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Determination of Available Chlorine in Bleaching Powder




Bleaching powder is the most commonly used material for bleaching cellulose, cotton and linen. It is also widely used as a disinfectant for potable water. It is prepared by passing chlorine gas over slaked lime at a temperature of 35 - 45°C. Majority of its reactions can be conveniently explained by assuming it to be a mixed calcium hypochlorite - chloride, Ca(OCl)Cl. However, detailed investigations have shown it to be a mixture of the composition

Ca(OCl)2 . CaCl2 . Ca(OH)2 . 2H2O

Bleaching powder evolves chlorine gas when treated with dilute acids.

Ca(OCl)Cl + H2SO4  CaSO4 + H2O + Cl2

The number at grams of chlorine liberated from 100 grams of bleaching powder when treated with dilute acid is referred to as the 'available chlorine'. Continuous decomposition of bleaching powder during storage is its main drawback. It absorbs moisture from atmosphere and evolves chlorine. Due to this deterioration, a little bleaching powder comprises of less quantity of chlorine than estimated. Therefore, each of the samples must be analyzed for its available chlorine.

Aim of the Investigatory Project

The aim of the project is to determine the percentage of 'available chlorine' in a given sample of bleaching powder.


The bleaching powder solution or suspension containing a known mass of bleaching powder is treated with an excess of KI solution and then acidified with dilute H2SO4. Iodine in liberated according to the equation

Ca(OCl)Cl + H2SO4 + 2KI   CaSO4 + 2KCl + H2O + l2

The extracted iodine is titrated by standard solution of sodium thiosulphate.

2Na2S2O3 +I2  Na2S4O6 + 2NaI

From the titre values, the percentage of available chlorine can be calculated.

Apparatus and Chemicals

1. Chemical balance

2. Weighing bottle

3. Standard flasks

4. Burette and pipette

5. Porcelain mortar

6. 0.1N Sodium thiosulphate solution

7. 10% Potassium iodide solution

8. Pure potassium dichromate

9. Starch solution


Mix the sample of bleaching powder well. Weigh accurately about 2 g of it and transfer to a porcelain mortar. Pour some water on to the sample in the mortar and rub with a pestle to get a smooth cream. Add more water, grind with the pestle and allow to settle for a while. Decant the milky solution into a 250 mL standard flask. Grind the residue with more water and pour the liquid into the standard flask as before. Repeat the operation until the sample is quantitatively transferred to the flask. Then make upto the mark and mix well.

Transfer 20 mL of the above solution or suspension to a 250 mL conical flask using a burette. Add 10 mL of 10% solution of potassium iodide and 10 mL of dilute sulphuric acid (These volumes need not be exact). The liberated iodine titrated against thiosulphate solution taken in a burette. When the colour of the solution becomes pale yellow, add 2 mL of freshly prepared starch solution and complete the titration till the blue colour disappears. Repeat to get concordant values.

Standardise the sodium thiosulphate solution using standard potassium dichromate as follows. Weigh accurately about 0.5 g of AR potassium dichromate and transfer to a 100 mL standard flask. Dissolve it in water and make upto the mark. Shake well. Pipette out 20 mL of this solution into a conical flask. Add 5 mL of con HCl and 10 mL of 10% potassium iodide solution. Dilute it to about 100 mL and titrate against the sodium thiosulphate solution taken in the burette. When the colour of the solution becomes pale yellow, add 2 mL of starch solution and continue the titration till the blue colour changes to green.


1. The solution of bleaching powder must be shakened well before every sample is taken out for titration.

2. Since Chlorine vapours are harmful, the solution must not be sucked into the pipette with mouth.

3. The solution being unstable, must be titrated instantly after its preparation.


1. Standardization of sodium thiosulphate solution

Mass of potassium dichromate in 100 mL = w1 g

Equivalent weight of K2Cr2O7 = 49.04

Normality of K2Cr2O7solution = w1 x 10/49.04 = N1

Volume of K2Cr2O7 solution pipetted out = 20 mL

Volume of sodium thiosulphate used up = v1 mL

Vol. of Na2S2O3 x Normality of Na2S2O3 = Vol. of K2Cr2O7 x Normality of K2Cr2O7

v1 x Normality of Na2S2O3 = 20 x N1

Normality of Na2S2O3 solution = 20 x N1/v1 = N2


2. Calculation of available chlorine


Mass of bleaching powder taken in 250 mL = w2 g

Volume of bleaching powder solution taken for each titration = 20 mL

Volume of Na2S2O3 solution used up = v2 mL.

Vol. of bleaching powder solution x Normality of bleaching powder solution = Vol. of Na2S2O3 x Normality of Na2S2O3

i.e., 20 x Normality of bleaching powder solution = v2 x N2

Normality of bleaching powder solution = v2 x N2/20 = N3

Equivalent weight of chlorine = 35.45

Mass of chlorine in the whole of the prepared solution = N3 x 35.45/4

Percentage of available chlorine = N3 x 35.45 x 100/4 x w2


Viva Questions and Answers

1. How is bleaching powder prepared ?

Ans: Bleaching powder is prepared by passing chlorine gas over dry slaked lime at a temperature of 35 - 45°C.

Ca(OH)2 + Cl Ca(OCI)Cl + H2O

2. What happens when bleaching powder is treated with dilute acids?

Ans: See 'introduction' of this project

3. Define 'available chlorine' in bleaching powder ?

Ans: See 'introduction'.

4. The 'available chlorine' in bleaching powder decreases on keeping it for a long time. Why?

Ans: Bleaching powder absorbs moisture from the atmosphere and evolves chlorine slowly. Hence the available chlorine decreases on keeping for a long time.

Ca(OCI)Cl + H2 Ca(OH)2+ Cl2

5. What happens when chlorine is bubbled through potassium iodide solution ?

Ans: Iodine is liberated.

2KI + Cl2  2KCl + I2

6. How does sodium thiosulphate react with iodine ?

Ans: Sodium thiosulphate reacts with iodine to form sodium tetrathionate and sodium iodide.

2Na2S2O3 + I2  Na2S4O6 + 2 NaI

7. How does potassium dichromate liberate iodine from potassium iodide ?

Ans: Acidified potassium dichromate is an oxidising agent. It oxidises KI to I2.

K2Cr2O7 + 4H2SO4  K2SO4 + Cr2(SO4)3 + 4H2O + 3[O]

2KI + H2SO4  K2SO4 + 2HI

2HI + [O]  H2O + I2


8. Explain the bleaching action of chlorine.

Ans: Chlorine reacts with moisture to form oxygen.

Cl2 + H2 HCl + [O]

This oxygen oxidises the colouring matter to colourless products. Thus chlorine bleaches by oxidation.

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