Hypochlorous acid, HOCl, is one of the simplest molecules in sanitation and one of the least understood. It is the same oxidant that white blood cells produce inside the body to destroy engulfed microbes. It forms whenever chlorine meets water. And it can be made on site, on demand, from nothing more than salt, water and electricity.
This page explains what HOCl is, how it is made, why its strength depends so heavily on pH, how it acts on microorganisms and why it does not last forever. For cell design, feed water and installation detail, see Technologies.
About claims on this page
EcoloxTech generators produce a hypochlorous acid solution on site. They are not EPA-registered disinfectants and we make no efficacy claims for them or for the solution they produce. The only EcoloxTech product with disinfection claims is HOCl-AS200 (EPA Reg. No. 101112-1), used according to its label. Research cited here describes published studies of hypochlorous acid and electrolysed water; it is not a claim for any product.
What hypochlorous acid is
Hypochlorous acid is a weak acid made of one hydrogen, one oxygen and one chlorine atom. It is the active form of what water chemists call free available chlorine. When chlorine gas dissolves in water it hydrolyses:
Cl₂ + H₂O ⇌ HOCl + H⁺ + Cl⁻
Hypochlorous acid then partly dissociates into a hydrogen ion and the hypochlorite ion, OCl−:
HOCl ⇌ H⁺ + OCl⁻
Both forms count as “free chlorine” on a test strip, but they are not equally effective. Which one dominates is decided by pH, and that single fact explains most of the difference between hypochlorous acid and bleach.
How electrolysis makes it from salt and water
An electrolytic cell is a chamber with two electrodes and a controlled electric current. Feed it a dilute solution of sodium chloride — ordinary salt in softened water — and two reactions take place at the same time.
| Electrode | Reaction | What it produces |
|---|---|---|
| Anode (+) | 2Cl⁻ → Cl₂ + 2e⁻ | Chlorine, which immediately hydrolyses to hypochlorous acid in the water |
| Cathode (−) | 2H₂O + 2e⁻ → H₂ + 2OH⁻ | Hydrogen gas and hydroxide, which raises pH |
What leaves the cell depends on how those two streams are handled. Membrane (two-chamber) cells keep them apart and produce a strongly acidic anolyte and an alkaline catholyte as two separate solutions. Single-cell (undivided) designs let them mix, and the hydroxide from the cathode partly neutralises the acid from the anode. With the right control of current, salt concentration and flow, the result is a near-neutral to slightly acidic solution in which most of the free chlorine is present as hypochlorous acid.
EcoloxTech systems use a single-pass design and produce a solution at pH 5.0–6.5. The concentration is set by the system: the EcoloxTech 300 produces 20–250 ppm at up to 300 L per hour, and the EcoloxTech 1200 20–600 ppm at up to 1,200 L per hour. The countertop Micro makes 300 ml in five minutes at up to 200 ppm from one gram of salt.
Why pH decides its strength
The balance between hypochlorous acid and hypochlorite shifts around a pKa of about 7.5. At pH 7.5 the two are present in roughly equal amounts. Every pH unit below that pushes the balance towards HOCl; every unit above pushes it towards hypochlorite.
| pH | Mostly present as | Typical example |
|---|---|---|
| 2–3 | HOCl, with some dissolved chlorine gas | Acidic anolyte from membrane cells |
| 5–6.5 | Almost entirely HOCl | Near-neutral electrolysed water, EcoloxTech output |
| 7.5 | Half HOCl, half OCl− | — |
| 11–13 | Almost entirely OCl− | Household and industrial bleach |
This matters for three reasons. HOCl is electrically neutral, so it passes through the negatively charged surfaces of microbial cells far more easily than the charged hypochlorite ion. HOCl is widely reported to be many times more biocidal than hypochlorite at the same free-chlorine reading (the research hub indexes a study comparing sodium hypochlorite adjusted to pH 12, 7.5 and 6.5). And below about pH 4, free chlorine starts to leave solution as chlorine gas, which is why the useful window sits in the slightly acidic to near-neutral range rather than as low as possible.
How it acts on microorganisms
Hypochlorous acid is an oxidiser. It does not bind to one target the way an antibiotic does; it reacts with many components of a cell at once:
- Cell wall and membrane. Oxidation of lipids and membrane proteins disrupts the barrier that holds the cell together.
- Proteins and enzymes. HOCl reacts readily with sulfur-containing amino acids and other groups, disabling enzymes the cell needs to produce energy and repair itself.
