Bleaching Powder

Bleaching Powder, chemically known as Calcium Oxychloride, is an important industrial chemical widely used for bleaching, disinfecting, and water purification. It is represented by the chemical formula Ca(OCl)₂ and is commonly referred to as chlorinated lime. The compound appears as a white or slightly yellowish powder, emitting a strong smell of chlorine due to its tendency to release chlorine gas slowly in moist air.
Bleaching powder is an essential compound in sanitation, textile processing, paper manufacturing, and public health. It serves as both a bleaching agent and a disinfectant, playing a vital role in maintaining hygiene and environmental safety.

Chemical Composition and Formula

The approximate composition of bleaching powder can be represented as:
Ca(OCl)2=CaCl2+Ca(OH)2+Cl2Ca(OCl)_2 = CaCl_2 + Ca(OH)_2 + Cl_2Ca(OCl)2​=CaCl2​+Ca(OH)2​+Cl2​
However, it is not a simple mixture of calcium chloride and calcium hypochlorite. The actual structure is more complex, with calcium cations and hypochlorite ions held within a solid matrix.
Chemical Name: Calcium OxychlorideChemical Formula: Ca(OCl)₂Molecular Weight: 142.98 g/mol

Preparation of Bleaching Powder

Bleaching powder is prepared by passing chlorine gas over dry slaked lime (Ca(OH)₂) at controlled temperatures in a special apparatus called a Chamber Process or Hasenclever Plant.
The chemical reaction involved is:
2Ca(OH)2+2Cl2→Ca(OCl)2+CaCl2+2H2O2Ca(OH)_2 + 2Cl_2 \rightarrow Ca(OCl)_2 + CaCl_2 + 2H_2O2Ca(OH)2​+2Cl2​→Ca(OCl)2​+CaCl2​+2H2​O
In this process, one part of the lime reacts to form calcium oxychloride (bleaching powder), while the other part produces calcium chloride as a by-product.
The reaction must be conducted under dry conditions, as the presence of excess moisture leads to the liberation of chlorine gas and decomposition of the product.

Physical Properties

• Appearance: White or pale yellow powder
• Odour: Characteristic smell of chlorine
• Solubility: Moderately soluble in water; forms a milky suspension due to insoluble impurities
• Stability: Decomposes on exposure to air and moisture, releasing chlorine gas
• Taste: Slightly bitter and irritating (though it should never be tasted directly)

When exposed to moisture or carbon dioxide in the atmosphere, bleaching powder gradually loses its chlorine content, making it less effective over time.

Chemical Properties

1. Decomposition in Moist Air:

   Ca(OCl)2+CO2+H2O→CaCO3+2HClOCa(OCl)_2 + CO_2 + H_2O \rightarrow CaCO_3 + 2HClOCa(OCl)2​+CO2​+H2​O→CaCO3​+2HClO
   The hypochlorous acid (HClO) formed is responsible for the bleaching and disinfecting properties.

2. Reaction with Dilute Acids: When bleaching powder reacts with dilute acids, it releases chlorine gas:

   Ca(OCl)2+2HCl→CaCl2+H2O+Cl2Ca(OCl)_2 + 2HCl \rightarrow CaCl_2 + H_2O + Cl_2Ca(OCl)2​+2HCl→CaCl2​+H2​O+Cl2​
   This reaction is used for producing chlorine gas in laboratories.

3. Reaction with Water: In water, bleaching powder produces hypochlorous acid, which acts as the active bleaching and disinfecting agent:

   Ca(OCl)2+H2O→Ca(OH)2+Cl2Ca(OCl)_2 + H_2O \rightarrow Ca(OH)_2 + Cl_2Ca(OCl)2​+H2​O→Ca(OH)2​+Cl2​

4. Oxidising Property: The hypochlorous acid generated oxidises coloured substances, converting them into colourless compounds. For instance, when used on coloured organic matter:

   Coloured compound+[O]→Colourless compound\text{Coloured compound} + [O] \rightarrow \text{Colourless compound}Coloured compound+[O]→Colourless compound

Uses of Bleaching Powder

1. As a Disinfectant:
   • Used for disinfecting drinking water and swimming pools.
   • Employed for sterilising public places, hospitals, drains, and sewage systems.
   • Widely used in sanitation drives and disease control programmes.
2. As a Bleaching Agent:
   • Used in the textile industry for bleaching cotton and linen fabrics.
   • Applied in the paper and pulp industry to whiten paper and wood pulp.
   • Employed in laundry work for whitening clothes.
3. In Chemical Industry:
   • Used in the manufacture of chloroform (CHCl₃), DDT, and other chlorinated organic compounds.
   • Acts as an oxidising agent in chemical processes.
4. In Water Treatment:
   • Commonly used for chlorination of drinking water to kill pathogenic microorganisms.
   • Helps control algae and bacterial growth in reservoirs and water storage tanks.
5. For Deodorising and Sanitising:
   • Used in toilets, garbage dumps, and animal shelters to reduce foul odours and kill germs.
6. For Emergency Use:
   • In epidemics or natural disasters, bleaching powder is distributed for water disinfection and sanitation due to its low cost and effectiveness.

Advantages

• Economical and easy to produce.
• Readily available and easy to handle.
• Effective both as a bleaching and disinfecting agent.
• Can be stored for a reasonable period under dry conditions.
• Useful for large-scale water purification and sanitation.

Disadvantages

• Instability: Decomposes on exposure to air and moisture, losing available chlorine.
• Corrosiveness: Can corrode metals and irritate skin or eyes upon contact.
• Chlorine Odour: The pungent smell may cause discomfort and respiratory irritation.
• Environmental Concerns: Excessive use may lead to the formation of chlorinated by-products harmful to the environment.

Available Chlorine and Strength

The effectiveness of bleaching powder is measured by its available chlorine content, representing the amount of chlorine that can be liberated from the compound under acidic conditions.
Typically, good quality bleaching powder contains about 33–36% available chlorine. Over time, due to decomposition, this percentage decreases, reducing its efficiency as a disinfectant.

Storage and Handling

• Should be stored in airtight containers in a cool, dry, and dark place.
• Must be kept away from moisture, acids, and organic materials.
• Contact with combustible substances should be avoided due to the risk of fire from oxidation.

Environmental and Safety Aspects

While bleaching powder is indispensable in sanitation and water treatment, its use must be carefully regulated:

• Excessive chlorination can lead to chlorinated hydrocarbons, which are toxic and persistent in the environment.
• Direct exposure can cause skin irritation, eye damage, and respiratory problems.
• Proper dilution and controlled use are essential to minimise health and environmental hazards.