A plain-language guide to hypochlorous acid (HOCl): what it is, how it relates to bleach, why pH matters, how generators make it, and what to know about testing, storage, and safe use.
On this page: What HOCl is · How it relates to bleach · Why pH matters · How generators make it · Bottled products vs. generators · Using it well · Frequently asked questions · Sources
The short answer
Hypochlorous acid (HOCl) is a form of free chlorine. It's one of the chemical species present when chlorine is dissolved in water, along with the hypochlorite ion (OCl⁻). The CDC's disinfection guideline attributes the germ-killing activity of chlorine largely to undissociated hypochlorous acid, and notes that how much is present as HOCl versus hypochlorite depends on pH.
HOCl is the active ingredient in some bottled sanitizers and disinfectants, and it's also what hypochlorous acid/electrolyzed water generators produce on site. The EPA notes that "electrolyzed water" is another name for hypochlorous acid.
How HOCl relates to bleach
Household bleach and HOCl are chemical relatives, not the same product. The CDC describes common household bleach as a 5.25%–6.15% sodium hypochlorite solution. Bleach solutions are alkaline, which shifts free chlorine toward the hypochlorite form; the CDC states that chlorine's disinfecting efficacy decreases as pH rises and HOCl converts to hypochlorite.
Concentration, pH, formulation, and intended use all differ between products. That's why you can't judge one product by another's label. For a fuller comparison, see our guide HOCl vs. Bleach: Chemistry, pH and Real-World Use Differences.
Why pH matters
In a free-chlorine solution, pH decides the balance between hypochlorous acid and hypochlorite. The CDC describes this pH dependence. The Eco One manual states that HOCl is most dominant in a chlorine solution between pH 4 and 6, and that the optional vinegar in its batch instructions lowers pH so HOCl is the dominant free chlorine molecule.
pH tells you about the form of chlorine, not how much is present. A pH reading alone does not tell you a solution's concentration or whether it will sanitize or disinfect a surface.
How generators make HOCl
Electrolysis uses electricity to drive a chemical reaction. In a hypochlorous acid generator, a current passes through water with dissolved salt in an electrolysis cell, producing free chlorine. The CDC guideline describes electrolyzing saline to produce a solution whose main products are hypochlorous acid and chlorine, and notes that, as with any germicide, its activity is strongly affected by the concentration of available free chlorine, and that its activity decreased substantially in the presence of organic material.
For example, the Eco One is a home generator that, per its manual, uses tap or purified water, 2 grams of food-grade non-iodized salt per liter, and optionally 1 teaspoon of distilled white vinegar to make a 1-liter batch.
Bottled products vs. generators
These are regulated differently. Under the EPA's guide to pesticide devices:
- Registered pesticide products, such as many bottled sanitizers and disinfectants, carry an EPA Registration Number, and the EPA reviews data supporting a product's registration.
- Hypochlorous acid/electrolyzed water generators are generally considered devices. However, the EPA notes that selling or distributing a generator with a substance can change how it's treated.
- The EPA distinguishes pesticide devices from registered chemical products. An EPA Establishment Number identifies the facility where a product was produced; it is not a product registration, approval, or endorsement.
- Device claims can't be false or misleading, and some states have their own requirements.
In practice: with a bottled product, follow its label. With a generator, follow the manufacturer's manual for making, testing, using, and storing the solution.
Using HOCl well
The same basics apply to any surface sanitizer or disinfectant. The CDC's home cleaning guidance says to:
- Clean surfaces first, because dirt can make it harder for chemicals to reach germs.
- Follow product directions, including the contact time, and keep the surface wet for the whole contact time.
- Not mix products or chemicals with each other.
- Store products out of reach of children and pets.
Don't mix a finished HOCl solution with other cleaners or chemicals. That's different from preparing a batch using only the ingredients a generator's manufacturer specifies, such as the optional vinegar step in the Eco One manual.
Frequently asked questions
1. Is hypochlorous acid the same as bleach? No. Both contain free chlorine, but household bleach is a concentrated, alkaline sodium hypochlorite solution, according to the CDC. pH, concentration, and formulation differ, and so do product directions.
2. Is HOCl safe? Safety depends on the specific product, its concentration, and how it's used. Read and follow the label or manual. For the Eco One, see its Safety Data Sheet and the precautions in its manual, which say the appliance should not be used by children.
3. Can I drink it or use generator output on my skin or eyes? No. The EPA recommends that electrolyzed water generated by these devices not be ingested or applied to the human body. Skin, eye, and wound products are separate products with their own formulations and labeling. Talk to a healthcare provider about any skin, eye, or wound concern.
4. Can I use it on pets or plants? This page doesn't cover pet or plant treatment. The CDC advises against wiping or bathing pets with disinfection products. For any animal- or plant-related use, follow a product label written for that use.
5. Does HOCl expire? How should I store it? Chlorine solutions can lose strength over time. As one example for a different kind of solution, the CDC reports that hypochlorite solutions in tap water at pH above 8, stored in closed opaque containers, can lose up to 40%–50% of their free available chlorine over a month. That figure isn't a shelf life for generated HOCl, and there's no single expiry date for every HOCl solution. Follow your product's storage directions; the Eco One SDS says to store in a cool area away from UV or direct sunlight with the container tightly closed. Test before use.
6. Can smell or pH tell me if it's still strong? No. Neither smell nor pH alone tells you concentration or effectiveness. pH indicates the chlorine form; concentration needs a chlorine test.
7. How do I check the concentration of generated HOCl? Use free-chlorine test strips. The Eco One manual describes chlorine test paper with color matches at 10, 50, 100, and 200 ppm. Strips give an approximate reading; they don't prove a surface has been sanitized or disinfected. The manual also notes ppm values are approximations because water quality varies.
8. Can I dilute it or make it stronger at home? Follow the manufacturer's instructions for the concentrations it describes rather than improvising. The Eco One manual lists settings and fill levels for specific approximate concentrations.
9. What kind of spray bottle should I use? Use the container the manufacturer provides or recommends. The Eco One product page lists an included 8 oz PET fine-mist sprayer.
10. Is my generator "EPA approved"? Generators are generally treated as devices, not registered products, per the EPA. An establishment number identifies a production facility; it isn't approval or endorsement.
Next steps
Thinking about making HOCl at home? Read How to Choose a Hypochlorous Acid Generator, or see the Eco One. For commercial applications, visit ewco.com.
This page is general information, not medical, legal, or professional advice. Always follow your product's label or manual.