Hypochlorous acid (HClO) is a chemical substance essential for numerous industrial, medical, and environmental . Although relatively unknown to the general public, this weak acid plays a crucial role as a disinfectant, oxidant, and chemical agent. In this article, we will explore its unique properties, production, and diverse uses, highlighting its growing importance in key areas such as hygiene, health, and environmental safety.
1. What is hypochlorous acid?
Chemical definition and properties
Hypochlorous acid, with the chemical formula HClO, is a weak acid composed of hydrogen, chlorine, and oxygen. It is formed by the dissolution of chlorine gas (Cl₂) in water according to the following chemical reaction:
Cl2+H2O⇋HClO+HClCl2+H2O⇋HClO+HCl
In this reaction, the chlorine atom is in the +1 oxidation state, giving hypochlorous acid its oxidizing properties.
It is an unstable compound that exists only in aqueous solution. It is sensitive to pH, light, and temperature, factors that influence its effectiveness and reactivity. At a pH of 7.5, hypochlorous acid is predominant compared to its conjugate anion, the hypochlorite ion (OCl⁻), which is less reactive.
Difference between hypochlorous acid and its derivatives
Hypochlorous acid is often confused with its salts, such as sodium hypochlorite (NaClO) used in bleach. The main difference lies in their electrical charge and effectiveness. Hypochlorous acid is electrically neutral, allowing it to easily penetrate the membranes of microorganisms to act as a disinfectant. In contrast, the hypochlorite ion carries a negative charge, which limits its penetration and reduces its effectiveness.
Physical properties
Hypochlorous acid is available as colorless solutions, sometimes slightly yellowish depending on the concentration. Its main properties are as follows:
- Molar mass : 52.46 g/mol.
- pKa : 7.5, which makes it a weak acid.
- Solubility : Totally soluble in water, but unstable in anhydrous form.
- Sensitivity : Degraded by light and heat, requiring specific storage conditions.
These characteristics make it an effective but demanding chemical agent, requiring precautions to optimize its use.
2. How is hypochlorous acid produced?
Natural production: a key element of the immune system
Hypochlorous acid is naturally produced by mammalian white blood cells, specifically neutrophils, during the immune response. When the body detects pathogens such as bacteria or viruses, these cells activate an enzyme called myeloperoxidase, which uses hydrogen peroxide (H₂O₂) and chloride ions (Cl⁻) to form hypochlorous acid. Hypochlorous acid then acts as a powerful biocide, destroying pathogens through oxidation.
This process, known as "oxidative burst", is a key natural defense, illustrating the effectiveness and safety of hypochlorous acid in biological systems.
Industrial production: chemical and electrochemical methods
The artificial synthesis of hypochlorous acid relies mainly on two processes: chemical reaction and electrolysis.
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Chemical reaction
Hypochlorous acid is formed by the dissolution of chlorine (Cl₂) in water, a reaction that also generates hydrochloric acid (HCl):
Cl2+H2O⇋HClO+HClCl2+H2O⇋HClO+HCl
This process is simple but requires rigorous control of pH and temperature conditions to avoid the degradation of hypochlorous acid.
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Electrochemical activation (ECA)
This modern process involves subjecting a saline solution (water + salt) to electrolysis, which simultaneously produces hypochlorous acid, hypochlorite ions (OCl⁻), and other oxidizing species. Electrolysis of chlorous acid (ECA) is a clean and environmentally friendly method widely used to produce disinfectant solutions.
- Membrane electrolysis : A membrane separates the anode and cathode, allowing the production of hypochlorous acid in the anodic compartment.
- Single-cell electrolysis : A simplified configuration where the anode and cathode share the same compartment, generating a directly usable hypochlorous acid solution.
Optimization and stability
Hypochlorous acid is an unstable compound that decomposes rapidly under the influence of light, heat, or pH fluctuations. To maximize its lifespan and effectiveness, stable solutions are obtained through specific conditions:
- Maintain a pH between 5.5 and 7.5, where hypochlorous acid is predominant.
- Store in opaque containers at controlled temperatures to reduce degradation.
3. Biocidal and disinfectant properties
Mechanism of action: powerful oxidation
Hypochlorous acid primarily works by destroying the cell membranes of microorganisms through oxidation. Due to its neutral charge, it easily penetrates the protective cell walls of bacteria, viruses, and fungi. Once inside, it damages vital components such as nucleic acids, enzymes, and lipids, leading to the death of pathogenic cells.
This mechanism, combined with its high oxidation potential, allows hypochlorous acid to rapidly neutralize a wide range of microorganisms, including resistant pathogens.
