Calcium hypochlorite

Calcium hypochlorite is a chemical compound with formula Ca(ClO)₂. It is widely used for water treatment and as a bleaching agent (bleaching powder). This chemical is considered to be relatively stable and has greater available chlorine than sodium hypochlorite (liquid bleach).

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Preparation

It is manufactured using the calcium process or the sodium process.

Calcium Process

2 Ca(OH)₂ + 2 Cl₂ → Ca(OCl)₂ + CaCl₂ + 2 H₂O

Sodium Process

2 Ca(OH)₂ + 3 Cl₂ + 2 NaOH → Ca(OCl)₂ + CaCl₂ + 2 H₂O + 2 NaCl

Bleaching powder is actually a mixture of calcium hypochlorite Ca(OCl)₂ and the basic chloride CaCl₂, Ca(OH)₂, H₂O with some slaked lime, Ca(OH)₂.

Properties

It is a yellow white solid which has a strong smell of chlorine. Calcium hypochlorite is not highly soluble in water. For that reason it should preferably be used in soft to middle hard water. There are two types of calcium hypochlorite - a dry form and a hydrated form. The hydrated form is safer to handle.

Calcium hypochlorite reacts with carbon dioxide to form calcium carbonate and release chlorine:

2Ca(OCl)₂ + 2CO₂ → 2CaCO₃ + 2Cl₂ + O₂

Calcium hypochlorite reacts with hydrochloric acid to form calcium chloride:

Ca(OCl)₂ + 4 HCl → CaCl₂ + 2 H₂O + 2 Cl₂

Extreme care should be used in handling this product. Always keep in a cool dry place away from any organic material. When mixing it with water, it is safest to add the calcium hypochlorite to water. This material has been known to undergo self heating and rapid decomposition accompanied by the release of toxic chlorine gas.

Uses

Calcium hypochlorite is used for the disinfection of drinking water or swimming pool water. For use in outdoor swimming pools, calcium hypochlorite can be used as a sanitizer in combination with a cyanuric acid stabilizer. The stabilizer will reduce the loss of chlorine because of UV radiation. Calcium does make the water 'hard' and tends to clog up some filters. However, some types of calcium hypochlorite do contain anti-scaling agents in order to prevent clogging up of pipes/filters. This grade of calcium hypochlorite can also be used in hard waters. The main advantage of calcium hypochlorite is that it is unstabilised unlike chlorinated isocyanurates such as sodium dichloroisocyanurate or trichloroisocyanuric acid. Latter products do contain cyanuric acid. If the level of cyanuric acid becomes too high, it will influence the performance of the chlorine. Pools running on chlorinated isocyanurates should maintain a free chlorine level between 2 and 5 ppm (mg/L), whereas pools running on calcium hypochlorite should have a chlorine level of 1-2 ppm (mg/L).

Calcium hypochlorite (known as 'bleaching powder') is also used for bleaching cotton and linen and used in the manufacture of chloroform.

References