HOCl

Hypochlorous acid (HOCl) is a weak inorganic acid and the primary active disinfectant form produced when chlorine is dissolved in water. In aqueous solution it exists in equilibrium with its conjugate base, hypochlorite (OCl−). At about 25 °C the pKa is near 7.5, so pH controls speciation: below this value HOCl dominates; above it, OCl− becomes more prevalent. HOCl is a stronger oxidant than OCl− and generally more effective at inactivating microbes because it is less charged and can more readily penetrate cell membranes.

Production and uses: HOCl can be generated by reacting chlorine gas with water (Cl2 + H2O → HOCl + HCl) or produced in situ by electrolyzing saline solutions. In drinking-water treatment, HOCl is the principal disinfectant formed from chlorine species; commercial products such as sodium hypochlorite or calcium hypochlorite release HOCl in water. In swimming pools, maintaining a pH that favors HOCl helps sustain disinfection. HOCl is also employed as a topical antiseptic, and as a sanitizer in food processing and healthcare environments.

Safety and limitations: HOCl is a strong oxidizer and corrosive. It reacts with ammonia and organic matter to form chloramines and various disinfection byproducts. Its stability and antimicrobial efficacy depend on pH, temperature, and exposure to light; higher pH and sunlight reduce HOCl levels.

Environmental and health notes: HOCl decomposes over time to chloride and oxygen. When used at appropriate concentrations, it is effective and relatively short-lived compared with other disinfectants, but its formation and byproducts must be managed to minimize ecological and health risks.