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Hypochlorous acid is a free chlorine molecule with the chemical structure HOCl. It is the dominant free chlorine species in chlorine solutions with a slightly acidic to neutral pH. HOCl is a more powerful oxidizer than sodium hypochlorite (or chlorine bleach).
Hypochlorous acid can be naturally produced by mammalian white blood cells. It is used by white blood cells to kill invading microbial pathogens
How does it kill microbial pathogens?
Hypochlorous acid (HOCl) is a neutrally charged molecule. Bacteria have negatively charged cell walls. Like magnets, molecules with the same electric charge also repel each other. For example, negatively charged bleach molecules (OCl-) repel each other from bacterial cell walls. HOCl has a neutral charge, so this is not the case. HOCl easily penetrates bacterial cell walls. HOCl can either oxidize the cell wall to kill bacteria or enter through the cell wall and destroy important components within the bacteria.
Are there any disadvantages?
Unlike other sanitation chemicals, hypochlorous acid does not have a sustained antimicrobial effect. In other words, once it lands on a surface, it reacts with any bacteria or organic matter on that surface and is immediately deactivated. This is good and bad. The good thing is that it doesn't need to be rinsed after use as there are no toxic chemicals left. The downside is that its application has to be guaranteed on an ongoing basis.
Hypochlorous acid is made through a process called electrolysis. Electrolyzed water is produced by passing a sodium chloride solution (NaCl) through an electrolytic cell containing an anode and a cathode. There are two commonly used electrolysis methods to generate hypochlorous acid, membrane electrolysis and single cell electrolysis. Membrane electrolysis converts brine into two solutions, a strongly acidic anolyte of hypochlorous acid and a strongly alkaline catholyte of sodium hydroxide. Single-cell electrolysis converts brine into a solution, the slightly acidic to neutral anolyte of hypochlorous acid.
Hypochlorous acid can be prepared from chlorine bleach by dilution, but there are limitations. Hypochlorous acid is almost absent in free chlorine solutions with a pH above 9. Chlorine bleach has a pH greater than 13. By diluting the chlorine bleach, the pH can be lowered, but the concentration of free chlorine will also be lowered. When diluting chlorine bleach to pH 8.5, the percentage of free chlorine in hypochlorous acid is less than 5%. Further dilution will dilute the free available chlorine concentration to useless levels. Attempting to lower the pH with an acidifier also won't help, as chlorine bleach reacts violently and free available chlorine product is lost as chlorine gas. Therefore, electrolysis is the only safe way to generate high concentrations of free chlorine solutions with hypochlorous acid as the predominant acid to neutral pH. At pH 5, the free chlorine percentage of hypochlorous acid is higher than 99%.
Depending on the process that produces hypochlorous acid, the solution can be stable. Membrane electrolysis produces two streams with opposite redox potentials and opposite pH values. This is done by forcing positively charged sodium ions across the membrane to the cathode side. On the anode side, a strongly acidic (~pH 3) highly concentrated anolyte is created. Hypochlorous acid production by this method is not as stable as that produced by single-cell techniques. The single-cell technique uses acidified saline and generates only one stream of solution in the pH range of 5-7. When hypochlorous acid is produced by a single cell, no high pressure is used and no ions are forced across the membrane. The hypochlorous acid produced is stable and does not constantly seek new balances like the anolyte produced by membrane electrolysis cell systems.
The shelf life can reach 3-6 months if stored in an airtight container to avoid exposure to oxygen in the air. UV-resistant containers may extend shelf life.
