The sterilization principle of chlorine
Chlorine (Cl2) has the effect of bleaching and sterilizing, but it is unstable and toxic because it exists in the atmosphere as a gas (Cl2), so it was difficult to put it into practical use.
Therefore, attempts were made to dissolve it in water to stabilize it and make it into an aqueous solution. When chlorine dissolves in water, it produces a small amount of hydrochloric acid (HCl) and hypochlorous acid (HOCl), and not all dissolved chlorine reacts with water. This process is a reversible reaction (a reaction in which one substance is oxidized and reduced at the same time), and the equilibrium point of the reaction is mainly on the left side, as shown in the reaction equation below (Ullmann, 1986). This reaction is dangerous because chlorine is very poorly soluble in water (0.3% to 0.7%) and dissolved chlorine is released as gas over time. In addition, as chlorine is released, the pH of the solution drops significantly due to the strong acid HCl, making it highly corrosive, making it difficult to easily use as a bleach or disinfectant.
Afterwards, chlorine was dissolved in a salt compound solution instead of water, and an alkali metal was substituted for the hydrogen of hypochlorous acid to create a stable and more concentrated form of effective chlorine solution. This was soon commercialized and became known as Odjabel. A representative hypochlorite is sodium hypochiorite (NaOCl), which is the ingredient in bleach that we use today.
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| The main ingredient in Clorox is sodium hypochlorite. |
When NaOCl reacts with water, it produces hypochlorous acid (HOCl) and sodium hydroxide (NaOH), which are effective chlorine. At this time, HOCl is a weak acid and reversibly dissociates into the hypochlorite ion (OCl-).
Chlorine is found in water in three forms: Cl2, HOCl, and OCl-. Cl2 is easily vaporized and disappears, so there are only two effective chlorine forms: HOCl and OCl-, both of which have the function of disinfecting. HOCl and OCl- can coexist, but their ratio varies greatly depending on the pH of the solution, which also affects the chemical stability of the solution (Biocontrol Sci. 2006).
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| The ratio of HOCl and OCl- varies with pH. |
The sterilizing activity of HOCl is 80-100 times more effective than that of OCl-, so the sterilizing activity of NaOCl is determined by the concentration of HOCl.
HOCl has a small molecular size and is electrically neutral, so it easily penetrates the cell wall and cell membrane of microorganisms and passively diffuses, affecting both the outside and inside of the cell, whereas OCl- cannot penetrate the cell wall, which is electrostatically repelled by the negatively charged cell wall, and thus only acts as an oxidizing agent on the cell surface.
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HOCl, OCl- of laxatives both have a bactericidal effect by damaging cell membranes and DNA and other cellular components inside and outside the cell mainly through oxidation (Arch. Biochem. Biophys 1999) and react with various biological molecules such as proteins, amino acids, peptides, and lipids, so they are greatly affected by organic matter.
First, fatty acid reacts with NaOH generated when NaOCl meets water, transforming into fatty acid salt (Soap) and glycerol (alcohol), reducing the surface tension of the solution. This is called saponification reaction, and the dissolution of organic tissue can be confirmed at this time.
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| Clorox breaks down the fat and saponifies it. (Saponification reaction) |
NaOH reacts with amino acids to form water and salt, and the pH drops as OH ions are removed (amino acid neutralization reaction).
 Clorox reacts with amino acids to form salts. (Amino acid neutralization reaction) |
When HOCl combines with an organic protein amino group (NH), chloramine is created. Chloramine is the substance that causes the smell of swimming pools, and it interferes with cell metabolism and has an antibacterial effect.
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| HOCl decomposes the protein components of organic matter. (chloramination reaction) |
Usually, since Clorox is used in a diluted form (pH > 8 when diluted), it goes through the same path as above, but if the concentration is high, the bactericidal effect is based on the high pH and OCl- oxidation acting on the cell surface (Estrela et al., 2002). Clorox is a strong alkaline substance with a pH of > 11, and when the pH balance of microbial cells is disturbed by the action of hydroxide ions (OH-), it can be expected to have a bactericidal and disinfecting effect by inhibiting biochemical enzyme reactions, inhibiting cell metabolism, and decomposing phospholipids of cell membranes to increase cell permeability or inducing cell lysis.
The higher the concentration, the better the antibacterial effect and organic matter decomposition ability, but it is inversely proportional to biocompatibility. In other words, it is more toxic to humans. Considering that Clorox can have an antibacterial effect even at low concentrations, it is appropriate to use 1% sodium hypochlorite (Braz Dent J 2002).
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