Sodium Hypochlorite
Properties
| State | Liquid (aqueous solution; solid pentahydrate is unstable) |
| Color | Pale greenish-yellow solution |
| Solubility | Miscible with water in all proportions |
| Melting Point | Pentahydrate melts/decomposes at 18 °C |
| Boiling Point | Decomposes before boiling; concentrated solutions release Cl2 and O2 |
About Sodium Hypochlorite
Sodium hypochlorite (NaClO, MW 74.439) is the active ingredient that put bleach in every laundry room and chlorine in every municipal water plant since Berthollet first prepared 'Javel water' in 1789 by passing Cl2 over Na2CO3 solution. Household Clorox is typically 6-8.25% NaClO, swimming-pool liquid chlorine is 10-12.5%, and commercial sanitizers run up to 15%. The chemistry that matters is the hypochlorite/hypochlorous acid equilibrium: ClO- + H2O <-> HClO + OH-, with pKa 7.53. Below pH 7.5 most of the active oxidant is HClO, the small neutral species that crosses microbial cell membranes and oxidizes thiol groups, iron-sulfur clusters, and DNA bases. Above pH 7.5 the equilibrium shifts to ClO-, which is more stable in the bottle but a weaker biocide. That's why pool operators target pH 7.2-7.6 and why sodium hydroxide is added to commercial bleach to keep it at pH 11-13 for shelf life (slowing 3 NaClO -> 2 NaCl + NaClO3 disproportionation). Industrially, NaClO is made by absorbing Cl2 from chlor-alkali plants into cold dilute NaOH: Cl2 + 2 NaOH -> NaCl + NaClO + H2O. Beyond disinfection it bleaches paper pulp (largely displaced by ClO2 to cut dioxin formation), oxidizes alcohols to ketones in the Anelli oxidation with TEMPO catalyst, and is the active in commercial mold removers and dental endodontic irrigants (3-6% NaClO dissolves necrotic pulp tissue during root canals).
Where you'll encounter it
If you've ever mopped a bathroom floor with diluted Clorox, jumped into a chlorinated pool, or had a root canal where the dentist flushed the canal with 'sodium hypo,' you've used NaClO. In a microbiology lab, the standard surface decon for a BSL-2 hood after a spill is freshly prepared 10% household bleach (giving roughly 0.5% NaClO, the CDC-recommended concentration) with a 10-minute contact time; bleach loses potency within days once diluted, so dilutions are made daily. In a municipal water plant, sodium hypochlorite has largely replaced chlorine gas because it's safer to truck and store; the dose is calibrated to leave a 0.2-0.5 ppm free chlorine residual at the customer tap. In an organic chemistry lab, NaClO with catalytic TEMPO and KBr in pH 8.5 buffer is the standard recipe for a clean, scalable oxidation of primary alcohols to aldehydes or carboxylic acids, depending on workup.
Common Uses
- Municipal drinking water disinfection at 0.2-0.5 ppm free chlorine residual
- Swimming pool chlorination (liquid chlorine, 10-12.5% NaClO)
- EPA-registered surface disinfectant for SARS-CoV-2, norovirus, and C. difficile spores
- Endodontic irrigant (3-6% NaClO) that dissolves necrotic pulp during root canal therapy
- Anelli/TEMPO co-oxidant for converting primary alcohols to aldehydes or acids
- Laundry whitener via oxidation of chromophores in stains
- BSL-2/BSL-3 spill decontamination at 0.5% (10% household bleach dilution)
- Cyanide destruction in mining tailings via alkaline chlorination
Safety Information
Corrosive to skin, eyes, and respiratory tract; severity scales with concentration. Household 6% bleach is an irritant; pool-grade 12.5% causes burns. Chlorine released on acidification is held to a ceiling exposure limit rather than an averaged one — brief peaks are what injure. Mixing with acidic toilet-bowl cleaners (HCl, NaHSO4) liberates Cl2 gas; mixing with ammonia (NH3, in many glass cleaners) generates chloramines (NH2Cl, NHCl2, NCl3) that have killed people in poorly ventilated bathrooms. GHS classifications: H290 (may be corrosive to metals), H314 (severe skin burns and eye damage), H400 (very toxic to aquatic life), H410 (with long-lasting effects), EUH031 (releases toxic gas in contact with acids). Pictograms: Corrosion, Environment. Decomposes on storage (slowly to NaCl + O2, faster at warm temperatures or in sunlight) so working concentration drops over months. Standard PPE: splash goggles, nitrile gloves, lab coat, fume hood for concentrated work.
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.