Bleaching experiment. Estimation of available chlorine in commercial bleaching solution.

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Title

Estimation of available chlorine in commercial bleaching solution.

Aim

Understand and handle iodometry. Learn to write a full report.

Introduction

For indirect determination of oxidizing agents, iodometry is widely applied. Here, standard solutions of sodium thiosulphate are required as the titrating agent. The procedure to determine an oxidizing agent is as follows: An oxidizing agent to be analyzed is added to an approximate concentration, excess of potassium iodide solution. The iodine liberated is then titrated with a standard solution of sodium thiosulphate until the end point is reached. In general, the reactions may be represented as follows:

Oxidizing agent ( to be analyzed ) + I- (excess ) → I2(aq) + other products

I2(aq) + 2S2O32-(aq) → 2I-(aq) +S4O62-(aq)

In this case, iodide ion acts as a moderately effective reducing agent. The quantity of iodine formed is chemically equivalent to the amount of oxidizing agent and thus serves as the basis for the analysis.

Similarly, for this experiment, the active ingredient is chlorate (I) ion in a bleach which will undergo redox reaction with excess of potassium iodine solution in the presence of acid, liberating iodine, which is then titrated against standard sodium thiosulphate solution. Besides, the available chlorine can also be obtained from the following ways:

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

Cl2(g) + 2I- (aq) +2H+ (aq)→ I2(aq) +H2O(l) + 2Cl- (aq)

I2(aq) + 2S2O32-(aq) → 2I-(aq) +S4O62-(aq)

Requirements

Volumetric flask(250cm3).

Burette(50cm3).

Pipette(25cm3).

Pipette filler.                              

 

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Conical flask (250cm3).

Dropper.

Wash bottle.

White tile.

Burette stand.

Balance.

Weighing bottle.

Beakers.

Measuring cylinders. (50cm3, 100cm3)

Commercial bleaching solution.

Potassium iodide crystals.

Ethanoic acid ( about 1M )

Standard sodium thiosulphate solution

Starch solution (Freshly prepared).

Procedure

  1. Volumetric flask, conical flask, burette, pipette, weighing bottle, beakers were washed with tap water.
  2. Step (1) was repeated but with deionized water at this time.
  3. The tissues were used to dry the apparatus.
  4. A 25cm3 pipette was rinsed with the bleach.
  5. 25cm3 of the bleach was pipetted into a clean 250cm3 volumetric flask. (Pipette filler was used.)
  6. The liquid level was made up to the graduation mark using deionized water.
  7. The volumetric flask was stoppered and was inverted it for several times. (to ensure the solution mix well)
  8. Step (1) was repeated for the pipette and then was rinsed with the diluted bleach.
  9. 25cm3 of the diluted bleach was pipettedinto a conical flask. (Pipette filler was used.)
  10. About2.0 g of KI crystals was weighed roughly in the weighing bottle by using the balance.
  11. The crystals was addedinto the solution in the conical flask.
  12. About 15 cm3 ethanoic acid (about 1 M) was added into the solution of conical flask.(Mesuring Cylinder was used.)
  13. The burette was rinsed with the standard sodium thiosulphate solution. (A funnel and beaker was used.)
  14. The standard sodium thiosulphate solution was added into the burette. (A funnel and beaker was used.)
  15. The solution in the conical flask was titrated against the standard sodium thiosulphate solution until the color of the reaction mixture turned pale yellow.
  16.  Few drops (2 cm3) of freshly prepared starch indicator (about 1 M) into the reaction mixture. (A dropper and measuring cylinder were used.)
  17. The titration was continued but the standard sodium thiosulphate solution was added to the reaction mixture drop by drop until the first complete decolorization of the blue color
  18. Steps (9) – (17) were repeated for few times.
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*For transferring or carrying different solutions, it was reminded that to repeat the step 1 to step 3.For burette or pipette, it was rinsed with the solutions which would be transferred. Besides, the conical flask should be swirled throughout the titration.                                        

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Data recording

The molarity of the standard sodium thiosulphate: 0.05018M

Calculation

ClO-(aq) +2H+(aq) + Cl-(aq) → Cl2(g) + H2O(l)------------(1)

Cl2(g) + 2I- (aq) +2H+ (aq)→ ...

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