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. OSHA PEL for chlorine vapor (the decomposition product) is 1 ppm ceiling. 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.