Question

Explain the chemistry underlying the disinfection of water by chlorination. What is the active agent in the destruction of the pathogens? What are the practical sources of the active ingredient?

Short answer

The active agent is hypochlorous acid, formed when chlorine reacts with water. Practical sources are gaseous chlorine, sodium hypochlorite, and calcium hypochlorite.

Step-by-step solution

Introduction to Water Disinfection

Water disinfection is an essential process to ensure that water is safe to drink by eliminating harmful pathogens. Chlorination is one of the most widely used methods for disinfection of water.

Understanding Chlorination

Chlorination involves the addition of chlorine or chlorine compounds to water. This process is effective in destroying a wide range of pathogens including bacteria, viruses, and protozoans.

The Chemistry of Chlorination

When chlorine is added to water, it reacts to form a variety of active species. The primary chemical reaction is between chlorine (Cl_2) and water (H_2O), resulting in hypochlorous acid (HOCl) and hydrochloric acid (HCl): \[ Cl_2 + H_2O \rightarrow HOCl + HCl \].

Role of Hypochlorous Acid

The hypochlorous acid (HOCl) formed is a weak acid and a strong oxidant. It is the principal active agent responsible for destroying pathogens by penetrating their cell walls and oxidizing their components, leading to cell death.

Sources of Chlorine

Practical sources of chlorine used for water disinfection include gaseous chlorine (Cl_2), sodium hypochlorite (NaOCl, found in household bleach), and calcium hypochlorite (Ca(OCl)_2, often used in granule or tablet form). These substances generate free chlorine that acts as a disinfectant when dissolved in water.

Key concepts

Chlorination

Chlorination is a widely adopted method for disinfecting water. It ensures the safety of drinking water by effectively reducing the presence of harmful microorganisms. The process involves the addition of chlorine or chlorine-based compounds to water.
Through this method, a wide array of pathogens can be neutralized, including bacteria, viruses, and protozoans. This makes it a crucial step in water treatment processes across the world.
Chlorination not only helps in maintaining the hygiene of public and private water systems but also significantly reduces the risk of waterborne diseases.

Hypochlorous Acid

Hypochlorous acid (HOCl) is the key player in the chemistry of water disinfection through chlorination. When chlorine is introduced into water, it reacts, producing hypochlorous acid among other compounds.
It is formed through the chemical reaction: \[ Cl_2 + H_2O \rightarrow HOCl + HCl \]
This compound is a weak acid yet a potent oxidizing agent capable of penetrating and disrupting the cell walls of various pathogens. Its ability to oxidize effectively destroys these harmful microorganisms, making the water safe to consume.

Pathogen Destruction

The destruction of pathogens is the primary goal of chlorination and is largely facilitated by hypochlorous acid. This compound acts by oxidizing the pathogen's cellular components.
Here’s how it works:

• HOCl penetrates the cell walls of pathogens
• It disrupts vital cellular processes through oxidation
• This oxidative reaction leads to the destruction or deactivation of the microorganisms

The breakdown of these pathogens ensures that the water is safe from potentially harmful biological contaminants.

Sodium Hypochlorite

Sodium hypochlorite (NaOCl) is a commonly used source of chlorine for water disinfection. Found in household bleach, NaOCl provides a practical way to generate free chlorine when dissolved in water.
Its popularity stems from its ease of handling and effectiveness in disinfection. Upon addition to water, sodium hypochlorite dissociates to release hypochlorous acid, which then facilitates the elimination of pathogens. This makes it an accessible and effective choice for both industrial and domestic water treatment applications.

Calcium Hypochlorite

Calcium hypochlorite (Ca(OCl)_2) is another popular source of chlorine for disinfecting water. Available in glassy granules or tablet forms, it is often used due to its high chlorine content and stability.
When dissolved in water, calcium hypochlorite releases chlorine, which forms hypochlorous acid essential for pathogen destruction.
Its use is prevalent in swimming pools and water systems where tablets or granular forms are convenient, providing a controlled and steady release of disinfectant over time.