What is hypochlorous acid used for?
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Hypochlorous acid (HOCl) is a weak chlorine acid used to sanitize and disinfect food-contact surfaces, produce, meat and poultry processing water, eggshells, animal housing and clinical surfaces1,5,4. It is made by electrolyzing salt water, it is the active germ-killing form of chlorine in solution, and it works best on cleaned surfaces at slightly acidic to neutral pH1,2,3.
What is hypochlorous acid?
Hypochlorous acid (HOCl, also written HClO) is an oxyacid of chlorine: a weak acid that is highly unstable and exists only in solution1. It forms whenever chlorine gas or hypochlorite salts, such as the sodium hypochlorite sold as bleach, dissolve in water, and it can also be generated directly from salt water by electrolysis, when it is often sold as electrolyzed water1.
The body makes it too. Neutrophils, a type of white blood cell, use the enzyme myeloperoxidase to turn hydrogen peroxide and chloride into hypochlorous acid, which binds to bacterial membranes and kills the bacteria1,4.
| Property | Value |
|---|---|
| Chemical formula | HOCl (HClO)1 |
| CAS number | 7790-92-31 |
| Molar mass | 52.46 g/mol1 |
| Appearance | Colorless aqueous solution1 |
| Acid dissociation constant (pKa) | 7.531 |
| Formation from chlorine gas | Cl2 + H2O gives HOCl + HCl1 |
How does hypochlorous acid kill germs, and why does pH matter?
Hypochlorous acid is an oxidizing agent. Active chlorine compounds damage microbial membranes, and other reported modes of action include decarboxylation of amino acids, reactions with nucleic acids and loss of key enzymes1. HOCl is generally believed to be the active germicidal species in chlorine solutions, while the hypochlorite ion (OCl-) is the main driver of cleaning action2.
Because it is a weak acid, HOCl dissociates into hypochlorite ion and a proton depending on pH2. It is the dominant chlorine species between about pH 2 and 7.5, and at pH 6.5 about 95% of the chlorine is in the HOCl form, the range that gives the greatest sanitizing effect1. Below about pH 4, dissolved chlorine gas can be lost by volatilization, which weakens the solution, creates health and safety issues and can corrode equipment1.
| Type | pH | Oxidation-reduction potential | Notes |
|---|---|---|---|
| Acidic (anode side) | 2 to 6 | 1,000 mV or higher | 10 to 90 ppm chlorine1 |
| Slightly acidic | 5.0 to 6.5 | 974 mV in the layer house study | Used for spray disinfection of poultry houses5 |
| Neutral | 6 to 7.5 | About 750 mV | Mainly HOCl, hypochlorite ion and trace chlorine; less corrosion and hand irritation concern1 |
| Basic (cathode side) | 7.5 to 13 | About -800 to -900 mV | Sodium hydroxide side of the cell1 |
How is hypochlorous acid made?
Most hypochlorous acid sold for sanitizing is made by electrolysis of a dilute sodium chloride (salt) solution1. In a two-chamber cell, a membrane separates the electrodes: chloride at the anode forms chlorine, hypochlorous acid, hypochlorite and hydrochloric acid, while the cathode produces hydrogen gas and sodium hydroxide1. Cells without a membrane produce neutral electrolyzed water1. Salt concentration, current, electrolysis time, temperature and pH all change the finished solution and its sanitizing effect1.
As of the 2015 USDA technical review, EPA registers sanitizers and disinfectants as pesticides, while on-site generators that make antimicrobial solutions from salt and water are not required to be registered as pesticides; their makers must document compliance with the federal pesticide device rules1.
What is hypochlorous acid used for in cleaning and sanitizing?
Electrolyzed water has been studied and used as a sanitizer for fresh produce, poultry carcasses, shell eggs, cutting boards and food-processing surfaces1. Federal rules exempt hypochlorous acid residues from food tolerances when it is used in sanitizing solutions on food-contact surfaces with adequate draining before food contact, provided the ready-to-use solution does not exceed 200 ppm total available chlorine6. USDA's Food Safety and Inspection Service has approved electrolytically generated hypochlorous acid in meat and poultry processing, including carcasses and poultry chiller water, at 5 to 50 ppm free available chlorine depending on the product1.