Comparison with other disinfectants
Hypochlorous acid stands out from many other disinfectants, particularly sodium hypochlorite (NaClO, bleach), due to its neutral charge and rapid action:
- Speed : Hypochlorous acid eliminates bacteria in seconds, while hypochlorite can take up to 30 minutes.
- Efficacy : It is up to 100 times more effective than sodium hypochlorite at killing microbial pathogens.
- Safety : At low concentrations, it is non-irritating to skin and mucous membranes, unlike hypochlorite, which can cause irritation.
Spectrum of action: broad-range effectiveness
Hypochlorous acid is effective against a wide range of microorganisms, including:
- Bacteria : Escherichia coli, Staphylococcus aureus, Salmonella spp.
- Viruses : Poliovirus, HIV, H1N1, coronavirus.
- Fungi : Candida albicans, Aspergillus spp.
Its broad spectrum of action makes it a preferred choice for applications requiring rapid and reliable disinfection.
Factors affecting efficiency
The effectiveness of hypochlorous acid depends on several parameters:
- pH : The ideal pH is between 5.5 and 7.5, where the HClO form is dominant. Above this threshold, effectiveness decreases because the hypochlorite ion (OCl⁻) becomes predominant.
- Concentration : An adequate concentration ensures rapid biocidal action without compromising safety.
- Presence of Organic Matter : Organic matter can react with hypochlorous acid, thus reducing its availability for pathogen elimination.
4. Applications of hypochlorous acid
Industrial applications
In the industrial sector, hypochlorous acid is primarily used as a disinfectant to ensure safety and hygiene:
- Food processing industry : Hypochlorous acid is used to disinfect food preparation surfaces, eliminate pathogens on fruits and vegetables, and ensure the microbiological quality of processed products. It is particularly valued for its lack of harmful chemical residues, which meets consumer demands for food safety.
- Water treatment : As a biocide, it is used to purify drinking water, reduce microbial contaminants in swimming pools and spas, and prevent the proliferation of bacteria such as Legionella spp. in cooling systems.
Medical and cosmetic applications
Hypochlorous acid's non-irritating properties and compatibility with human tissues make it an ideal choice in the medical and cosmetic fields:
- Wound treatment : Used to clean wounds, hypochlorous acid helps reduce infection while promoting healing. Its effectiveness against a broad spectrum of pathogens makes it particularly useful in hospital settings.
- Personal disinfectants : incorporated into hand sanitizing solutions, it offers a safe and non-toxic alternative to alcohol-based disinfectants. It is also used in baby products, thanks to its lack of irritants.
- Skin care : Hypochlorous acid is included in some cosmetic products to moisturize and soothe the skin while eliminating potential pathogens.
Environmental applications
environmental impact of hypochlorous acid makes it a responsible choice for applications related to environmental protection:
- Environmentally friendly disinfection : It is used to treat industrial effluents and reduce the bacterial load in wastewater. Its biodegradability ensures that it leaves no residues harmful to aquatic ecosystems.
- Soil cleaning : Hypochlorous acid is effective in decontaminating soils polluted by microbial agents, thus strengthening environmental safety.
Innovations and domestic uses
Increasingly, hypochlorous acid is being adopted in domestic settings due to its safety and effectiveness:
- Surface cleaning : Hypochlorous acid-based solutions are ideal for cleaning countertops, door handles, and other surfaces exposed to bacteria.
- Applications in household appliances : it is integrated into certain devices such as humidifiers or steam cleaners to disinfect the air or textiles.
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5. Laboratory analysis of hypochlorous acid
Dosage methods: common techniques
Hypochlorous acid analysis relies on precise techniques to measure its concentration and assess its stability. Common methods include:
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Photometry :
This method relies on measuring the absorbance of a hypochlorous acid solution at a specific wavelength. It allows for the rapid determination of the free chlorine concentration, including hypochlorous acid. This technique is used in laboratories for environmental and industrial analyses.
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Chemical titration :
Titration is a reliable method for quantifying active chlorine. It involves adding potassium iodide (KI) to a solution containing HClO. Free chlorine oxidizes the iodide to form iodine, which is then titrated with a sodium thiosulfate solution. This process allows for the precise calculation of the HClO concentration based on the amount of titrant used.
Importance of sampling conditions
To ensure the reliability of the analyses, samples must be collected and stored under optimal conditions:
- Airtight and inert packaging : samples must be stored in airtight containers to prevent contamination or degradation by air or light.
- Temperature control : Temperature fluctuations can accelerate the decomposition of hypochlorous acid. Therefore, maintaining a stable temperature during the transport and storage of samples is crucial.
Applications of the analysis results
The data obtained through these techniques is essential for:
- Food processing industry : verification of the concentration of hypochlorous acid used in the disinfection of food or surfaces.
