No. 19. Water purification using bleaching powder

Aim

To determine the amount of bleaching powder required to purify the given sample of contaminated water.

Practical Significance

The increasing global population has significantly increased the demand for potable water, emphasizing the necessity of identifying suitable water sources for drinking purposes. As many existing water sources are not naturally suitable for consumption, the purification and distribution processes must be carefully managed and organized to fulfill this demand. This practical will focus on teaching students the water purification technique using bleaching powder.

Theory:

Water Purification

Water purification involves the removal of common impurities harmful to human and domestic use, making water safe, clear, colorless, and odorless. Groundwater from deep wells typically requires minimal treatment, whereas surface water sources like rivers need purification to ensure safety.

Chlorination

Polluted water requires disinfection to eliminate pathogenic microorganisms, often achieved through chlorination. Chlorine, in forms such as liquid sodium hypochlorite, gas, or bleaching powder, is a widely used and cost-effective disinfectant. It functions by halogenation, acting as an oxidizing agent that targets microorganisms, iron, manganese, and hydrogen sulfides. Chlorine also neutralizes substances causing taste and odor issues.

Upon adding chlorine to water, it reacts with pollutants, including microorganisms, during a specified contact time. The remaining chlorine, termed residual chlorine, serves as a disinfectant until the water reaches consumers, safeguarding it against potential pathogens. Water’s chlorine demand, indicating the required chlorine amount for microbial and organic matter oxidation, should be assessed: Chlorine demand = Amount of chlorine added – Residual chlorine

Bleaching powder, or chlorinated lime, is a white powder with a distinctive odor used for disinfecting wells, tanks, rivers, and canals. The Horrock’s Test quantifies the bleaching powder needed for effective water disinfection, employing a specialized apparatus with cups for the powder and water samples, along with a starch-iodide indicator solution.

Requirements

Contaminated water sample, black cup (200 mL), 6 white cups (200 mL), 7 glass rods, pipette, droppers, starch-iodide indicator solution, bleaching powder, two metal spoons.

Procedure:

1. Take 1 level spoonful (2 gm) of bleaching powder in the black cup and make it into a thick paste. Add water up to the circular mark. Allow it to settle. This is the Chlorine Stock Solution.
2. Fill all six white cups with water to be tested and label them from 1 to 6.
3. Add one drop of chlorine stock solution to cup No. 1, 2 drops to cup No. 2, 3 drops to cup No. 3, and so on (Use a special pipette for adding the stock solution).
4. Stir each cup with a separate glass rod.
5. Allow them to stand for 30 minutes for the chlorine to take effect.
6. Add a drop of starch-iodide indicator solution to each white cup and stir again (Starch-iodide solution: Dissolve 0.5 gm of soluble starch in 10 mL of water containing 0.5 gm of potassium iodide).
7. A blue color will develop in some cups, indicating the presence of free residual chlorine.
8. Note the first cup number showing a distinct blue color. For example, if cup No. 3 shows the color first, then 3rd level, i.e., 6 gm of bleaching powder, is required to disinfect 455 liters of water (1 level = 2 gm of bleaching powder).
9. Calculate the amount of bleaching powder required to disinfect the given volume of contaminated water.

Observation Table

Cup Number	Appearance of blue colour
1	Yes / No
2	Yes / No
3	Yes / No
4	Yes / No
5	Yes / No
6	Yes / No

Volume of contaminated water given (V) = _________________________ mL

First cup number in which blue colour appeared (N) = ________________

Calculations:

Amount of bleaching powder required to disinfect 455 litres of water (A) grams.

A = N × 2

A = ______× 2

A = ___________ gm.

Amount of bleaching powder required to disinfect V litres of contaminated water (X)

X = (A x V) /455

X =

X = ___________ gm

Result

The amount of bleaching powder required to disinfect a given volume of contaminated water is ________g.

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