Here's an earlier post on the subject: It may help understand? Duraleigh posted it on another thread...
What does Shock mean?
Quote:
Insufficient chlorination of ammonaceous or ammoniacal waters leads to the formation of a mix of chloramines (monochloramine, dichloramine, and even nitrogen trichloride) which irritate the skin and eyes and impart an unpleasant odor to the water.
At sufficiently high concentrations of active chlorine (hypochlorite, hypochlorous acid, and molecular chlorine), a phenomenon known as breakpoint chlorination occurs. In breakpoint chlorination, ammonaceous and ammoniacal materials are completely oxidized to dinitrogen and the active chlorine is simultaneously reduced to chloride. In order to accomplish breakpoint chlorination, swimming pools are shock-treated. Over the winter, a large amount of organic matter has accumulated in most swimming pools. At the beginning of the swimming pool season, the chlorine demand is too high for normal amounts of chlorine to completely destroy all the organic matter. Consequently, significant concentrations of chloramines and other partial oxidation products are present. In shock-treatment, a large amount of active chlorine is added so as to completely oxidize all of these products.