Shock dosing or superchlorination is a process by which bather impurities are removed from the pool water. When chlorine combines with amines it forms chloramines. Hence, the term combined chlorine. Chloramines have little or no sanitising value. They are the cause of odour and irritation complaints. Often this is a sign that there is too little chlorine in the pool, not too much. Because it is a strong oxidising agent, Calcium Hypochlorite is good for shocking to destroy chloramines.

　　In the water, various reactions are slowly taking place.

　　Chlorine + Ammonia > Hydrochloric Acid + Monochloramine

　　Monochloramine sanitises a little and does not irritate

　　Chlorine + Monochloramine > Hydrochloric Acid + Dichloramine

　　Dichloramine irritates the eyes and nose and does not sanitise. Fortunately, it is unstable and in the presence of chlorine the chloramines breakdown to form hydrochloric acid and nitrogen. With the production of hydrochloric acid however, the pH will fall and unless corrected could lead to a situation where nitrogen trichloride is formed. This is the most irritant of all the chloramines.

　　When the shock dose of Calcium Hypochlorite is added to the water, the initial reaction is for the combined chlorine to rise. After this the free chlorine continues to rise but the combined chlorine does not, indicating that all the nitrogenous matter has been oxidised. This is known as breakpoint chlorination. Usually, adding sufficient Calcium Hypochlorite to raise the free chlorine by 10 parts per million is sufficient for this.