Hypochlorous acid

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Hypochlorous acid
IUPAC name
Hypochlorous acid
Other names
Chlorine hydroxide
Hydrogen hypochlorite
Molar mass 52.46 g/mol
Appearance Unstable colorless liquid (solution)
Odor Strong chlorine odor
Melting point Decomposes
Boiling point Decomposes
Solubility Reacts with bases
Acidity (pKa) 7.53
Safety data sheet None
Flash point Non-flammable
Related compounds
Related compounds
Hydrochloric acid
Chlorous acid
Chloric acid
Perchloric acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Hypochlorous acid (HClO) is a weak acid that forms when chlorine dissolves in water, and itself partially dissociates, forming the hypochlorite anion ClO.



Hypochlorous acid is extremely unstable and rapidly decomposes, releasing oxygen gas:

2 HOCl → 2 HCl + O2

The reaction is catalyzed by light and metallic impurities.

HClO reacts with HCl to form chlorine gas:

HClO + HCl → H2O + Cl2

HClO reacts with ammonia and amines form chloramines and water.

NH3 + HClO → NH2Cl + H2O

HClO is a stronger oxidant than chlorine under standard conditions.

2 HClO(aq) + 2 H+ + 2 e- ⇌ Cl2(g) + 2 H2O E = +1.63 V


Hypochlorous acid only exists as solution, and thus its properties are derived from the solution.


Hypochlorous acid is not sold by anybody because it's too unstable to be of any use.


Addition of chlorine to water gives both hydrochloric acid and hypochlorous acid:

Cl2 + H2O ⇌ HClO + HCl
Cl2 + 4 OH ⇌ 2 ClO + 2 H2O + 2 e
Cl2 + 2 e ⇌ 2 Cl

When acids are added to aqueous salts of hypochlorous acid (such as sodium hypochlorite in commercial bleach solution), the resultant reaction is driven to the left, and chlorine gas is formed. Thus, the formation of stable hypochlorite bleaches is facilitated by dissolving chlorine gas into basic water solutions, such as sodium hydroxide.

The acid can also be prepared by dissolving dichlorine monoxide in water; under standard aqueous conditions, anhydrous hypochlorous acid is currently impossible to prepare due to the readily reversible equilibrium between it and its anhydride:

2 HOCl ⇌ Cl2O + H2O




Hypochlorous acid is a powerful oxidizer and is highly corrosive.


Hypochlorous acid decomposes rapidly and cannot be stored. Hypochlorite salts are more stable, and can be kept as precursor for the acid when needed.


Sodium thiosulfate can be used as neutralizing agent.


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