine ions from potassium iodide in the following way:
ClO- + 2I- + 2H+ ———>I2 + H2O + Cl-
When sodium thiosulphate solution is added into this reacted solution, a further reaction occurs:
I2 + 2S2O32- ————–>2I- + S4O62-
This reaction could be used in titration to find out the number of moles of thiosulphate ions, thus the concentration of hypochlorite ions in the bleaching solution.
Procedure
1. 10 cm3 of Kao Bleach was pipette into a volumetric flask.
Distilled water was added until the meniscus reaches the graduation point.
2. 25 cm3 of the titrated bleach was pipette into a conical flask. About 10 cm3 of potassium iodide and dilute sulphuric acid was added into the conical flask.
3. The solution was titrated with sodium thiosulphate solution until the brown colour of the iodine fades.
4. Starch solution was added into the conical flask, and the solution was further titrated until the dark-brown colour of the starch-iodine complex turns to colourless. The volume of sodium thiosulphate solution required to reach the end point was recorded.
5. Steps 1 to 4 were repeated 3 more times.
6. Steps 1 to 5 were repeated using Clorox Bleach.
Data and Calculation
Molarity of standard Na2S2O3 solution = 0.05182M
Brand A: Kao
Price: $11.9/ 1500ml
Trial
1
2
3
Final reading/cm3
26.8
23.1
25.7
26.0
Initial reading/ cm3
4.1
0.4
2.9
3.2
Volume of Na2S2O3
22.7
22.7
22.8
22.8
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