Analysis of commercial bleaches.

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Tittle: Experiment 2  Analysis of Commercial Bleaches

Objectives:

To determine the concentration of two bleaching solutions by titration with standardized thiosulphate solution.

Introduction:

The bleaching action of commercial bleaches is due to its oxidizing power of sodium hypochlorite (NaClO(aq)).

We cannot determine the concentration of this ingredient directly. We can firstly add excess acidified potassium iodide to the bleaches. The iodide would reduce to form iodine.

NaClO +KI(aq)+ H2SO4(aq) —I2(aq)+ H2O(l)+ NaCl(aq)+ K2SO4(aq)

The concentration of iodine generated in situ is directly proportional to the concentration of the NaClO.

The iodine is titrated against standardized sodium thiosulphate solution (2NaS2O3(aq)).

I2(aq) +2NaS2O3(aq)  —Na2S4O6(aq)  +2NaI(aq)

Starch is used as an indicator in this titration. When the concentration of iodine is low (pale yellow), the starch indicator is added and the solution turns bluish black. The titration is continued until the color of the solution becomes colorless.

So, the concentration of the active ingredient NaClO(aq) in bleaches can be determined form this back titration.

 

Procedure:

  1. 25 cm3 of each of the commercial bleaches (Clorex and Kao) was pipetted to the volumetric flask. And was diluted with deionised water to 250 cm3.
  2. The diluted bleach solutions were shook to mix.
  3. 0.1M of sodium thiosulphate solution was poured into burette with a filter funnel. The initial read of the burette was recorded.
  4. A white tile was put under the burette.
  5. 25 cm3 of the diluted bleach solution was pipetted into a clean conical flask.
  6. 10 cm3 of diluted sulphuric acid and 10% potassium iodide solutions were added by measuring cylinders,
  7. The content was swirled after the addition. The mixed solution was titrated against the sodium thiosulphate solution until the solution turned pale yellow.
  8. A few drop of starch solution was added to the solution.
  9. The titration was continued until the blue color just turned colorless.
  10. The final reading of the burette was recorded.
  11. Steps 3-10 were repeated twice.
  12. Steps 3-11 were repeated using another brand of bleach solution.
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Result:

Brand: Clorex

Table 1

Average titre: 32.70cm3

Brand: Kao

Table 2

Average titre: 25.67cm3

Questions:

  1. Determine the concentration of each of the two brands of bleaches provided.

Ans: The molar mass of NaClO =35.5+16+23=74.5

        No. of mole of Na2S2O3: No. of mole of NaClO= 2:1

The Clorex conc. is

Avg. Na2S2O3 used is 32.7cm3

        No. of Mole/volume=M

The no. of mole of diluted NaClO

        1/2X0.1X32.7/1000=1.635X10-3

The no. of mole of NaClO=1.635X10-3X10

                                           =1.635X10-2mole        

        mole X molar mass/volume=g/cm3

        1.635X10-2X74.5/25=0.0487 g/cm3

The Kao conc. is

Avg. Na2S2O3 used is 25.67cm3

        No. of Mole/volume=M

The no. ...

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