- Water treatment : monitoring of free chlorine levels in drinking water or industrial effluents to ensure compliance with environmental regulations.
- Health and cosmetics : monitoring of products containing hypochlorous acid to ensure they comply with safety thresholds.
Standards and certifications
Hypochlorous acid analyses must meet strict standards to ensure their accuracy and compliance:
- ISO 17025 : This international standard certifies laboratories capable of performing reliable and traceable analyses.
- COFRAC : In France, laboratories accredited by COFRAC offer additional guarantees regarding the quality of results.
By combining robust analytical methods and strict adherence to sampling conditions, it is possible to fully exploit the potential of hypochlorous acid while ensuring its safety and effectiveness in various applications.
6. Advantages and limitations of hypochlorous acid
The advantages of hypochlorous acid
Hypochlorous acid offers a unique combination of effectiveness, safety and environmental friendliness, making it a preferred disinfectant in many contexts:
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Exceptional biocidal efficacy :
Hypochlorous acid is capable of rapidly eliminating a wide range of bacteria, viruses, and fungi due to its strong oxidizing power. It is particularly useful for combating resistant pathogens.
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Safety for humans and animals :
At low concentrations, it is non-irritating to the skin and mucous membranes, unlike many other disinfectants. This characteristic makes it suitable for medical, cosmetic, and household applications.
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Biodegradability and ecology :
Hypochlorous acid decomposes naturally into water, oxygen, and chloride ions, minimizing its environmental impact. It is therefore ideal for environmentally friendly uses, such as wastewater treatment.
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Compatibility with surfaces and products :
Unlike some harsh disinfectants, it does not corrode metals and does not damage textiles or plastics when used correctly.
The limitations of hypochlorous acid
Despite its many advantages, hypochlorous acid presents challenges related to its stability and use:
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Chemical instability :
Hypochlorous acid is sensitive to light, heat, and pH changes. At high pH levels or after prolonged exposure, it degrades into less effective hypochlorite ions. This instability necessitates strict storage conditions, such as opaque containers and temperature-controlled storage.
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Reactivity with organic matter :
In the presence of organic matter, hypochlorous acid can form undesirable by-products such as chloramines, which reduce its disinfectant effectiveness.
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Limited lifetime :
Hypochlorous acid solutions have a relatively short shelf life. This limits their storage and necessitates immediate production or use.
How can these limitations be overcome?
To overcome the limitations of hypochlorous acid, several solutions are implemented:
- Optimization of production methods : Modern processes such as membrane electrolysis make it possible to produce more stable hypochlorous acid solutions.
- Improving storage conditions : Using suitable, opaque and airtight containers extends the shelf life of solutions.
- Innovative formulations : Some manufacturers combine hypochlorous acid with stabilizers to improve its longevity and effectiveness, even under suboptimal conditions.
7. Impact of hypochlorous acid in industry
Role in the agri-food industry
In the food industry, hypochlorous acid is used to ensure food safety and compliance with health regulations:
- Disinfection of fresh produce : Hypochlorous acid is used to eliminate microbial contaminants on fruits, vegetables, and meats, while preserving their quality. It is a superior alternative to other, more aggressive disinfectants because it leaves no toxic residues.
- Equipment cleaning : Processing surfaces and equipment are disinfected with hypochlorous acid solutions, reducing the risk of cross-contamination and increasing the lifespan of installations due to its low corrosive potential.
Applications in public health
Hypochlorous acid plays a key role in protecting public health by acting as a safe and effective disinfectant:
- Drinking water treatment : Used to eliminate bacteria, viruses and other contaminants, hypochlorous acid ensures drinking water quality that meets international health standards.
- Nosocomial disease control : in hospitals, it is used to disinfect surfaces and medical equipment, thus limiting the spread of bacterial and viral infections.
Applications in the environment
The use of hypochlorous acid also contributes to environmental preservation:
- Wastewater treatment : It is used to reduce bacterial load and disinfect wastewater before it is released into the environment. Its biodegradability limits its impact on aquatic ecosystems.
- Pollutant reduction : In contaminated soils, hypochlorous acid helps to neutralize harmful microorganisms without introducing persistent compounds.
Prospects for innovation and development
Current research and new technologies are opening up exciting prospects for hypochlorous acid:
- Advanced formulations : stabilized solutions extend the shelf life of hypochlorous acid, facilitating its transport and storage.
- Integration into new sectors : Hypochlorous acid is increasingly used in emerging industries, such as the manufacture of air purification devices and the disinfection of ventilation systems.
- Portable production equipment : Portable hypochlorous acid generators allow small businesses and individuals to produce fresh and effective solutions on-site, reducing costs and logistical needs.
