Question

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

Answer 1

Chlorination is the process of adding chlorine to water in order to kill germs. Chlorine is oe of the most common and effective disinfectant for water disinfection. It is also widely used because of low cost involved in its application.

Chlorine as a disinfectant

Chlorine kils germs and pathogens (viruses and bacteria) by breaking the chemical bonds present in their molecules. When chlorine is added to water, it starts reacting with the active enzymes present in bacterial cells and one or more hydrogen atoms of the enzyme are effctively replaced by Chlorine atoms.This causes the entire molecule to distort and its structure may fall apart. Ultimately, as enzymes do not function properly, the bacterial cells die.

When chlorine is added to water, it forms hypochlorous acid

Cl₂ + H₂O $\rightarrow$ HOCl + H⁺ + Cl^-

HOCl is a strong disinfectant and is the active ingredient that kills pathogens. HOCl reacts to form OCl^- ions at different pH values

HOCl + H₂O $\rightarrow$ OCl^- + H₃O⁺  

OCl^- (hypochlorite ions) is also an active killing agent. However, HOCl is more effective (approximately eight times more) than OCl^- because it is uncharged and penetrates the cell wall of bacterias easily