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The three most common chlorine-containing substances used in water treatment are chlorine gas, sodium hypochlorite, and calcium hypochlorite.
Hypochlorite solution gradually releases chlorine into water. Hypochlorite anion,ClO-, changes the oxidation-reduction potential of the cell, and resulting in the inactivations of the micro-organism's function. Calcium Hypochlorite quickly forms hypochlorous acid (the killing form of chlorine) in water. It is used in water treatment as a general biocide. Calcium hypochlorite is used by many paper conservationists to bleach archival papers, and it is used as an oxidizing agent to reduce the effects of foxing.