Hypochlorous Acid

HOCl production in neutrophils depends on the activity of myeloperoxidase (MPO), the in vitro activity of which is optimal at body temperature (36–37°C) and is diminished at lower or higher temperatures (Severns et al., 1986).

From: Current Topics in Membranes, 2014

Related terms:

Superoxide, Antioxidant, Enzyme, Protein, Hydrogen Peroxide, Oxidizing Agent,

Myeloperoxidase, Chlorine, Reactive Oxygen Metabolite

Sodium hypochlorite and hypochlorous acid

In Meyler’s Side Effects of Drugs (Sixteenth Edition), 2016

Hypochlorous acid

Hypochlorite is very unstable, but hypochlorous acid is stable and is highly micro- bicidal, active against bacteria, viruses, and fungi. It has been used in 30 patients to  treat venous leg ulcers that had not healed with conventional treatment; 10 achieved a 44% ulcer reduction after 3 weeks [2]. The other 20 patients were then treated for 12 weeks; in nine cases there was full healing and in five the ulcers were reduced in size by over 60%. All the patients became free of pain. Adverse reactions were not reported.

Free and Combined Chlorine

Nicholas F. Gray, in Microbiology of Waterborne Diseases (Second Edition), 2014

Free Chlorine

Chlorine gas rapidly reacts with water molecules in an aggressive reaction to form hypochlorous acid (HOCl) and hydrochloric acid (HCl; Eq. 31.1). The former is a weak acid which rapidly dissociates to give hypochlorite ions (OCl−) with a chemical equilibrium developing between the associated and unassociated forms (Eq. 31.2). Both HOCl and OCl− have disinfection properties but the acid only dissociates under alkaline conditions. The hydrochloric acid also dissociates resulting in a reduction in both pH and the alkalinity of the water. 

The degree of dissociation of hypochlorous acid is pH dependent being gradually suppressed as the pH falls. At a pH < 5.0 then approximately 100% of the chlorine is in HOCl form which falls to 50% at pH 7.5, while at pH > 9.0 100% of chlorine is present as OCl− (Table 31.2). Hypochlorous acid has much faster reaction rates than hypochlorite, being approximately 80 times more effective. Therefore, chlorination is most effective at a slightly acidic pH (pH < 7.0). Temperature also has a small effect on the dissociation of HOCl causing the reaction to occur at slightly higher pH values at lower temperatures. Chlorine will react with any reducing agents or organic matter in the treated water, so that free chlorine residuals may not remain for very long within the distribution network. The decay rate of free chlorine in the distribution network follows a first order kinetics. In practice, the cleaner the water the longer the residual effect lasts. Hypochlorite ions rapidly react with the photons in sunlight producing oxygen, chlorite and chloride. The rate of this reaction is very pH dependent with the half life of free chlorine falling from 50 minutes at pH 6 to 10 minutes at pH 8 (Black and Veatch Corporation, 2010).

Table 31.2. Variation of the Percentage of Hypochlorous Acid (HOCl) with Temperature and pH as a Percentage of Free Chlorine in Drinking Water

HOCl (%)