Question

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

Short answer

The active agent in chlorination is hypochlorous acid (HOCl). Practical sources include chlorine gas, sodium hypochlorite, and calcium hypochlorite.

Step-by-step solution

Understanding Chlorination

Chlorination is the process of adding chlorine or chlorine compounds to water to disinfect it by killing bacteria, viruses, and other pathogens. This makes it safe for drinking and other uses.

Identifying the Active Agent

In the disinfection process, the active agent is hypochlorous acid (HOCl), which is formed when chlorine is added to water. Hypochlorous acid is a powerful oxidizing agent that can penetrate the cell walls of microorganisms and destroy them by disrupting their cellular processes.

How Chlorine Works

When chlorine is added to water, it reacts to form hypochlorous acid and hypochlorite ions. The chemical equation is: \[ Cl_2 + H_2O \rightarrow HOCl + H^+ + Cl^- \]Hypochlorous acid is the most effective form for disinfection, as it is more efficient than hypochlorite ions, especially at neutral pH levels.

Practical Sources of Chlorine

Practical sources of chlorine for water disinfection include elemental chlorine (Cl₂), sodium hypochlorite (NaOCl, commonly found in bleach), and calcium hypochlorite (Ca(OCl)₂). These compounds are added to water to release chlorine, which forms hypochlorous acid, the active disinfecting agent.

Key concepts

Chlorination

Chlorination is a widely utilized method for purifying water by eliminating harmful microorganisms. It involves introducing chlorine or its compounds into water to act as a disinfectant. This process is crucial as it helps in making water safe for human consumption and use in daily activities. When chlorine is added to water, it begins a reaction that results in the formation of hypochlorous acid and hypochlorite ions, responsible for killing pathogens. These tiny invaders can cause diseases, so it's essential to remove them from water supplies.

A few benefits of using chlorination include:

• Effectively kills bacteria, viruses, and other pathogens.
• This method has been used for over a century, which establishes its reliability.
• It's adaptable to various sizes of water treatment facilities, from small community systems to vast urban water supplies.

Chlorination remains a preferred choice due to its effectiveness and cost-efficiency.

Hypochlorous Acid

When chlorine interacts with water, a crucial component known as hypochlorous acid (HOCl) is formed. Hypochlorous acid is the active ingredient responsible for pathogen destruction in the chlorination process. Its formation begins with the addition of chlorine to water:\[ Cl_2 + H_2O \rightarrow HOCl + H^+ + Cl^- \]This chemical reaction highlights how chlorine turns into active forms that play a significant role in water disinfection.

Hypochlorous acid stands out because of the following reasons:

• It is a powerful oxidizing agent capable of penetrating the cell walls of microorganisms.
• It disrupts vital cellular processes in pathogens, leading to their destruction.
• Compared to hypochlorite ions, HOCl is more effective, especially at neutral pH levels, increasing its efficiency in disinfecting water.

Understanding the chemistry of hypochlorous acid helps in appreciating its crucial role in maintaining water quality.

Pathogen Destruction

Pathogens are microorganisms like bacteria and viruses that can cause diseases if present in drinking water. The goal of water disinfection, such as chlorination, is to destroy these harmful entities. Hypochlorous acid is exceptionally efficient at breaking down these microorganisms. Here's how it does it:

When hypochlorous acid comes in contact with pathogens, it penetrates their cell membranes due to its neutral charge and small size. Once inside, it causes biochemical reactions that disturb their normal functioning, leading to their eventual death.

Pathogen destruction through chlorination offers several advantages:

• It provides reliable and rapid inactivation of harmful microorganisms.
• Maintains the safety and quality of water over extended periods, even after initial treatment.
• Reduces the risk of waterborne diseases and keeps communities healthier.

By understanding these destruction mechanisms, it's easier to see why chlorination is a cornerstone in water treatment processes.

Chlorine Compounds

In chlorination, various chlorine compounds are used to release chlorine into water, where it forms hypochlorous acid, integral to the disinfection process. Some of the primary compounds include:

• Elemental Chlorine (Cl₂): Directly added to water, it promptly reacts to form hypochlorous acid and hypochlorite ions.
• Sodium Hypochlorite (NaOCl): Often found in household bleach, it's a liquid form of chlorine that's easy to apply and handle.
• Calcium Hypochlorite (Ca(OCl)₂): Available in a solid form, this compound steadily releases chlorine into water as it dissolves.

Sourcing chlorine using these compounds is practical and convenient, each having unique benefits that suit different applications. For instance, sodium hypochlorite is commonly used for small-scale operations due to its ease of use. On the other hand, elemental chlorine is preferred for larger, centralized water treatment plants.

Different situations demand tailored chlorine solutions, but their collective goal remains the same: ensuring safe, clean water for all.