How Does Chlorine Work to Kill Bacteria?

Ann Johnson

Chlorine was first made in 1774 by Swedish chemist, Carl Wilhelm Scheele, who believed it contained oxygen. He did this by treating muriatic acid with manganese dioxide.

How Does Chlorine Work to Kill Bacteria?

What is Chlorine

Thirty-six years later the English chemist, Sir Humphry Davy insisted it was a chemical element and gave it its name, which is derived from a Greek word meaning greenish-yellow. The substance is a poisonous gas, yet when combined with metal sodium it makes table salt. Chlorine is found in chloride minerals, which occur naturally in salt lakes, sea water and in rock salt deposits. It is a member of the halogen group of elements.

How it is Used

Chlorine is commonly used to kill bacteria in water. It is widely used to purify swimming pool, spa and drinking water. When it is dissolved in sodium hydrate it can be made into chlorine bleach or disinfectant. Disinfectant is used to kill germs, and chlorine bleach is used to whiten clothes and to disinfect. Chlorine bleach can also be used to sanitize well water.

How it Works

When chlorine is poured into water it breaks into several chemical compounds, including hypochlorous acid and hypochlorite ion. The combination of hypochlorous acid and hypochlorite ion is a reaction called "free chlorine." Both of these substances attack the microorganisms and bacteria in the water by going after the lipids in their cell walls and destroying the enzymes. As they destroy the structure inside the cells the chemical compounds leave the bacteria cells oxidized, which kills the cell, leaving it harmless.

Hypochlorous Acid vs. Hypochlorite Ion

Hypochlorite ion carries a negative electrical charge, while hypochlorous acid carries no electrical charge. The hypochlorous acid moves quickly, able to oxidize the bacteria in a matter of seconds, while the hypochlorite ion might take up to a half hour to do the same. Germ surfaces carry a negative electrical charge which results in a repulsion of the negatively charged hypochlorite ion to the area of the germ surfaces, making hypochlorite ion less effective at killing germs. The ratio of the two compounds is determined by the relative acidity (pH) of the water. Water treatment specialists can adjust the pH level to make hypochlorous acid more dominate, as it is more efficient at killing bacteria. The hypochlorous acid's lack of electrical charge allows it to more efficiently penetrate the protective barriers surrounding germs.