Hypochlorous acid is a strong oxidant that will always try to get an electron from another molecule. Synthetic surfaces have difficulty getting electrons from organic matter, microbial pathogens, or oxygen in the air. Once hypochlorous acid gets an electron, it binds to that molecule and forms a new one, converting to hypochlorite, or to saltwater
Chlorine is a very effective disinfectant for inactivating bacteria. A study conducted in the 1940s investigated time-dependent inactivation levels of Escherichia coli, Pseudomonas aeruginosa, Salmonella typhi and Shigella Shigella (Butterfield et al., 1943). The results of the study showed that HOCl was more effective than OCl - (aka chlorine bleach) is more effective at inactivating these bacteria. These results have been confirmed by several researchers, who concluded that HOCl is 70-80 times more efficient than Ocl in inactivating bacteria. (Culp/Wesner/Culp, 1986). Hundreds of publications have confirmed the superiority of HOCl to OCl- since 1986 (click here to access the research database). HOCl may be more effective than OCl- for two reasons, first Because it has a neutral charge, it can easily penetrate the negatively charged cell walls of bacteria. The second reason is because HOCl has a much higher oxidation potential than OCl-.
Hypochlorous acid (HOCl) has been studied and shown to be effective against many viruses Click here to visit Virus Research.
Yes, hypochlorous acid is very effective at removing biofilms and preventing their formation. Click here to visit Biofilm Research.
The concentration that should be used depends on which application is applied. In the 20-30 ppm concentration range, it is very effective to sanitize foods such as fruits and vegetables, as well as fish and seafood, and the FDA allows the use of concentrations up to 60 ppm, which does not require a post-sanitization rinse. Sanitizing food contact surfaces is also effective at 20-30ppm, however the FDA allows concentrations up to 200ppm. Water disinfection is effective at 1-2ppm concentrations, but the EPA allows up to 4ppm. When deciding which concentration to use, it is best to refer to research results. Over 300 published research articles covering virtually all applications.
How to measure concentration?
Hypochlorous acid is measured with standard test strips used to measure free chlorine in swimming pools. The test strip will turn a different purple to indicate
Concentrations are between 10 and 200ppm. For higher concentrations, the test solution can be diluted. (Example: A 1000ppm solution can be diluted 1:10 and the test strip reads 100ppm, indicating the original solution was 1000ppm).
What applications have been studied?
The most studied application is the use of hypochlorous acid in the food industry for food hygiene and food contact surface hygiene. Other research applications include disinfection and sterilization of healthcare equipment, wound care, and general sanitation in healthcare facilities against MRSA and spore-forming organisms. In addition, studies have been carried out in industries such as animal husbandry, agriculture, water treatment and disinfection. Click here to access the industry organization's research database.
Is it effective against Listeria, Salmonella and E. coli?
Yes, most of the research on the effects of hypochlorous acid has focused on the microbial pathogens Listeria, Salmonella and Escherichia coli Click here to access a database of research organized by microbial pathogens.
Is it effective against MRSA and Clostridium difficile?
Hypochlorous acid is very effective against MRSA. Click here to view research on MRSA. Since Clostridium species are specifically difficult to grow in the laboratory, Bacillus species (which are also spore-forming bacteria and are more difficult to kill) were used as experimental surrogates.
Hypochlorous acid is non-toxic and harmless. Unlike most chemical disinfectants, hypochlorous acid has no effect on the eyes, skin and respiratory tract. Even if accidentally ingested, it will not cause harm.
Yes, most of the research on hypochlorous acid uses hypochlorous acid directly on food. FDA's Food Contact Notice 1811 allows hypochlorous acid to be used in raw or processed fruits and vegetables, fish and seafood, meat, poultry and eggs at levels up to 60 ppm. Click here to view FCN 1811 on the FDA website.
Hypochlorous acid does not alter the taste or smell of food when used at FDA-approved concentrations
FDA requirements for food contact substances are that they do not leave any harmful residues. Hypochlorite can be used at concentrations up to 60 ppm.
Yes, hypochlorous acid is 100% safe and non-irritating. It's safe enough to use on personal items like toothbrushes, baby pacifiers, and pet toys.
Hypochlorous acid is much less aggressive to fabrics than peroxide or chlorine bleach. While hypochlorous acid generally does not cause bleaching or discoloration, some low-quality dyes may bleed when exposed to hypochlorous acid.
Hypochlorous acid is a strong oxidant that can cause corrosion if exposed to brass, copper, iron or lower quality steel for extended periods of time. Stainless steel can also be corroded by prolonged immersion in high concentrations of hypochlorous acid (> 200 ppm)
