Hypochlorous Acid
Properties
| State | Liquid (exists only in aqueous solution) |
| Color | Colorless to pale yellow-green |
| Solubility | Miscible with water |
| Melting Point | Not applicable (aqueous solution) |
| Boiling Point | Not applicable (decomposes) |
About Hypochlorous Acid
Hypochlorous acid is what actually kills the pathogens when you chlorinate a pool, sanitize a cutting board, or treat municipal water. Bleach (NaOCl) is a delivery system for it: dissolve hypochlorite in water, drop the pH, and you shift the OCl-/HOCl equilibrium toward the neutral acid form. With a pKa of 7.53, the speciation flips right around physiological pH — at pH 6 you're mostly HOCl (the active killer), at pH 8 you're mostly OCl- (about 80x less effective per mole at biocidal endpoints). That's why pool operators run pH 7.2–7.6 and not 8.0, and why electrolyzed water systems for hospital and food-service sanitation are engineered to deliver stable dilute HOCl at near-neutral pH instead of generic bleach. The biology is the same trick: neutrophils generate HOCl in the phagosome via myeloperoxidase, which couples H2O2 from the respiratory burst with chloride: H2O2 + Cl- → HOCl + OH-. The HOCl chlorinates and oxidizes everything in reach — protein cysteines and methionines, bacterial cell-wall components, viral envelope lipids — which is why pathogens haven't evolved meaningful resistance the way they have to most antibiotics.
Where you'll encounter it
If you've ever shocked a pool, used a hospital-grade Aniosyme spray, or watched a wound dressed with Dakin's solution, you've used HOCl chemistry — sometimes labeled as bleach, sometimes as electrolyzed water, but always the same molecule doing the work. Pool operators who chase 'free chlorine' on a DPD test kit are actually measuring the HOCl + OCl- pool, and when they hold the bather load down by keeping pH at 7.4 they're maximizing the HOCl fraction at roughly 60% — drift to 8.0 and that fraction collapses to about 25%. Hospital infection-control teams switched to electrolyzed-water generators in surgical suites for the same reason: a benchtop unit that produces 200 ppm HOCl at neutral pH gives the kill rate of bleach without the corrosive residue or the chloramine smell that triggers respiratory complaints.
Common Uses
- Active disinfectant species in chlorinated drinking-water and pool treatment
- Hospital surface and instrument sanitization via electrolyzed water systems
- Wound irrigation (Dakin's solution and modern stabilized HOCl products)
- Produce-rinse sanitation in commercial food processing
- Mild oxidant for laboratory chlorination and ClO+ transfer reactions
Safety Information
Concentrated HOCl solutions are corrosive to skin, eyes, and respiratory tract. Mixing with acids (toilet-bowl cleaner, vinegar) liberates Cl2 gas — a classic and frequently lethal household-chemistry accident; never combine bleach-type products with acids or with ammonia (the latter generates chloramines). GHS varies with concentration: 200 ppm wound-care solutions are recognized as safe for direct tissue contact, while 5–6% pool-shock concentrates are corrosive Cat 1B. Chlorine vapor, the decomposition product, carries a low ceiling-type exposure limit of its own. Store cool and dark — UV and heat both accelerate decomposition to chloride and oxygen.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.