Chlorine is one of the most widely used disinfectants. It is very applicable and very effective for the deactivation of pathogenic microorganisms. Chlorine can be easily applied, measured and controlled. It is fairly persistent and relatively cheap.
Chlorine has been used for applications, such as the deactivation of pathogens in drinking water, swimming pool water and wastewater, for the disinfection of household areas and for textile bleaching, for more than two hundred years. When chlorine was discovered we did not now that disease was caused by microorganisms. In the nineteenth century doctors and scientists discovered that many diseases are contagious and that the spread of disease can be prevented by the disinfection of hospital areas. Very soon afterward, we started experimenting with chlorine as a disinfectant. In 1835 doctor and writer Oliver Wendel Holmes advised midwives to wash their hands in calcium hypochlorite (Ca(ClO)2-4H2O) to prevent the spread of midwives' fever.
However, we only started using disinfectants on a wider scale in the nineteenth century, after Louis Pasteur discovered that microorganisms spread certain diseases.
Chlorine has played an important role in lengthening the life expectancy of humans. Surfaces can be disinfected by bleaching. Bleach consists of chlorine gas dissolved in an alkali solution, such as sodium hydroxide (NaOH). When chlorine is dissolved in an alkalic solution, hypochlorite ions (OCl-) are formed during an autoredox reaction. Chlorine reacts with sodium hydroxide to sodium hypochlorite (NaOCl). This is a very good disinfectant with a stable effect.
Bleach cannot be combined with acids. When bleach comes in contact with acids the hypochlorite becomes unstable, causing poisonous chlorine gas to escape. The accompanying hypochloric acid is not very stable.
Bleaching powder (CaOCl2) can also be used. This is produced by directing chlorine through calcium hydroxide (CaOH). The benefit of bleaching powder is that it is a solid. This makes it easier to apply as a disinfectant in medical areas, next to its use as a bleach. When bleaching powder dissolves, it reacts with water to hypochloric acid (HOCl) and hypochlorite ions (OCl-).
How does chlorine disinfection work?
Chlorine kills pathogens such as bacteria and viruses by breaking the chemical bonds in their molecules. Disinfectants that are used for this purpose consist of chlorine compounds which can exchange atoms with other compounds, such as enzymes in bacteria and other cells. When enzymes come in contact with chlorine, one or more of the hydrogen atoms in the molecule are replaced by chlorine. This causes the entire molecule to change shape or fall apart. When enzymes do not function properly, a cell or bacterium will die.
Underchloric acid (HOCl, which is electrically neutral) and hypochlorite ions (OCl-, electrically negative) will form free chlorine when bound together. This results in disinfection. Both substances have very distinctive behavior. Underchloric acid is more reactive and is a stronger disinfectant than hypochlorite. Underchloric acid is split into hydrochloric acid (HCl) and atomair oxygen (O). The oxygen atom is a powerful disinfectant.
The disinfecting properties of chlorine in water are based on the oxidizing power of the free oxygen atoms and on chlorine substitution reactions.
The cell wall of pathogenic microorganisms is negatively charged by nature. As such, it can be penetrated by the neutral hypochloric acid, rather than by the negatively charged hypochlorite ion. Underchloric acid can penetrate slime layers, cell walls and protective layers of microorganisms and effectively kills pathogens as a result. The microorganisms will either die or suffer from reproductive failure.
The effectivity of disinfection is determined by the pH of the water. Disinfection with chlorine will take place optimally when the pH is between 5.5 and 7.5. Hypochloric acid (HOCl) reacts faster than hypochlorite ions (OCl-); it is 80-100% more effective. The level of underchloric acid will decrease when the pH value is higher. With a pH value of 6 the level of hypochloric acid is 80%, whereas the concentration of hypochlorite ions is 20%. When the pH value is 8, this is the other way around.
When the pH value is 7.5, concentrations of hypochloric acid and hypochlorite ions are equally high.
Ankur Choudhary is an experienced pharmaceutical blogger.