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Desinfeccion - Lejia - Mecanismo de desinfeccion - Cloro activo - Dosis - Descomposicion en la cloronizacion
Concentraciones - efectividad - Efectos para la salud - Legislacion
Figure 1: Carl Wilhelm Scheele discovered chlorine in 1774
Scheele discovered that chlorine gas was water-soluble and that it could be used to bleach paper, vegetables and flowers. It also reacted with metals and metal oxides. In 1810 sir Humphry Davy, an English chemist who tested fundamental reations of chlorine gas, discovered that the gas Scheele found must be an element, given that the gas was inseperable. He named the gas ‘chlorine’ (Cl), after the Greek word ‘chloros’, which means yellow-greenish and refers to the color of chlorine gas (White, 1999. Watt, 2002)
Figure 2: chlorine atoms contain 17 electrons
Chlorine can form very stable substances, such as kitchen salt (NaCl). Chlorine can also form very reactive products, such as hydrogen chloride (HCl). When hydrogen chloride dissolves in water it becomes hydrochloric acid. The hydrogen atom gives off one electron to the chlorine atom, causing hydrogen and chlorine ions to form. These ions react with any kind of substance they come in contact with, even metals that are corrosion resistant under normal circumstances. Concentrated hydrochloric acid can even corrode stainless steel. This is why it is stored either in glass or in plastic.
Figure 3: products containing chlorine
Chlorine-based bleach is applied as a disinfectant on a large scale. The substances are also used to bleach paper. Bleaching occurs as a result of chlorine or hypochlorite oxidation.
For more information about pathogens in aquatic systems, please take a look at pathogens in freshwater ecosystems
Figure 4: chlorine is often used as a bleach
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 underchloric acid (HOCl) and hypochlorite ions (OCl-).
Figure 5: the neutral underchloric acid can better penetrate cell walls of pathogenic microorganisms that the negatively charged hypochlorite ion
The cell wall of pathogenic microorganisms is negatively charged by nature. As such, it can be penetrated by the neutral underchloric 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.
Underchloric acid (left) : hypochlorite ions (right)
What is free and bound active chlorine?
Table 1: disinfection time for several different types of pathogenic microorganisms with chlorinated water, containing a chlorine concentration of 1 mg/L (1 ppm) when pH = 7,5 and T = 25 °C
What are the health effects of chlorine?
More information on water disinfection?:
What is water disinfection? Necessity of drinking water disinfection History of water disinfection Waterborne diseases Factors that influence disinfection Conditions of water disinfection Regulation drinking water disinfection EU USA