Hypochlorous acid is an inorganic compound with the chemical formula ClOH, also written as HClO, HOCl, or ClHO.

[6] In living organisms, HOCl is generated by the reaction of hydrogen peroxide with chloride ions under the catalysis of the heme enzyme myeloperoxidase (MPO).

[9] Hypochlorous acid was discovered in 1834 by the French chemist Antoine Jérôme Balard (1802–1876) by adding, to a flask of chlorine gas, a dilute suspension of mercury(II) oxide in water.

It is not until recent years that scientists have been able to cost-effectively produce and maintain hypochlorous acid water for stable commercial use.

Thus, the formation of stable hypochlorite bleaches is facilitated by dissolving chlorine gas into basic water solutions, such as sodium hydroxide.

This is because HClO oxidises sulfhydryl groups, leading to the formation of disulfide bonds[43] that can result in crosslinking of proteins.

Sulfinic acid and R−S(=O)2−OH derivatives are produced only at high molar excesses of HClO, and disulfides are formed primarily at bacteriocidal levels.

[44] Chlorinated amino acids rapidly decompose, but protein chloramines are longer-lived and retain some oxidative capacity.

Consistent with these results, it was later proposed that the chloramine undergoes a molecular rearrangement, releasing HCl and ammonia to form an aldehyde.

A second slower reaction that results in cleavage of the pyridine ring occurs when excess HClO is present.

[33] Hypochlorous acid also reacts with a subclass of glycerophospholipids called plasmalogens, yielding chlorinated fatty aldehydes which are capable of protein modification and may play a role in inflammatory processes such as platelet aggregation and the formation of neutrophil extracellular traps.

[48][49][50] E. coli exposed to hypochlorous acid lose viability in less than 0.1 seconds due to inactivation of many vital systems.

[40] In 1948, Knox et al.[40] proposed the idea that inhibition of glucose oxidation is a major factor in the bacteriocidal nature of chlorine solutions.

[56] In agreement with this, McFeters and Camper[57] found that aldolase, an enzyme that Knox et al.[40] proposes would be inactivated, was unaffected by HClO in vivo.

The discovery that HClO blocks induction of β-galactosidase by added lactose[58] led to a possible answer to this question.

The uptake of radiolabeled substrates by both ATP hydrolysis and proton co-transport may be blocked by exposure to HClO preceding loss of viability.

Venkobachar et al.[60] found that succinic dehydrogenase was inhibited in vitro by HClO, which led to the investigation of the possibility that disruption of electron transport could be the cause of bacterial inactivation.

Later studies[52] revealed that Ubiquinol oxidase activity ceases first, and the still-active cytochromes reduce the remaining quinone.

The cytochromes then pass the electrons to oxygen, which explains why the cytochromes cannot be reoxidized, as observed by Rosen et al.[54] However, this line of inquiry was ended when Albrich et al.[37] found that cellular inactivation precedes loss of respiration by using a flow mixing system that allowed evaluation of viability on much smaller time scales.

So it was proposed that modification of some membrane-bound protein results in extensive ATP hydrolysis, and this, coupled with the cells inability to remove AMP from the cytosol, depresses metabolic function.

[45][63] During bacterial genome replication, the origin of replication (oriC in E. coli) binds to proteins that are associated with the cell membrane, and it was observed that HClO treatment decreases the affinity of extracted membranes for oriC, and this decreased affinity also parallels loss of viability.

Rosen's group proposed that inactivation of membrane proteins involved in DNA replication are the mechanism of action of HClO.

[68] Commercial disinfection applications remained elusive for a long time after the discovery of hypochlorous acid because the stability of its solution in water is difficult to maintain.

The active compounds quickly deteriorate back into salt water, losing the solution its disinfecting capability, which makes it difficult to transport for wide use.