Hypochlorous acid
An inorganic acid used as a disinfectant and generated by white blood cells.
Hypochlorous acid is an inorganic compound with the formula ClOH, often written as HClO, HOCl, or ClHO, and its structure is H−O−Cl. It is an acid produced when chlorine dissolves in water, and it partially breaks down into a hypochlorite anion (ClO⁻). Both HClO and ClO⁻ act as oxidizers and are the main disinfecting agents in chlorine solutions. Because it rapidly reaches equilibrium with its precursor chlorine and its anhydride dichlorine monoxide, HClO cannot be isolated from these solutions.
Due to its strong antimicrobial properties, related compounds like sodium hypochlorite and calcium hypochlorite are key ingredients in many commercial bleaches, deodorants, and disinfectants. Mammalian white blood cells, including those in humans, also produce hypochlorous acid to fight off foreign bodies. In living organisms, HOCl is created when hydrogen peroxide reacts with chloride ions, catalyzed by the heme enzyme myeloperoxidase. Like many disinfectants, hypochlorous acid solutions can destroy pathogens on surfaces, including COVID-19.
The French chemist Antoine Jérôme Balard discovered hypochlorous acid in 1834 by adding a dilute suspension of mercury(II) oxide in water to a flask of chlorine gas. He also named the acid and its compounds. While relatively easy to make, maintaining a stable solution is difficult, and only in recent years have scientists found cost-effective ways to produce and keep hypochlorous acid water stable for commercial use.
In organic synthesis, HClO converts alkenes into chlorohydrins. In biology, it is generated in activated neutrophils through myeloperoxidase-mediated peroxidation of chloride ions, helping destroy bacteria and other microbes. Medically, hypochlorous acid water has been used as a disinfectant and sanitizer. As of early 2016, the U.S. Food and Drug Administration approved products with hypochlorous acid as the main active ingredient for treating wounds and infections in humans and pets, and it is also approved as a preservative for saline solutions. For disinfection, it comes in liquid sprays, wet wipes, and aerosolized forms. Recent studies have found it suitable for fog and aerosol application in disinfection chambers and for disinfecting indoor spaces like offices, hospitals, and clinics.
- Discovered
- 1834
- Discoverer
- Antoine Jérôme Balard
- Discoverer nationality
- French
- Chemical formula
- ClOH (HClO, HOCl, ClHO)
- Known for
- Strong antimicrobial properties; primary disinfectant in chlorine solutions; used in commercial bleaches, deodorants, and disinfectants; generated by white blood cells to fight pathogens
Lore & Background
Hypochlorous acid was discovered in 1834 by the French chemist Antoine Jérôme Balard. He added a dilute suspension of mercury(II) oxide in water to a flask of chlorine gas, and he also named the acid and its compounds. The acid is relatively easy to make but difficult to maintain a stable solution; only in recent years have scientists been able to cost-effectively produce and maintain hypochlorous acid water for stable commercial use.
In living organisms, HOCl is generated by the reaction of hydrogen peroxide with chloride ions under the catalysis of the heme enzyme myeloperoxidase (MPO). The white blood cells of mammals, such as humans, contain hypochlorous acid as a tool against foreign bodies. In biology, hypochlorous acid is generated in activated neutrophils by myeloperoxidase-mediated peroxidation of chloride ions and contributes to the destruction of bacteria and other microbes.
Hypochlorous acid reacts with a wide variety of biomolecules, including DNA, RNA, fatty acid groups, cholesterol, and proteins. It oxidizes sulfhydryl groups, leading to disulfide bonds that can crosslink proteins. It also reacts with amino groups to form organic chloramines, and with unsaturated bonds in lipids. These reactions contribute to its antimicrobial effects.
Reader's Guide
Hypochlorous acid is significant primarily for its role as a powerful disinfectant. It is the active sanitizer in hypochlorite-based products used in water treatment, swimming pools, and marine sanitation devices. In medicine, the U.S. Food and Drug Administration has approved products whose main active ingredient is hypochlorous acid for treating wounds and various infections in humans and pets, and as a preservative for saline solutions. It has been used in liquid spray, wet wipes, and aerosolised application for disinfecting indoor settings such as offices, hospitals, and healthcare clinics. In food service and water distribution, specialized equipment generates weak solutions of HClO from water and salt to treat food preparation surfaces and water supplies. It is also used in restaurants due to its non-flammable and nontoxic characteristics. In deodorization, hypochlorous acid has been tested to remove up to 99% of foul odours. Like many other disinfectants, hypochlorous acid solutions will destroy pathogens, such as COVID-19, adsorbed on surfaces. Its legacy lies in its broad utility across sanitation, healthcare, and food safety, stemming from its strong antimicrobial properties and relative safety.
Did You Know?
- Hypochlorous acid was discovered in 1834 by French chemist Antoine Jérôme Balard by adding a dilute suspension of mercury(II) oxide in water to chlorine gas.
- The white blood cells of mammals generate hypochlorous acid via the enzyme myeloperoxidase to destroy bacteria and other microbes.
- Hypochlorous acid cannot be isolated from solution due to rapid equilibration with chlorine and dichlorine monoxide.
Frequently Asked Questions
What is Hypochlorous acid?
Hypochlorous acid is a weak inorganic acid with the molecular formula HClO (also written as HOCl or ClOH), structured as H−O−Cl. It forms naturally when chlorine gas dissolves in water and exists in dynamic equilibrium with its conjugate base, the hypochlorite ion.
What role does Hypochlorous acid play in disinfection?
Both HClO and its anion ClO⁻ serve as powerful oxidizing agents that destroy microbial cell walls and proteins, making them the primary active disinfectants in household bleach and commercial sanitizer products. This same oxidative chemistry is why sodium hypochlorite solutions are so effective against bacteria and viruses.
Why can't Hypochlorous acid be isolated as a pure substance?
HClO is locked in a rapid chemical equilibrium with dissolved chlorine and its anhydride, dichlorine monoxide (Cl₂O), so it cannot be separated from those species in solution. Practically, this means chemists work with it only as a component of aqueous chlorine mixtures rather than as a standalone bottle of liquid.
How does the human body use Hypochlorous acid?
White blood cells synthesize HClO as part of the innate immune response, releasing it to oxidize and kill invading bacteria and other pathogens. This biological production mirrors the same oxidative chemistry exploited in commercial disinfectants.
Who discovered Hypochlorous acid and when?
The compound was identified in 1834 by the French chemist Antoine Jérôme Balard. His work helped establish the broader family of chlorine oxyacids and their reactivity patterns.
More in Inorganic Compounds & Materials 1-20
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