Chlorine is used in pools to kill harmful germs and algae. Chlorine can be delivered to the water in several forms. Free chlorine gas has storage and handling problems as the gas is toxic when inhaled, indeed the gas was used as a chemical weapon in WWI. When chlorine (or Cl2) is added to water the following disproportionation reaction occurs:
HOCl is called hypochlorous acid and is the main sanitising agent in pool water. The most common way of adding this to water usually as sodium or calcium hypochlorite. The compound is unstable and dissociates in water to form HOCl or 'free chlorine'. Alternatively di- or trichlorocyanuric acid can be used to add Cl2 to the water (see other pool chemicals page for more on this)
The free chlorine can then break down bacteria by entering the cell wall and disrupting protein and enzyme functions. When hypochlorous acid reacts with ammonia (present in sweat and urea) chloroamines are produced:
The distribution of the three types of chloroamines is a function of pH5. Chloroamines are combined chlorine residuals and the concentration of chloroamine in the water can easily be calculated using;
Total chlorine = Free chlorine + Combined chlorine
As there are simple tests (using small tablets that dissolve in the pool water producing colours - the colours relate to concentration) for the amounts of free chlorine and total chlorine in the water the combined chlorine concentration can be calculated by subtracting the amount of free Cl from the total amount of Cl.
Dichloroamine has a disagreeable taste and odour - the characteristic 'chlorine' smell of swimming pools. In order to break down chloroamines more chlorine must be added. This is known as shock treatment where a large dose of free chlorine is added to break down unwanted compounds an leave a chlorine residual. Contrary to popular belief the smell that many people find unpleasant in pools is actually caused by too little chlorine in the water!