Calcium hypochlorite is an inorganic compound with formula Ca(ClO)2. It is the main active ingredient of commercial products called bleaching powder, chlorine powder, or chlorinated lime, used for water treatment and as a bleaching agent. This compound is relatively stable and has greater available chlorine than sodium hypochlorite (liquid bleach). It is a white solid, although commercial samples appear yellow. It strongly smells of chlorine, owing to its slow decomposition in moist air. It is not highly soluble in hard water, and is more preferably used in soft to medium-hard water. It has two forms: dry (anhydrous); and hydrated (hydrous).
Hypochlorous acid calcium salt
Bleaching powder, Calcium oxychloride or chloride of lime
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||142.98 g/mol|
|Density||2.35 g/cm3 (20 °C)|
|Melting point||100 °C (212 °F; 373 K)|
|Boiling point||175 °C (347 °F; 448 K) decomposes|
|21 g/100 mL, reacts|
|Solubility||reacts in alcohol|
|Safety data sheet||ICSC 0638|
|R-phrases (outdated)||R8, R22, R31, R34, R50|
|S-phrases (outdated)||(S1/2), S26, S36/37/39, S45, S61|
|NFPA 704 (fire diamond)|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|850 mg/kg (oral, rat)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Calcium hypochlorite is commonly used to sanitize public swimming pools and disinfect drinking water. Generally the commercial substances are sold with a purity of 65% to 73% with other chemicals present, such as calcium chloride and calcium carbonate, resulting from the manufacturing process. As a swimming pool chemical, it is blended with other chemicals less on than other forms of chlorine, due to dangerous reactions with some common pool chemicals. In solution, calcium hypochlorite could be used as a general purpose sanitizer, but due to calcium residue, sodium hypochlorite (bleach) is usually preferred.
Calcium hypochlorite is a general oxidizing agent and therefore finds some use in organic chemistry. For instance the compound is used to cleave glycols, α-hydroxy carboxylic acids and keto acids to yield fragmented aldehydes or carboxylic acids. Calcium hypochlorite can also be used in the haloform reaction to manufacture chloroform.
Calcium hypochlorite is produced industrially by treating lime (Ca(OH)2) with chlorine gas. The reaction can be conducted in stages to give various compositions, each with different concentration of calcium hypochlorite, together with unconverted lime and calcium chloride. The full conversion is shown
- 2 Cl
2 + 2 Ca(OH)
2 → Ca(ClO)
2 + CaCl
2 + 2 H
Bleaching powder is made with slightly moist slaked lime. It is not a simple mixture of calcium hypochlorite, calcium chloride, and calcium hydroxide. Instead, it is a mixture consisting principally of calcium hypochlorite Ca(ClO)2, dibasic calcium hypochlorite, Ca3(ClO)2(OH)4 (also written as Ca(ClO)2 · 2 Ca(OH)2), and dibasic calcium chloride, Ca3Cl2(OH)4 (calcium hydroxychloride also written as CaCl2 · 2 Ca(OH)2).
A confusion sometimes reigns between calcium oxychlorides and calcium hypochlorite. Indeed, the name calcium oxychloride (or calcium hydroxychloride) does not immediately refer to calcium hypochlorite, but is only applicable to the mixed calcium basic chloride compounds remaining unreacted in the bleaching powder, such as, e.g. CaCl2 · 2 Ca(OH)2.
Calcium oxychloride may also be formed in concrete in roads and bridges when calcium chloride is used as deicing agent during winter. Calcium chloride then reacts with calcium hydroxide (portlandite) present in cement hydration products and forms a deleterous expanding phase also named CAOXY (abbreviation for calcium oxychloride) by concrete technologists. The stress induced into concrete by crystallisation pressure and CAOXY salt expansion can considerably reduce the strength of concrete.
A calcium hypochlorite solution is basic as the hypochlorite anion can easily accept a proton from a water molecule leaving an hydroxyl anion in solution. This basicity is due to the hydrolysis undergone by the hypochlorite anion, as the hypochlorous acid is a weak acid. As a result, the hypochlorite anion is a strong conjugate base:
+ H2O → HClO + OH−
The hypochlorite anion is also a strong oxidizing agent containing a chlorine atom at the valence I (redox state: Cl+1) which reacts under acidic conditions with the reduced chloride species (Cl–, here the reducing agent) present in hydrochloric acid to form calcium chloride, water and gaseous chlorine. The global redox reaction involving an electron transfer from the chloride anion (e– donor) towards the hypochlorite anion (e– acceptor) is the following:
- Ca(ClO)2 + 4 HCl → CaCl2 + 2 H2O + 2 Cl2
Calcium hypochlorite is stored dry and cold, away from any acid, organic materials, and metals. The hydrated form is safer to handle. If accidentally mixed with a strong acid as e.g., hydrochloric acid, a sudden outgassing of toxic and corrosive elemental chlorine can occur. Chlorine emanations are harmful and can create severe pulmonary problems.
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- Nwaukwa, Stephen; Keehn, Philip (1982). "Oxidative cleavage of α-diols, α-diones, α-hydroxy-ketones and α-hydroxy- and α-keto acids with calcium hypochlorite [Ca(OCl)2]". Tetrahedron Letters. 23 (31): 3135–3138. doi:10.1016/S0040-4039(00)88578-0.
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- "Calcium-munching bacteria could be a secret weapon against road salt eating away at concrete roads and bridges". The Conversation. Retrieved 7 April 2019.
- Suraneni, Prannoy; Monical, Jonathan; Unal, Erol; Farnam, Yaghoob; Weiss, Jason (2017). "Calcium Oxychloride Formation Potential in Cementitious Pastes Exposed to Blends of Deicing Salt". ACI Materials Journal. 114 (4). doi:10.14359/51689607.