Figure 1- Illustration of chlorine's mode of action on a bacterial cell
Once inside the cell, the oxidation damages important cellular components such as the mitochondrion, which is the powerhouse of the cell. Just a few “hits” of HOCl will inactivate the cell. While -OCl has a negative charge and is repelled by the cell membrane, hypochlorite will oxidize components of the cell wall. This creates significant damage, but it takes many more “hits” before the sufficient damage is done that would inactivate the cell.
Importance of pH
Hypochlorous acid is a weak acid (pKa = 7.5) in aqueous solution and is dissociated readily to -OCl and H+ depending on the pH. At a pH of 7.5, half of the chlorine present is in the form of HOCl and half in the form of -OCl. As shown in Figure 2, with an example of 5 ppm chlorine, at a pH of 7.5, the amount of HOCl, the most active form of chlorine, is at 2.5 ppm; the lower the pH, the higher the concentration of HOCl. Chlorine solutions with pH above 8 are relatively ineffective against pathogens because most of the chlorine is in the ion form. As the pH rises above 7, available chlorine drops rapidly. Additionally, chlorine gas predominates once you get to pH below 4.0, which will not readily stay in solution and can be corrosive to equipment surfaces and materials.
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