- Nucleic acids. DNA and RNA are damaged, preventing replication.
- Viral envelopes and capsids. In viruses, oxidation damages the envelope (where present) and the protein coat, preventing infection of host cells.
Because it attacks so many targets at once, oxidative chemistry of this kind is not generally associated with the acquired resistance seen with antibiotics. That does not mean all organisms are equally susceptible: spores, protected biofilm communities and organisms buried in soil need more concentration, more time or mechanical help. See kill times for the factors that decide speed.
Why it does not last, and why that is useful
The same reactivity that makes hypochlorous acid effective makes it unstable. Available chlorine declines over time, faster when the solution is exposed to:
- Light, particularly sunlight and ultraviolet;
- Heat;
- Air, through open containers and agitation;
- Organic matter and metals, which consume it;
- Incompatible containers — the HOCl-AS200 label recommends white HDPE for secondary containers.
As it reacts, hypochlorous acid is reduced back towards chloride — in effect, dilute salt water. That short life is the reason onsite generation exists: instead of buying a concentrated product formulated to survive shipping and storage, you make what you need, at the strength you need, close to where you use it. It is also why any stored solution should be tested before use.
How it is measured
| Measurement | What it tells you | How |
|---|---|---|
| Free available chlorine (ppm) | How much active chlorine is present | Test strips, test paper or a photometer |
| pH | How much of that chlorine is present as HOCl | pH paper, pH pen or meter |
| ORP (mV) | The solution’s oxidising potential; a useful trend indicator | ORP probe |
A ppm reading without a pH reading tells you only half the story. Together they tell you what you actually have in the bottle.
Onsite generation versus bottled HOCl
Hypochlorous acid reaches the point of use in one of two ways. It can be generated on site from salt and water, or bought as a finished, packaged product. Both are the same molecule; what differs is how the solution is controlled, how long it has to survive, and what it is legally allowed to claim.
| Generated on site | Bottled, registered product (e.g. HOCl-AS200) | |
|---|---|---|
| Made | On demand, from salt, water and electricity | In a registered establishment, then packaged |
| Strength | Set on the system for each task | Fixed by the formulation on the label |
| Shelf life needed | Hours to days: used close to production | Months: must survive shipping and storage |
| Claims | None — generators are devices | Registered claims, as printed on the label |
| Best for | Steady, high-volume daily use at one site | Tasks that require a registered disinfectant, occasional use, remote locations |
Many sites use both: generated solution for the high-volume daily work, and a registered product where a label claim is required. The sanitation audit sorts tasks into the two.
Where hypochlorous acid is used
Because it is made from inexpensive inputs and breaks down to salt water, hypochlorous acid is used across a remarkably wide range of settings: food processing and packing, seafood and poultry plants, dairies and beverage lines, restaurants and commercial kitchens, schools, hotels, cruise ships, healthcare and senior care, agriculture and livestock, controlled-environment growing and restoration work. Each has its own tasks, soils, surfaces and regulatory expectations. Our industry programmes cover fourteen of them in detail, and the research hub can be browsed by sector.
What hypochlorous acid is not
- It is not a detergent. It does not replace cleaning. Soil must be removed first.
- It is not a sterilant for medical devices. The HOCl-AS200 label explicitly excludes use as a terminal sterilant or high-level disinfectant on critical and semi-critical devices.
- It is not the same as bleach at a lower strength. Different pH, different dominant species, different behaviour.
- It is not compatible with everything. Do not mix it with acids, ammonia or other household or industrial chemicals; doing so can release hazardous gas.
Frequently asked questions
Is electrolysed water the same as hypochlorous acid?
Not exactly. “Electrolysed water” is any solution made by passing current through brine, including acidic anolyte, alkaline catholyte and near-neutral solutions. Hypochlorous acid is the active molecule. Near-neutral electrolysed water is the form in which HOCl dominates.
What do you need to make it?
Softened water, food-grade salt and electricity, in a properly controlled electrolytic cell. Ready-mixed brine is available for the commercial systems.
Why is pH 5.0–6.5 the target?
Because almost all of the free chlorine is then present as hypochlorous acid, while staying clear of the low pH at which chlorine gas starts to come out of solution.
How long does it keep?
It depends on storage. Kept cool, dark and sealed it lasts far longer than in an open container in sunlight. Test before use rather than relying on a date.
Where can I read the research?
The research hub indexes more than 300 published studies, searchable by organism and by industry.