Calcium hypochlorite is made up of the calcium salts of hypochlorous acid. It is produced by dissolving chlorine gas (Cl2) into a solution of calcium oxide (CaO) and sodium hydroxide (NaOH). Calcium hypochlorite is a white, corrosive solid that comes either in tablet form or as a granular powder. Calcium hypochlorite is very stable, and when packaged properly, large amounts can be purchased and stored until needed. The chemical is very corrosive however, and thus requires proper handling when being used to treat water. Calcium hypochlorite needs to be stored in a dry area and kept away from organic materials. It cannot be stored near wood, cloth or petrol because the combination of calcium hypochlorite and organic material can create enough heat for an explosion. It must also be kept away from moisture because the tablets/granular powder readily adsorb moisture and will form (toxic) chlorine gas as a result. Calcium hypochlorite has a very strong chlorine odour – something that should be kept in mind when placing them in storage.
When treating water, a lesser amount of calcium hypochlorite is needed than if using chlorine gas. Compared to the 1-16 mg/L required with chlorine gas, only 0.5-5 mg/L of calcium hypochlorite is required. When calcium hypochlorite is added to water, hypochlorite and calcium ions are produced.
Ca(OCl)2 -> Ca+2 + 2OCl-
Instead of decreasing the pH like chlorine gas does, calcium hypochlorite increases the pH of the water (making the water less acidic). However, hypochlorous acid and hypochlorite concentrations are still dependent on the pH of the water; therefore by decreasing the pH of the water, hypochlorous acid will still be present in the water. As a result, calcium hypochlorite and chlorine gas both produce the same type of residuals.