In animal housing, slightly acidic electrolyzed water at 250 mg/L available chlorine and pH 6.19 inactivated all the bacteria and fungi in dust, feces, feathers and feed sampled from a layer house, and outperformed benzalkonium chloride and povidone-iodine solutions5. It also significantly reduced microbes on floors, walls, feed troughs and water pipe surfaces, and wetting surfaces with tap water before spraying played an important role in the result5. In clinical settings, a 2020 review reports that 200 ppm HOCl decontaminated surfaces carrying noroviruses and other enteric viruses within a 1-minute contact time4.
| Setting | Use | Concentration reported |
|---|---|---|
| Food-contact surfaces | Sanitizing solution, drained before food contact | Up to 200 ppm total available chlorine6 |
| Meat and poultry processing | Carcasses, parts, chiller and processing water | 5 to 50 ppm free available chlorine1 |
| Layer houses | Spraying surfaces, dust, litter and equipment | 250 mg/L available chlorine at pH 6.195 |
| Clinical surfaces | Hard surface decontamination | 200 ppm with 1 minute of contact4 |
| Hand hygiene | Hand sanitizer | 100 to 200 ppm4 |
Does a hypochlorous acid spray work, and what weakens it?
A spray works when enough active solution reaches the surface for long enough. Contact time and concentration are the main factors in its efficacy, and the method of application also matters4. Fogging reduced available free chlorine by about 70% and raised pH by about 1.3 in the studies a 2020 review describes, so strength and pH have to be adjusted before fogging4.
HOCl is easily used up. Proteins, carbohydrates and other nitrogen- or aldehyde-containing organic compounds, and ions such as nitrite, sulfite, iron and copper, consume it rapidly and significantly reduce its activity against bacteria3. In processing plants, debris and soil reduce sanitizer efficacy, so solutions need monitoring and refreshing1. Clean and rinse surfaces first, then follow the product label for concentration and contact time.
Light, air and heat break it down as well: HOCl is unstable under ultraviolet light, sunshine, contact with air and temperatures of 25 °C (77 °F) and above3.
Is hypochlorous acid hazardous to people, animals or the environment?
The USDA technical review describes hypochlorous acid at pH 6.5 to 7.5 as less hazardous to use than other chlorine-containing disinfectants, with corrosion and hand irritation being lesser concerns for neutral electrolyzed water1. Chlorine disinfectants have still caused occupational dermatitis and skin irritation, and very acidic solutions can release chlorine gas1.
It is very toxic to aquatic life: EPA levels of concern cited in the review are 0.009 ppm for aquatic invertebrates and 0.023 ppm for freshwater fish1. Free residual chlorine in released effluent dissipates with a half-life of 1.3 to 5 hours, and diluted solutions decompose faster in sunlight1. Keep used solution out of ponds, streams and drains that lead to surface water.
Neutral and near-neutral HOCl solutions are generally accepted not to form trihalomethanes and haloacetic acids, but HOCl at low pH and high concentration can decompose to halogenated species including chlorate and trihalomethanes1.
Key takeaways
- Hypochlorous acid (HOCl) is the active germ-killing form of chlorine in solution, with a pKa of 7.53.
- Most sanitizing HOCl is made on site by electrolyzing salt water.
- It works best at slightly acidic to neutral pH and on precleaned surfaces, because organic matter consumes it.
- Food-contact sanitizing solutions are capped at 200 ppm total available chlorine.
- Store it cool and dark, and keep used solution out of surface water.
References
- Hypochlorous Acid, Handling: Technical Evaluation Report
- Mechanisms of Actions of Sodium Hypochlorite in Cleaning and Disinfection Processes
- Stability of Weakly Acidic Hypochlorous Acid Solution with Microbicidal Activity
- Hypochlorous Acid: A Review
- Application of slightly acidic electrolyzed water for inactivating microbes in a layer breeding house
- 40 CFR 180.940 Tolerance exemptions for active and inert ingredients for use in antimicrobial formulations (Food-contact surface sanitizing solutions)
Frequently asked questions
Is hypochlorous acid the same as bleach?
No, but they are closely related. When hypochlorite salts such as sodium hypochlorite dissolve in water, they form hypochlorous acid in balance with hypochlorite ion1. HOCl is the main germicidal form and hypochlorite ion is the main cleaning form, and pH decides which dominates2. Electrolyzed HOCl solutions are made at acidic to neutral pH, where HOCl is the predominant species1.
How long does hypochlorous acid last after it is made?
Is hypochlorous acid used on skin and eyes?
Yes, beyond surface disinfection. A saline eyelid hygiene solution with 100 ppm HOCl reduced staphylococcal load on the skin around the eye by more than 99% within 20 minutes, and HOCl at 100 to 200 ppm is used in hand sanitizers4. Use any HOCl product only for the purpose stated on its label.
Does hypochlorous acid leave residues on food?
At about 200 ppm and above, chlorine from HOCl has been found to bind at low levels to the lipids, proteins and carbohydrates of immersed meat, poultry and fish1. For food-contact surfaces, federal rules cap the ready-to-use solution at 200 ppm total available chlorine and require adequate draining before food contact6.