analysis of two commercial brands of bleaching solution

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F.6A Lam Pik Sum (10)

Title: analysis of two commercial brands of bleaching solution

Date: 6/10/2008

Objective: to find out which of the two brands of bleach is cheaper base on their actual bleaching strength.

Introduction

Redox titration can be divided into two parts: in one half the reducing agent loses electrons and in other half the oxidizing agent gains electrons. The stoichiometric equation for a redox reaction may then derive using the fact that all the electrons lost by the reducing agent must be gained by the oxidizing agent.

In this experiment, we need to find out the cost of active ingredient per gram in two different brands of household bleaches in other to find out with bleach is cheaper. The two bleaches we use are KAO ($13.9 for 1.5L) and Best Buy ($11.9 for 2L).

Sodium hypochlorite forms the basis of most of commercial bleaches. In this analysis, the sodium hypochlorite is allowed to react with an excess of potassium iodide solution in the presence of acid, liberating iodine, which is then titrated against standard sodium thiosulphate solution.

Reactions involved:         ClO + 2 I + 2 H →  I2 + H2O + Cl

        I2 + 2 S2O32- → 2 I + S4O62-

In this redox titration, potassium iodide in the acidic medium acts as a reducing agent is added to the bleach solution to generate the iodine by the reduction of the hypochlorite ions. The formed iodine is then back-titrated with sodium thiosulphate to reduce iodine to iodide ions while sodium thiosulphate is being oxidized in order to determine the amount of hypochlorite ions originally present.

When the brown colour of iodine fades as the end point approaches, a little amount of starch solution is added. Starch solution acts as an indicator is added to clarify the end point of the titration. Since starch forms a dark blue aqueous complex with iodine, the disappearance of the dark blue colour indicates that there is no iodine in the reaction mixture, that means the end point of the titration is reached.

Procedures

For each brand of bleach, carry out the following:

  1. Both volumes and the prices of the bleaches were recorded.
  2. 10 cm3 of bleach was measured into a volumetric flask using a pipette.
  3. Distilled water was added to the volumetric flask until reaching the graduation mark.
  4. 25 cm3 of this solution was pipetted into a 250 cm3 conical flask.
  5. 10-15 cm3 of 1M potassium iodide solution was added to the conical flask.
  6. 10-15 cm3 of 1M potassium iodide solution was added to the conical flask.
  7. Standard sodium thiosulphate solution was added to the burette.
  8. The initial reading on the burette was recorded
  9. Standard sodium thiosulphate solution was run out from the burette to the conical flask until the colour of the solution in the conical flask changed from reddish brown to pale yellow..
  10. Few drops of starch solution were added to the solution in the conical flask.
  11. Standard sodium thiosulphate solution was run out from the burette to the conical flask until the colour of the solution in the conical flask changed from blue-black to colourless.
  12. The final reading on the burette was recorded.
  13. Titrations were repeated until the normal degree of consistency is obtained.
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Data of results

Titration table of KAO bleach with KI and H2SO4 against standard 0.07676M sodium thiosulphate

Average volume of sodium thiosulphate used

= (19.9 + 20.0 + 19.9) ÷ 3

= 19.93 cm3

20.1 cm3is rejected because this is just a trial.

Titration table of Best Buy bleach with KI and H2SO4 against standard 0.07676M sodium thiosulphate

Average volume of sodium thiosulphate used

= (14.1 + 14.2 + 14.2) ÷ 3

= 14.167 cm3

14.5 cm3 is rejected because this is just a trial.

Calculation

KAO

In ...

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