Sodium hypochlorite
Alkaline compound used as bleach and disinfectant since the 18th century.
Sodium hypochlorite is an alkaline inorganic chemical compound with the formula NaOCl (also written as NaClO). It is commonly known in a dilute aqueous solution as bleach or chlorine bleach, and is the sodium salt of hypochlorous acid, consisting of sodium cations (Na+) and hypochlorite anions (−OCl). It has been widely used since the 18th century as a disinfectant and bleaching agent, and remains the most important chlorine-based bleach.
- chemical_formula
- NaOCl (also NaClO)
- common_name
- Bleach or chlorine bleach
- appearance
- Pale greenish-yellow dilute solution; anhydrous form is white solid; pentahydrate is pale greenish-yellow solid
- crystal_structure
- Orthorhombic
- key_property
- Unstable; decomposes explosively when anhydrous; liberates chlorine as active principle
- known_for
- Household disinfectant and bleaching agent since the 18th century
Lore & Background
Sodium hypochlorite is most often encountered as a pale greenish-yellow dilute solution referred to as chlorine bleach. The anhydrous compound is unstable and may decompose explosively, but it can be crystallized as a pentahydrate NaOCl·5H2O, a pale greenish-yellow solid that is not explosive and is stable if kept refrigerated. The pentahydrate crystals contain 44% NaOCl by weight and melt at 25–27 °C, decomposing rapidly at room temperature, but reportedly only 1% decomposition after 360 days at 7 °C. A 1966 US patent claims that stable solid sodium hypochlorite dihydrate NaOCl·2H2O can be obtained by carefully excluding chloride ions, which catalyze decomposition into chlorate and chloride.
Reader's Guide
Sodium hypochlorite is significant as the most important chlorine-based bleach, widely used in households and industry for disinfection and bleaching since the 18th century. Its corrosive properties, common availability, and reaction products make it a significant safety risk. Mixing liquid bleach with acids releases toxic chlorine gas, and mixing with ammonia produces dangerously toxic chloramines such as nitrogen trichloride. In solution, the compound is unstable and easily decomposes, liberating chlorine as the active principle. Its stability is pH-dependent: solutions are fairly stable at pH 11–12, but at pH below 2, chlorine gas is evolved. Decomposition to chlorate or oxygen occurs, with rates maximized at pH around 6. The compound reacts with carbon dioxide to form sodium carbonate, and with nitrogen compounds to form volatile chloramines. Its legacy includes industrial production of sodium chlorate and use in oxidation of organic compounds, such as starch modification and alcohol oxidation.
Did You Know?
- Anhydrous sodium hypochlorite is a white solid with orthorhombic crystal structure that decomposes explosively on heating or friction.
- The pentahydrate NaOCl·5H2O is stable if kept refrigerated, with only 1% decomposition after 360 days at 7 °C.
- Mixing bleach with ammonia produces dangerously toxic chloramines such as nitrogen trichloride, not chlorine gas.
- At pH below 2, sodium hypochlorite solutions evolve chlorine gas; at pH above 7.4, the majority is in the form of hypochlorite anions.
More in Sodium Compounds 1-24
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