Calcium hypochlorite

Calcium hypochlorite is an inorganic compound with formula Ca(OCl)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.[1] This compound is relatively stable and has greater available chlorine than sodium hypochlorite.[2] It is a white solid, although commercial samples appear yellow. It strongly smells of chlorine, owing to its slow decomposition in moist air.

History
Charles Tennant and Charles Macintosh developed an industrial process for the manufacture of Chloride of Lime in the late 18th Century.[3] It was patented in 1799 and used heavily during World War I for disinfecting the trenches and wounds.
Uses

Sanitation

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 often 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,[4] but due to calcium residue, sodium hypochlorite (bleach) is usually preferred.
Organic chemistry
Calcium hypochlorite is a general oxidizing agent and therefore finds some use in organic chemistry.[5] For instance the compound is used to cleave glycols, α-hydroxy carboxylic acids and keto acids to yield fragmented aldehydes or carboxylic acids.[6] Calcium hypochlorite can also be used in the haloform reaction to manufacture chloroform.[7] Calcium hypochlorite can be used to oxidize thiol and sulfide byproducts in organic synthesis and thereby reduce their odour and make them safe to dispose of.[8]
Production
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[1]
2 Cl
2 + 2 Ca(OH)
2 → Ca(OCl)
2 + CaCl
2 + 2 H
2O
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(OCl)2, dibasic calcium hypochlorite, Ca3(OCl)2(OH)4 (also written as Ca(OCl)2 • 2 Ca(OH)2), and dibasic calcium chloride, Ca3Cl2(OH)4 (calcium hydroxychloride also written as CaCl2 • 2 Ca(OH)2).[9]
Calcium oxychlorides[edit]
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 deleterious 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.

Safety
It is a strong oxidising agent, as it contains a hypochlorite ion at the valence +1 (redox state: Cl+1).
Calcium hypochlorite should not be stored wet and hot, or near any acid, organic materials, or metals. The unhydrated form is safer to handle.

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